SME Mining Engineering Handbook 3ed Deawatering - Darling Peter

CHAPTER 9.5

Dewatering Surface operations Paul R. Peppers

inTRoDuCTion

flow-gauging station data. These can show up as drift in the mean runoff levels and intensity over time because removal of ground cover and construction of paving tend to limit infiltration and reduce the concentration time. Some examples of these factors follow:

Hydrology is an imprecise science developed by empirical observations by many people over the course of 3,000 years. Complex nonlinear systems are involved; many of the factors that go into the computations are probability based, time variable, imprecise, or uncertain. Identifying and quantifying the risks associated with system failure usually sets the level of effort that is justified. More precise calculation methods are evolving, but, considering the uncertainty of the data, approximations are commonly used. The locations, quantities, and detrimental effects associated with water in the operating areas are continually changing as the mine development progresses. This effect renders the precise calculation of the performance characteristics of the pumping system at any particular point of limited operational value. Computer-based systems hide the complexity of the calculations and allow more rapid solutions of the empirical equations. Complex computational fluid dynamic calculations are usually justified only if the costs and development times are substantial. For example, a manufacturer of a new class of pumps could justify the effort because upfront computer modeling may well reduce the cost and time needed to build prototypes. However, a mine engineer trying to decide if that pump would have sufficient capacity to pump from his pit probably could not justify that level of computation. Some factors from the mine planning process are predictable with reasonable precision. For example, the hydrostatic lift necessary to pump water from a planned shovel face is predictable because engineers design the elevations of the pit face and pit rim in advance. Changes in these factors are predictable as long as mine operations follow the plans. The size and shape of the drainage basins that contribute to the runoff into the mine area are measurable in advance, but other critical factors affecting the quantity estimation of runoff are difficult to measure accurately. Meteorological factors, climatology, and actual rainfall can vary significantly from the predictions. Changes in land use and development over time reduce the predictive ability of long-term rainfall monitoring and

• Natural basins and channels are nonhomogeneous in their ground cover, slopes, soil type, and moisture content. • The variability of rainfall intensity over a basin increases with the size of the basin. Larger basins are subject to locally heavier precipitation intensities. • Most precipitation records are point measurements. Rainfall gauges are small samples of the larger precipitation event. Spatial variability of storm patterns makes even nearby locations difficult to predict without on-site data. • Streamflow gauges are better indicators than rainfall gauges, where these records are available, because of the integration of the runoff from the upstream basins, as long as the characteristics have not changed over time. Historic observations used to determine the probabilistic frequency of rainfall events are uneven. In some places, reliable precipitation records may have recorded the variability over several centuries, while in others, only for decades. In the case of some greenfield developments, no historic data may exist at all. In semiarid regions, empirical methods are commonly used to predict rainfall runoff events, which make it difficult to quantify the magnitude of potentially damaging high-return period storms such as 100-year or 500-year events. Coverage is definitely better in the developed world than in remote areas.

Mine DeWATeRing TeAM

Few people think of mine dewatering at a surface operation as a major issue when compared to other production tasks such as fragmentation, loading, hauling, crushing, or milling. If neglected, however, no other operations are possible in the mine. Mine dewatering requires applying knowledge from many fields to create practical, cost-effective, and efficient

Paul R. Peppers, Supt. Central Maintenance & Projects, Sierrita Operations, Freeport-McMoRan Copper & Gold Co., Green Valley, Arizona, USA

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SMe Mining engineering handbook

systems to manage the water. This chapter discusses practical field methods that provide sufficient levels of accuracy and precision to limit risk. The objective is to put tools in the hands of the engineers and technicians to manage mine water inflow and accomplish production effectively and economically. All of the following specialists have a role to play in managing water, but it is the role of the generalist to understand the big picture and know when to call the experts. • Mining engineers overlap and coordinate with other specialty engineering disciplines to develop a cross-disciplinary engineering approach to the issues involved with prediction, collection, and handling of water in mine workings. • Meteorologists track the data related to historic precipitation patterns, utilizing rainfall-measuring stations, streamflow gauges, radar, satellites, and modern database systems that supply the data to predict probable precipitation values and the associated statistical risks. • Civil engineers/hydrologists predict the runoff recurrence and design the surface drainage controls, storage basins, and conveyance structures. • Mechanical/structural engineers provide the expertise to design pumps, pipelines, and mechanical aspects of pumping stations, including concrete, supports, and pump engineering. • Electrical/instrumentation engineers design the electrical power, distribution, instrumentation, and controls required to run, interlock, and control pump stations and dewatering systems. • Environmental engineers provide the knowledge of federal, state, and local regulations for obtaining permits prior to commencing operations. They assist mines to predict, evaluate, and mitigate pollution from operations. • Land, legal, and water rights professionals manage the legal aspects of obtaining permission to extract and use specific water quantities for industrial purposes. They also help miners manage disputes over historic ownership and water rights law. • Government agencies fund and manage the basic research and development that provide much of the background hydrology data available. In the United States, the National Weather Service and the National Oceanic and Atmospheric Administration (NOAA) collect and distribute much of the precipitation data that are critical for predicting runoff. The U.S. Geological Survey provides a database of streamflow records for drainage basins and groundwater levels in monitoring wells located throughout the country.

RevieW of SuRfACe hyDRAuliC CAlCulATionS

This sections gives a brief outline of the process of designing a runoff control system for a mine. The process can be broken down into a series of estimates required to scope the problem. Detailed explanations for these estimates are in Chapter 16.4. Precipitation/Storm events To determine the amount of protection necessary for the various mine operations structures, the risk level of the proposed structure can be estimated based on failure consequences. The appropriate design storm point precipitation frequency (PPF) event can be selected from NOAA or other sources to determine precipitation amount and intensity (NOAA 1980).

An example of a design storm selection based on risk follows: Risk

Design Storm

Low risk

10 years: 6-hour storm

Medium risk

100 years: 24-hour storm

Catastrophic risk

1,000 years: 24-hour storm or probable maximum precipitation

Basin Runoff Calculation Using the rational method or one of a number of computational runoff-calculating models (U.S. Army 1971): • Basin characteristics for each basin in the watershed are measured: shape, area, channel length, ground cover, and relief. • PPF from the design storm for the area is applied: A measure of precipitation (millimeters or inches) that falls within the storm period (e.g., 25 mm or 1 in. of rainfall in 6 hours expected once every 10 years). • Peak flow rate Q, time of concentration, and total runoff volume are calculated. • Routing network is designed. Using the flow information from the individual basins, the runoff protection network can be designed. This may consist of a number of structures to route the water from the source to the discharge point: open channels, culverts, detention and sediment control basins, spillways, weirs, and flumes. The U.S. Army Corps of Engineers Hydraulic Engineering Center (HEC 2009) tools are helpful, as is the American Iron and Steel Institute handbook (Spindler 1971).

SAfeTy ASPeCTS of DeWATeRing

Water in mine workings presents a number of hazards to the safe operation of the mine. Sudden precipitation events result in rapid, high-volume runoff into surface mines along the road network and over the pit rim. Water allowed to run over highwalls can cause erosion, bringing down debris, talus, and raveling, thereby creating hazardous conditions to mining operations below. Ponding, erosion, and degradation of the haulage surfaces and working benches mean that all operations slow down to maintain control of the equipment. Slides and rockfall risks increase when water percolates into cracks in the highwall. In cold climates, ice and freeze–thaw cycles increase the risk of raveling and rockfalls as the freezing water swells inside the fractures, wedging the blocks apart. Ice builds up where water seeps out of the highwall. When these overhangs fail and fall, they can cause extensive damage, especially if the ice mass is large. Seasonal thawing causes these ice buildups to break loose and rain rocks and ice into the pit. Drainage control maintenance should be conducted prior to the arrival of seasonal precipitation in areas where this is predictable. Such control measures include filling surface cracks around the pit perimeter and diverting runoff from the pit rim to limit water infiltration. Regular inspection of ditches, culverts, and drainage channels for damage and cleaning away debris to allow maximum water flow should become standard operating procedure. Scheduling of major construction or modifications should allow for completion during the dry seasons, avoiding critical system downtime on dewatering systems.

Dewatering Surface operations

High-volume precipitation events cause flash flooding, particularly in mountainous areas and arid climates, such as in the American Southwest. Proper planning of the pit location and associated mine facilities must take into account drainage protection. Significant storm runoff can be controlled by properly designed drainage controls that direct runoff away from the pit to prevent problems before they occur. In-pit haulage roads and benches should be sloped to prevent ponding and direct water through perimeter ditches to sumps for collection. Culverts must be adequately sized to convey water where roads intersect drainage crossings. emergency Planning Contingency plans must exist to protect personnel from entrapment if flooding from sudden inflow of runoff results in loss of the main access route. This can happen if the haul road washes out or slides off the wall because of slope failure. Ensuring that the pit designs include emergency evacuation routes must be part of the mine planning process. Surface mines usually have more than one escape path from the pit. Ramping down to start a new bench in the pit bottom, commonly called drop cutting, can be particularly hazardous during monsoon season. It may be necessary during intense precipitation events to abandon mining equipment in the drop cut and evacuate personnel to a safe location until the runoff subsides. Escape route planning is even more important in underground mines that may be subject to rapid inundation if old, flooded workings exist in the area. Mine pumping systems cannot keep up with storm runoff in real time due to extremely high inflow rates that are usually at least one to two orders of magnitude higher than economically feasible pumping rates. In severe events, power may be disrupted, and sumps designed to capture the entire runoff volume of a particular design storm can be overwhelmed. Most dewatering systems cannot function during heavy rains and must be shut down to prevent damage from excessive sediment and debris. Floating barge pumps handle these events much better than fixed pump stations as long as their sumps can handle the runoff. After significant precipitation, operators may have to excavate mine sumps in order to recover the original volume lost due to sediments washed in from erosion of the roads and walls above. Pumps may be inundated or buried under sediment and may need to be replaced before pumping from the pit can resume. impoundments An embankment that impounds water at a level above its previous channel elevation creates a potentially hazardous condition. Failure of the impoundment can release a tremendous amount of stored energy, which is then directed at anything below. Embankments, dams, and diversion dikes require higher than normal safety factors if that structure’s failure could lead to loss of life or significant property damage. This higher safety factor is based on risk assessments of downstream structures and populations. Periodic inspections, monitoring, and maintenance of operating impoundments must be carried out to ensure that the structure remains safe and its operation is within its designed limits. The size of the impoundment often determines the frequency and level of inspections required by the permitting agencies. Knowing the applicable regulations can reduce the costs of monitoring and inspections. For example, under the U.S. Mine Safety and Health Administration regulations for

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coal mining, embankments larger than 6 m (20 ft) high and ones with a capacity exceeding 24,670 m3 (20 acre-ft) require inspections every 7 days. Impoundments only slightly smaller have less onerous inspection requirements (MSHA 1977). Dam breaks are now rare because of regulatory vigilance imposed in the wake of significant disasters in the past. The potential damage from a failure can be very high due to the high volumes of water that can be released in a short time and the proximity of population and structures downstream. When impoundments fail, investigations into the causes usually find that the fault lies with improper design, construction, operation, inspection, and/or poor maintenance practices. Slope failures into impoundments Evaluation of the potential for slope failure should be part of the design criteria for all deep sumps adjacent to highwalls. Backfilling operations into flooded pits create the potential for dump failures to produce large damaging waves. The volume of sumps built under slopes with a history of stability problems should be as small as possible. Shallow sumps with large surface areas are preferred to smaller deep-water sumps. Miners working near large in-pit ponds and sumps must be extra vigilant if any risk exists of slope failure into the water. Procedures for real-time monitoring of wall stability can mitigate the hazards to provide advance warning if raveling develops. If personnel are required to work near the shoreline of sumps, shallow areas such as old ramps are most hazardous because a wave approaching from deeper water can rise significantly. An example of slide-induced waves occurred at the Valdez Creek mine in the Alaska Range (United States) during backfilling into a previously mined pit, when a dump failed and created a very large wave. The area, a long, sinuous valley 67 m (220 ft) deep by 457 m (1,500 ft) wide and 1.6 km (1 mi) long, was concurrently collecting tailings and dump material from the stripping operations. The pit had a standing water pond 53 m (175 ft) deep against the toe of the dump. Mine dumping was from the original pit rim elevation in a single lift about 24 m (75 ft) above the water surface. Tension cracks developed behind the dump crest, and a large section 18 m (60 ft) thick slid into the water. The dump material displaced the water at the toe, resulting in a wave 11 to 12 m (35 to 40 ft) high that hit the wall across the pit. At the far end of the valley 1.6 km (1 mi) away, the wave was still more than 2 m (6 ft) high where it damaged a pump station. Fortunately, no one was hurt, but the potential for severe injury existed for anyone working on the ground near the surface of the pond. As a historic note, in 1958, the largest wave ever recorded, at 533 m (1,750 ft) high, was caused by a slope failure into deep standing water in a fjord in Lituya Bay, Alaska (BBC 2000). Access Safety and Maintenance Planning Heavy machinery, deep water, limited access, and people in close proximity create significant safety challenges that must be mitigated when considering floating barge applications. Long-term sumps must have access structures designed into the system. These can include floating gangways complete with handrails, cableways, and pipeline flotation systems. Anyone working around the barge must wear appropriate flotation protective devices. Whenever the barge and pump draft are too deep to allow the barge to be pulled close to shore, docks may need to be built. In sumps with shorter design lives,

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the barge may be accessed by boat, providing that its weight limits and stability are not exceeded as a work platform. The following are important factors to be considered: • Servicing plan for pump/control maintenance • Weight and center of gravity for the barge, crane reach limitations, and control access requirements • Occasional operator access for pump maintenance • Support for pipelines and electric cables to account for water level changes Moving equipment and machinery on barges creates the possibility of capsizing the barge. To avoid tipping, major shifts in the center of gravity during maintenance or operations need to be carefully planned. Special marine engineering expertise may be required to identify hydrodynamic, stability, and buoyancy issues before construction or major component replacements. Buoyancy control adjustments can be accomplished with air chambers that allow the deck level adjustments. Pipeline supports must include additional buoyancy where the discharge pipe connects to the barge to compensate for the weight where the pipe rises above the water. lifting Plans, Weight, and Crane Capacity Site access must be designed to allow pumps, motors, and other equipment to be successfully lifted from the barge to transport equipment onshore. Pump and barge draft requirements may limit the ability to drag the barge close enough to the bank for service. The slope of the sump bank frequently causes the barge to run aground well before it is close enough to shore to enable convenient access. For small, steep-sided sumps, this is less of a problem due to the higher wall angles. Large sumps or pit lakes require the development of standard operating procedures that include the weights of the barge, pumps, switchgear, and pipe fittings that will need to be placed both during construction and changed later during maintenance cycles. Some sumps have access limitations that preclude using shore-based lifting equipment. In these specialized cases it may be necessary to employ sky-crane heavylift helicopter services to fly the equipment to the operating location. Generous factors of safety are needed to account for the barge weights that will increase when mud, water-logged floats, and scale accumulate over time. The suction force that has to be added to the weight to break free of the mud if the barge settles to the bottom should not be underestimated. The critical factors in these plans are the crane locations, reach limits, and estimates of the weight that must be lifted. Bank stability is also a concern in sumps with fluctuating water levels.

in-PiT WATeR ConTRol

The primary objective of the in-pit water control network is to expeditiously convey water through the workings to the temporary collection sumps for removal without adversely affecting the mining activity. Runoff sources include the point sources from groundwater infiltration, streams entering the pit, and rainfall collection on the surfaces inside the pit’s perimeter. Factors that water control system designers must consider as engineering trade-offs are the cost/performance designs involving the pump capacity, pipeline diameter, power requirements, system duty cycle, and number of lift stages.

Large-capacity pump/pipeline systems with few stages are designed to run intermittently compared with smaller systems with larger sumps that run almost continuously. The large system has a higher factor of safety to handle unusually large inflows but comes at a higher capital cost than the smaller system, which accepts higher risks of being overwhelmed in infrequent precipitation events but handles normal dewatering needs better. Smaller, redundant parallel pumping systems can provide the surge capacity of a larger system while increasing system availability. Increasing sump capacity is one of the least expensive ways of reducing risk without sacrificing the efficiency of the pumping system. indirect Costs of Water in the Pit Water in the rock fractures adversely affects the geotechnical stability of the pit walls. Mining pits at the steepest stable pit highwall angles is the most profitable because it lowers the stripping ratio. Proper water control increases the pit wall stability, thus maximizing the wall angles. Preventing water from infiltrating the fractures in the rock is critical to maintaining maximum stability. Horizontal drain holes drilled to intercept the fractures can successfully reduce the pore pressure. Water in the mine working areas causes inefficiency in all materials handling operations. Water buildup at the oreloading face degrades the crushing and conveying efficiencies. Dewatering efforts should move the water to local sumps that follow the loading equipment close enough to prevent digging in wet ore. Fines and water combine to form a highly abrasive grinding medium that destroys moving parts. Excessive moisture can cause backsliding on inclined belts where the feeders drop ore onto a moving belt. The steeper the belt, the worse the problem becomes. When conveying saturated material, water seeps out of the muck, running down the belt until it spills and builds up in the drives and under the rollers. Particularly in cohesive ores with many fines, wet material causes a buildup in chutes, resulting in plugging and costly downtime. Slabs from material buildup on the walls of the chutes can break off, fall onto the belt, and cause adverse wear or torn belts. haulage Road Runoff Control Haulage roads act as arcs in the flow network in the pit because they are continuous and interconnected, and lead from the pit’s upper reaches to the bottom working levels, making them ideal channels for water. Haul truck tires are becoming one of the major costs of mining; wet roads, tire slippage, and sharp rocks are primary causes of premature failure. Drivers must reduce their speeds to maintain control under wet road conditions. Maintaining good drainage control prevents visibility degradation and reduces spray from haulage roads. Muddy conditions create poor traction, resulting in increased stopping distances for mobile equipment. Properly designed haul roads should be crowned or slanted to prevent ponding on the road surface. Crowned roads with at least 2% cross grades have fewer problems (than surfaced roads without drainage grades) with washboard ruts because the road surface retains most of its strength by having the cross grade eliminate standing water. Perimeter ditches concentrate the water and direct it down toward the sumps. On long ramps, the water volume can build up significantly before it reaches the bottom, as smaller tributary streams and rivulets join the main flow. It is desirable to divide the flow before the water accumulation gets deep enough to overflow the ditch


into the haulage lane. Where the ditch is located on the pit side of the road, creating holes in the berm allows part of the runoff to escape the ditch and sheet flow down the wall, reducing the size of the ditch necessary for the remaining portion of the flow. The water flows over the highwall, travels to the next lower area in the pit, and from there to an intermediate sump or another berm relief on its journey toward the bottom. Switchbacks in the haul road present challenges to handling the water collected in the ditches along the highwall. Haul roads usually have superelevated curves raising the outer edge of the road perimeter. If the straight road section is crowned or sloped back toward the highwall, water runs in a V-ditch between the road and the wall. When approaching the switchback, the ditch elevation increases as the road nears the superelevated curve, and the water escapes the ditch, running across the active traffic lanes to continue downhill. The result is erosion of the travel surface, creation of washboard ruts, and tendency of the water to meander over the entire road surface. Several ways are suggested to mitigate these problems: • A trench between the highwall and the superelevated curves should be created to allow water to continue past the curve and cascade over the wall. This has the effect of moving the water off the road but does so by directing it to the pit bottom. Most pit designs do not allow enough width to accomplish this because the trench takes up too much room. • Diagonal swales, grooves, or water bars should be cut into the road surface prior to the curve to force the water to cross the road at a designated location. Aggregate can be selectively placed and compacted to prevent erosion in these designated water crossings. Approach transitions must be gradual enough to ensure no adverse effects on the speed of trucks traversing the swales. • The most expensive way to move the water is to divert the ditch into a culvert running under the superelevated fill. The culvert can run along the same route that the trench would have taken but without the width “penalty.” Alternatively, the culvert can cut the corner under the lanes, thereby diverting the water to the inside edge below the switchback, continuing the V-ditch down the road. The culvert entrance design usually involves a small sump but must prevent water from backing up and flooding over the road. There is a width penalty to construct this, and sufficient fill depth must be available under the road to prevent the weight of a loaded haul truck from crushing the culvert pipe. Sumps Sumps capture and temporarily hold runoff to allow for sediment control and for pumping the clarified water. Properly designed water collection networks concentrate the accumulated runoff as high in the pit as possible. Sumps can be located in a pit bottom, a wide spot in the bench along the wall, or at a switchback along a haul road. Obviously, pumping costs increase with the elevation difference between the sump location and the discharge elevation. Sumps should be large enough to ensure that the active volume will keep the pump running for a reasonable period. Efficient sump design matches the pump capacity to the inflow rate for normal groundwater infiltration, reducing the need for periodically cycling the pump. If a sump has low or infrequent flows, it may not be in the right place. In such instances, it

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may be necessary to combine the inflow from other ditches to increase the utilization or consider eliminating the sump by rerouting flows elsewhere. Removal of expected sediment and debris must be part of any sump design. The extent of the cleanout structure depends on the planned life of a sump, the velocity of the water in the approaches, and the presence of sediment traps in channels. Sediment traps have weirs and larger-flow cross sections in the channel, which slow the water flow, allowing the entrained solids to drop out of suspension. Sumps collecting runoff from high-intensity storms can rapidly fill with sediment if the velocities are erosive because of steep channel grades or easily eroded soils. It is easy and advisable to inspect the sumps periodically to ensure that necessary capacity exists. Slope-stability effects must be part of the design and location of the sump. Geologic structures known to have stability issues should be avoided. Long-term sumps on the highwalls are a source of water infiltration that percolates into the rock, resulting in an increase in the pore pressure along the fractures. This increases the probability of slope-stability problems as mentioned in the safety section.

gRounDWATeR

The “Review of Surface Hydraulic Calculations” section discussed handling runoff water from meteorological sources that originated outside the mine. This section will give a brief outline of the process of identifying, quantifying, and controlling groundwater infiltration into the mine. Groundwater infiltration from aquifers, water-filled cavities, or other sources exists in nearly all mining operations. A more detailed treatment for these issues is found in Loofbourow 1973 and in Chapter 16.4. The process can be broken down into a series of estimates required to scope the problem depending on the status of the project. Premining feasibility studies: • The subsurface geologic structure and hydrology of the deposit are mapped and understood. • Exploration wells are drilled to locate and quantify potential water resources for processing and potential mining hazards. • The wells are tested to model flow rates, porosity, temperature, and chemistry, looking for possible adverse issues. • The quality and chemical makeup of the water are assessed in order to design the treatment methodology. • The interception strategy is chosen for handling the predicted water inflow rates: – Interception wells in the aquifer are pumped to dewater outside the mining area. – Inflow outside the mine perimeter is blocked by reducing porosity. – Inflow into the mine is accepted and handled along with other water sources. Mine development phase: • Predictions from the pre-mine feasibility studies are verified. • The flow outside the mine perimeter is blocked through grouting, freezing, or other methods to reduce porosity of aquifers or fractures identified as high risks. • Water is intercepted and drawn down using well fields, preventing quality degradation and avoiding treatment issues.

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Mine operation: • Groundwater infiltration is routed into sumps and handled through the in-pit pumping systems. • Water quality issues are treated along with affected run-on water prior to discharge or reuse. • Pump system redundancy and capacity are maintained to handle unexpected changes in flow rates. (See the Valdez Creek case study later in this chapter.) • Interception well fields and blocking structures are maintained and expanded as mining progresses through the reserve. The presence of groundwater in a mining claim has both beneficial and detrimental effects on the economic viability of the operation. All mining operations and processing plants need water to function, and it must be imported if insufficient supplies exist on the property. However, groundwater intercepted in the pit is usually in the wrong place with inconsistent flow rates and may have quality problems that must be handled before it can be used in the process. The main objective in mine dewatering is to economically capture the water, route it around the operations with minimal disruption, and store it for beneficial use; or, failing that, treat the water prior to discharge. Wet Drilling and Blasting Drilling under wet conditions is more expensive and less productive than if the operation is dry. One method of limiting the effects of water on the drill bench is by digging perimeter trenches into the bench. This is particularly useful in deposits that have considerable water present in the rock but relatively low permeability. Perimeter trenching would not be effective if the ore is highly porous because this allows water intercepted by the trench to recharge back into the drill pattern. Even if it is not possible to pump or drain the perimeter trench, it may lower the local water table, sufficiently drawing the water from the bench’s upper couple of feet to improve the conditions for starting the hole (collaring). Fracturing from blasting the previous bench is usually sufficient to provide the additional permeability to draw down the local water table from the drill area to the perimeter trench. The permeability in the unfractured, surrounding rock is lower, preventing excessive flow from the walls back into the drilling area. If the bench conditions are wet, redrilling is often required and may or may not be successful because the same water conditions that caused the initial problems still exist. It is not uncommon to drill a pattern while drop cutting only to find that 90% of the holes are collapsed or plugged by the time the blast is scheduled. After the good holes are loaded and shot to create additional porosity to pull down the water table, redrilling starts again. Under wet conditions like this, it can take a month to develop a 152-m- (500-ft-) long drop cut that might take only a week if the bench were dry. Although factors such as energy density, detonation velocity, or energy distribution usually dominate the decision about the explosive type, the presence of water in the blastholes prevents using the least expensive blasting agents. Commonly used ammonium nitrate and fuel oil (ANFO) blasting agents will not detonate or will misfire if the powder gets wet. Water-resistant emulsions must be used if the water cannot be removed, but they are nearly twice as expensive to use as dry-hole, ANFO-based products of equivalent energy. Blasthole dewatering with hydraulic-driven pumps can offset some of the emulsions costs if the water infiltration

rates are slow enough. Pumping the holes and using impermeable polyethylene sleeves allows the placement of inexpensive water-sensitive ANFO in areas that otherwise would require emulsions. The cost of the sleeves adds to the blasting costs but significantly offsets the cost of using expensive emulsions. Case Study: valdez Creek groundwater Control Between 1993 and 1995, Cambior Alaska’s Valdez Creek mine in the Alaska Range was reaching the end of the mine’s reserves. This large placer gold mine operated year-round above timberline in a remote subarctic stream valley. This mine operation’s primary defining challenge was groundwater infiltration. Flow in Valdez Creek ran year-round across the entire length of the mining claim and continually recharged the subsurface aquifer. Groundwater flowed through the gravel deep in the deposit year-round, although the pit walls froze solid to a depth of 3 to 4.6 m (10 to 15 ft), blocking the flow during the winter. Blasting occasionally breached this frozen wall, resulting in sudden inflows of water and sediment into the pit. The inflow increased during the annual spring breakup with a significant spike in water volume as the walls thawed, draining the water held back during the winter. Management scheduled closing of the mine by freezeup in the fall of 1995 due to reserve exhaustion and marginal economics. The company was under pressure to limit spending, particularly on fixed infrastructure that it would have to abandon or scrap at closure. The remote location required more time and logistical effort than most mines to make major improvements. Adding incremental diesel-powered pumps was feasible with some notice (weeks), but adding highdensity polyethylene (HDPE) pipeline capacity was difficult, expensive, and required a long lead time (months). The mine experienced increasing water inflow rates. In 1993, the pit had an approximately 69-L/s (1,100-gpm) inflow with a pumping capacity of 82 L/s (1,300 gpm). By mid-1995, the inflows had increased to 454 L/s (7,200 gpm). Pipeline and pumping capacities were barely adequate for initial conditions. Surges overwhelmed the system, flooded the pit floors for weeks, and interrupted ore production until the inflow subsided. The initial 20-cm- (8-in.-) diameter pipeline was not adequate beyond about 63 L/s (1,000 gpm), requiring two pumps in series to keep up and maxing out pipeline capacity. Because the surge capacity of the system was limited, the mine added a second 20-cm (8-in.) pipeline. Flows increased in 1993 to nearly 126 L/s (2,000 gpm) by freeze-up, interrupting production more frequently. Management agreed to fund an upgraded 30.5-cm- (12-in.-) diameter pipeline because it was essential to regain control over the water in the pit. This pipeline was in place before breakup in 1994. The new pipeline worked well for about a year before it was deemed insufficient to keep up with increasing demands. The inflow tripled over this period to 391 L/s (6,200 gpm). The pumps that were on-site or readily available had to be pressed into service even though they were not well matched. The system started with one large diesel-powered pump and then added a large electric slurry pump and finally an additional large diesel pump in series. Two of the pumps operated at poor efficiencies, well beyond their curves due to flows above their designed flow rate. Only one pump was still operating on its curve. Adding the second and third pumps in series to the pipeline resulted in smaller incremental flow increases. Extremely high flow velocity in the pipe, approaching 6 m/s


(21 ft/s), resulted in high friction head losses (3 m [10 ft] of loss per 30.5 m [100 ft] of pipe), causing the hydraulic grade line (HGL) to exceed 10% slope. Pumping in series adds the pressure of the last pump to the discharge pressure of the previous stages, so the sequential order of the pumps is important. Pumps, like pipelines, have maximum design internal case pressure ratings to avoid overpressuring and cracking the cases. Additionally, the discharge pressure exceeded the recommended working pressure for the HDPE pipeline, which was operating at a reduced safety factor. The operating point for this system was only possible because it pumped cold, near-freezing water, doubling allowable working pressure. The last incremental 63 L/s (1,000 gpm) required additional pipeline capacity 6 months before closure. Because no additional funds were available, the pump crew salvaged the original 20-cm- (8-in.-) diameter pipeline from the 1993 pit and relocated it to the final pit. Two pumps in the series pushed the incremental flow up the wall, extending production nearly to the end of the mine’s life. Ultimately, the lack of a pump caused abandonment of a portion of the final ore face in the last 3 months. The pit filled with water because of the lack of a 189-L/s (3,000-gpm) capacity cross-pit pump and the availability of a short pipeline to move water from the shovel-loading area. This inundation resulted in the loss of approximately 3,000 to 4,000 oz of gold contained in ore that was uncovered and awaiting haulage to the wash plant. The mine lost the water battle in late summer when an additional 442-L/s (7,000-gpm) inflow occurred through the pit wall, immediately forcing the operation to abandon production efforts and to commence reclamation. When considering the increasing magnitude of the water inflows over a short period, the mine did an outstanding job managing the pumping problems with limited resources available. It managed to mine more than 95% of the planned tonnage from the final pit even with seriously deteriorating pit water conditions.

SPeCiAlizeD PiT PuMPing PRoBleMS

Because mines are located where economically recoverable minerals are found, they are frequently in inhospitable locations with extreme climates. Mountainous areas have high relief, topographically controlled rainfall, and steep channels that cause erosive flow velocities. Jungles frequently add extremely high rainfall and seasonal monsoonal patterns. Arctic areas have extremely cold climates, ice, glaciation, and permafrost. Deserts defined by low annual precipitation can be very hot (Sahara), high altitude and cold (Antarctica, Atacama in Chile), or dry with most of the annual precipitation in a few intense storms (Arizona’s Sonoran Desert in the United States). Each of these climates has unique challenges that affect the dewatering system designs. Arctic Conditions and Permafrost Permafrost is the condition in the soil below the seasonal active layer where the temperature never rises above the freezing point of water. This is not a problem by itself, but if the soil contains moisture and fine-grained soils, it tends to create thick lenses of clear ice. Permafrost that exists in foundation areas must either be kept frozen or removed. If the conditions that formed the permafrost such as ground cover and shading are changed, thawing is possible. Fine-grained soils must

749

be removed from the foundations of the pump stations and replaced with coarser, nonwicking material. Any source of heat, such as flowing water in pipelines, must be insulated to prevent thawing around the pipe. If the ground thaws, the bearing capacity may be lost, causing collapse of the overlying structures. Buried pipes in frozen ground can melt an annulus around the pipe that changes the permeability of the soil. In the case of an impoundment, sump, or artesian well, this can result in uncontrolled flow outside (and around) the pipeline. Above about 60° latitude, permafrost may exist in all subsurface areas unless the area has been tested and been found to be clear. Even in areas generally free of widespread permafrost, there may be sporadic permafrost containing clear ice lenses in local areas. Cold climate areas without permafrost have seasonal freeze–thaw cycles that occur in the active layer. Depending on the severity of the climate, these can affect ground support stability as much as 3 m (10 ft) below the surface. Haulage traffic can push the frost line considerably deeper than usual, up to 5 m (18 ft) under the roadway, because of increased compaction, reduced moisture content, and the complete elimination of snow or ground cover. In permafrost areas, most pipelines are installed aboveground in insulated utilidors for small-diameter installations. Pumps, valves, and other control stations should be enclosed, heated, and insulated to prevent freezing. Steel has a much higher heat-transfer rate and is usually much thinner than HDPE, so the pipes will begin to form ice at the fittings and flanges quicker. The pump crew should be equipped with a propane-fired burner to thaw valves that have frozen in place. Water standing in dead zones will freeze rapidly, preventing the actuation of the valves. The time that it takes pipes to freeze depends on initial water temperature, pipe-insulating value, heat transfer rate, outside temperature, heat capacity (Cp) of the water, and the latent heat of fusion. Once the water reaches the freezing point, the time required for freezing the pipeline is shown by the first term in Equation 9.5-1. The second term in Equation 9.5-1 is the time required to lower the temperature of the water from an initial temperature Ti to the freezing point. If at some point the pump becomes available before the pipe freezes solid, water could flow and begin thawing the ice by applying the wasted energy from the system inefficiency. Large-diameter (40.6-cm [16-in.]) pipe can be laid on the surface in very cold areas if outages are kept within the heat capacity of the water. Even a small flow will prevent freezing in the pipeline; however, exposed valves and metal fittings may initiate freezing faster. Snow piled on a pipeline acts as additional insulation, further adding to the time available to get the pumping system back in service. time = c where

R ^mass h H f R ^mass h C p ^Ti − Tf h m+e o area ^averageΔT h areaΔT

(9.5-1)

R = insulation value per inch, 0.17611 ((m2K)/W), ((1 h-ft2°F)/Btu) mass = fluid mass per unit length of pipe Hf = latent heat of fusion area = outside area of pipe ∆T = temperature differential between outside air and fluid

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Cp = heat capacity of water 4.187 kJ/(kg-K), (1 Btu/°F-lbm [pounds-mass]) (Ti – Tf) = difference between initial temperature and freezing point Example 1. The following example of Equation 9.5-1 calculates the time to freeze the entire mass of 0°C (32°F) water setting in a pipeline at an ambient temperature of –40°C (–40°F). Pipe characteristics (derivation not shown) for this example: • SDR 15.5, 40.6-cm (16-in.) outside diameter pipe, wall thickness 26 mm (1.03 in.) • Internal diameter, 353 mm (13.9 in.) • Mass, 29.7 kg (65.7 lbm) water per foot length • Latent, Hf for water, 334 kJ/kg (144 Btu/lbm) • Outside area of pipe, 0.39 m2 (4.18 ft2/ft length) • Insulation, R = 0.053 (m2K)/W (0.3 (h-ft2°F)/Btu) Solution. Time to freeze solid 2 = c 0.3 h-ft °F mc 1 linear 2ft m` 65.7 lbm water j` 144 Btu j` 1 j Btu 1 linear ft lbm water 72°F 4.18 ft = 9.4 hours

Pipes begin freezing where the sections are most exposed and where internal structures allow ice to anchor, such as elbows, fittings, and valves. As freezing progresses, these areas fill with ice and push water pockets ahead of them. This usually means that the last place liquid water pockets exist is inside walls where the insulation slows down the freezing process and the pipe usually ruptures in the area where it is most difficult to repair. Desert Pumping Deserts uniquely challenge pit-dewatering operations in several ways. Extreme swings in seasonal rainfall require system designers to consider both too much and too little water. Water is usually (but not always) in short supply, except when flooding occurs. The sources are frequently deep wells drilled into alluvial valley deposits and may require long overland pipelines to bring the water to the mine site. Because the supply is usually short, the systems design must avoid waste or contamination wherever possible. The temperature range of deserts has extremely wide variations, both seasonal and diurnal. Electrically operated pumps need additional care to ensure that insufficient cooling does not shorten component life. Evaporation and reusing water can cause undesirable concentration of contaminants. It is necessary to derate the HDPE pipe working pressure for conditions above 22.8°C (73°F). The working pressure ratings must be reduced 50% at 60°C (140°F). Pipe lying in the sun while empty or with the water not flowing can quickly reach this temperature. Pressure surges on start-up can cause the hot pipeline to rupture. Anchoring pipe along narrow rights-of-way prevents thermal expansion cycles from expanding and deforming into sinusoidal loops (snaking), causing pipe encroachment onto the road. Deserts frequently experience large temperature swings exceeding ∆22.2°C (∆40°F) daily. Allowing the pipe to lie in the sun with no flow to stabilize the temperature simply exacerbates thermal expansion issues.

Alkalinity or other mineral content of the water in the sumps increases with evaporation. Recycling increases this effect because limited freshwater sources are available for dilution because of arid conditions. To prevent processing issues, the buildup of dissolved minerals must be managed. Bleeding a small split off the main process stream and replacing it with fresh makeup water may prevent undesirable concentration of minerals. If the quality reaches unacceptable levels, it may be necessary to segregate water for less critical uses—for example, haul road dust suppression. Seasonal monsoon rains can create an excess water volume that overwhelms the sump capacity. Storage reservoirs should be as large as practicable to capture and hold the excess runoff for use during the dry season. If too much water becomes a problem, an alternative to pumping from the pit can be spraying water into the air to enhance evaporation. This sometimes becomes a dilution issue with large collection areas on top of the leach dumps in solvent extraction/ electrowinning circuits.

oPen ChAnnel DeSign

Open channel designs must include adequate safety factors to carry the required peak flow without eroding the channel and while maintaining sufficient freeboard to prevent overtopping. Manning’s equation is the primary tool available to design open channels flowing under gravity at atmospheric pressure. This method covers the design of ditches, rivers, and culverts flowing partly full. Equation 9.5-2 is the basic velocity form. When combined with the flow discharge Equation 9.5-3, it creates the flow quantity Q, Equation 9.5-4, that the channel will carry. Manning’s equation: V = b k l R 2/3 S 1/2 n

(9.5-2)

Q = AV

(9.5-3)

Q = A b k l R 2/3 S 1/2 n

(9.5-4)

where V = velocity, m/s (ft/s) k = dimension conversion factor—1 in metric (1.486 English) n = Manning’s friction factor, a unitless characteristic of surface roughness of the channel material normally ranging from 0.010 to 0.050 R = hydraulic radius, defined as the (area/wetted perimeter), m (ft) S = channel slope, m/m (ft/ft) Q = flow rate, m3/s (ft3/s) A = flow area, m2 (ft2) From the basin runoff outline given, the normal calculation sequence is to determine the maximum flow rate Q, then design the channel to carry the flow. However, Equation 9.5-4 assumes that the flow depth and channel geometry are known. Hydraulic radius, depth, and area are interrelated variables and are dependent on the shape of the channel. Depth is the usual unknown value and is a non-isolatable factor in both the hydraulic radius and the area calculations. Because the flow depth cannot be solved directly, the usual methods


which were widely used prior to the availability of modern computers, are safer. Nomographs for Manning’s equation solutions are complex, making assumptions of the specific channels shapes, and are sometimes only valid for particular flow conditions. When using them, it is important to understand the underlying assumptions.

Trapezoidal Channel 1 h

d Wb

PuMP SeleCTion

figure 9.5-1 Trapezoidal cross section

of solution are to assume a channel width and depth, and then iteratively solve the equation. A = W b d + hd 2 R=

W d + hd 2 W b + 2d ^h + 1h1/2

2/3 W b d + hd 2 Q = ^W b d + hd 2h k = ^ S h1/2 G / 1 2 n W b + 2d ^h + 1h

751

(9.5-5) (9.5-6) (9.5-7)

where Wb = bottom channel width d = flow depth h = side slope ratio in the design channel in the form h:1, horizontal to vertical S = Manning’s channel slope Most channels built for mine drainage use trapezoidal channel cross sections. Figure 9.5-1 shows the general trapezoidal section parameters used to develop Equation 9.5-7. The hydraulic radius (Equation 9.5-6) and area (Equation 9.5-5) calculations are expressed in terms of the channel parameters: bottom width, depth, and side slopes. If field solutions are necessary, spreadsheet programs with a solver function running on laptop computers or programmable calculators can be set to do these computations. The normal solution starts with Q as a known value from the basin design, and field constraints on Wb, d, S, n, and h. Example 2. Parameters for a typical trapezoidal channel construction design might be the following: • Required flow rate Q, 14.15 m3/s (500 ft3/s) • Minimum bottom channel width (equipment width limited), 4.53 m (15 ft) • Three to one (3:1) side slopes (h = 3) • Channel slope, 0.005 (ft/ft) (0.5%) • Minimum freeboard above the water surface, 0.61 m (2 ft) • Maximum flow velocity to minimize erosion of 1.52 m/s (5 ft/s) • Coarse gravel bottom Manning’s roughness coefficient n, 0.045 Solution. From the preceding example, an iterated solution is Wb = 5.3 m (18.8 ft) d = 1.05 m (3.46 ft) V = 1.51 m/s (4.96 ft/s) Channel depth with freeboard = 1.66 m (5.46 ft) The use of Manning’s equation in the field by technicians or less experienced designers may lead to errors. Nomographs, graphical representations of the underlying relationships,

Most surface mine–dewatering systems use centrifugal pumps because of their wide range of operating characteristics. The required head (H) and flow rate (Q) will determine the pump design scheme. Operating pumps in parallel allows additional flow capacity beyond what is practicable for a single pump and provides system redundancy. Higher head requirements than single-stage pumps develop can be handled by utilizing multistage pumps driven from a single shaft or by using several single-stage pumps in series. Extremely high head applications such as grout pumping or underground dewatering may require positive displacement pumps: piston, diaphragm, variable cavity, or gear pumps. The flow pulsates because of the periodic nature of the pump mechanisms and requires dampening to prevent shock damage to system components (see “Issue: Water Hammer” section later in this chapter). Systems employing positive displacement pumps require more care matching the system’s power, flow, and pressure capacity to the required operating point and are less flexible in changing conditions than centrifugal pumps. Centrifugal Pumps Commonly available centrifugal pumps have head ranges up to about 107 m (350 ft) in single-stage configurations with flow rates up to 681 m3/h (3,000 gpm). Pumps are available that have capabilities exceeding these but are less common. Centrifugal pumps can be grouped into the following categories based on their configurations: • End suction pumps use a single open suction eye from the side of the impellor with the drive motor on the other side. These are the most commonly used single-stage pumps but have high net positive suction head required (NPSHR). • Double-suction pumps use a manifold to route suction from both sides of the impeller. The drive motor axis passes through both sides of the pump case and can be driven from either side. These are moderate NPSHR pumps. • Vertical turbines use multiple stages driven from a common shaft and generate very high discharge heads. These usually mount through a horizontal deck with the motor above and require that the suction bell be submerged under the barge or in a deep well. These pumps require the least NPSHR because the suction and the first pump stages are submerged. Pump Curves The primary source of pump data is the manufacturer’s data sheets. Manufacturers will usually supply a line card showing ranges of each family of pumps they offer, covering a wide range of head/flow requirements. This is the tool most designers use for preliminary screening of the specifications to find a pump with the appropriate capacity for the application. After a general pump family is selected, it is time to get the detailed data specifications from the manufacturer.

752


Sy ste

m

400

ttle

A'

Sy

1,600 rpm

Head TDH, ft

B

70% 60%

1,400 rpm

200

Cu

A

300

250

m

ste

Th ro

1,800 rpm

e

rv

d

Pump Speed

350

50% 150

HP 300 200 Horsepower

50

0

500

1,000

1,500

2,000

2,500

3,000

30

0

20 10

NPSHR 0

100

4,000

4,500

NPSHR, ft

100

5,000

Flow Rate Q, gpm

figure 9.5-2 Pump system head curve

The following are some of the most important specifications that the system designer must determine from the manufacturer’s pump operating characteristics curves: • • • • •

Total dynamic head (TDH)/flow (Q) range requirements Revolutions per minute (rpm)—motor rotational speed Driver type, coupling, and efficiency Power NPSHR

System operating Point example Figure 9.5-2 is a typical system head curve (system curve) showing the pressure–flow characteristic curve for a pumping system. This system consists of a single-stage pump with field-variable rpm. The pipeline system has both a hydrostatic head (elevation lift) of 52 m (170 ft) and friction losses that increase as the flow rate increases. A family of curves that are rpm dependent represents the pump’s TDH versus flow rate Q output. The system curve lines represent the head and flow relationship of the pipeline, valves, and fittings. The system curve intersects the left side of the chart where Q is 0 at 52 m (170 ft) total static head. The pump must develop at least this amount of TDH to overcome the hydrostatic head and begin pumping water. As the system curve moves right with increasing Q, the curve starts to rise as friction losses increase. The difference between the total static head and the system curve is the energy either lost to friction in the system or used accelerating the water column in the pipe (velocity head). The pump system operating point is the intersection of the system curve and the pump TDH curve: Point A, where the 1,800-rpm pump curve and the system curve intersect at 88 m (290 ft) TDH pumping 170 L/s (2,700 gpm). This pump works at its best efficiency point (BEP) at a Q of around 177 L/s (2,800 gpm). Ideally, the designer would

select a pump for the system that matches the operating point as close to the BEP as possible. Because of internal pump dynamics, the efficiency is not only a function of Q, but there is usually a shift in efficiencies when changing the rpm. Pump efficiency is overlaid on the pump curve and resembles a bull’s-eye with the BEP in the center. Each pump is designed to work within a particular flow range. Centrifugal pumps have a limited ability to continue to pump water past designed upper limits for the pump family. Flow in a pump has a maximum point beyond which TDH drops off rapidly. Above 659 m3/h (2,900 gpm), the efficiency drops, and, by 1,022 m3/h (4,500 gpm), the pump’s TDH begins to fall rapidly. At zero flow, the TDH rises to the maximum head that the pump can develop (deadhead pressure)—98 m (320 ft). However, the pump is doing no work so the efficiency is zero; all of the input energy is wasted as heat. Throttling There are several ways to change the system flow to accommodate variable demand. In a fixed-speed system, such as one driven directly by an electric motor, the most common way to reduce the flow is by installing a control valve in the discharge pipe. The system head curve is modified by partially closing a valve, shifting the operating point from A to A' as the valve creates additional frictional losses, and reducing the flow from 613 to 454 m3/h (2,700 to 2,000 gpm). The pump is still working hard, but the energy lost across the valve causes the hydraulic efficiency to drop. In a variable-speed system, similar changes in the operating point require reducing the pump’s speed from 1,800 to 1,600 rpm, which would shift the pump curve. This would move the operating point from point A to point B at TDH of 75 m (245 ft)


Table 9.5-1 Pump affinity laws Characteristics

Speed Ratio

impeller Diameter Ratio

Flow

Q1/Q2 = (N1/N2)

Q1/Q2 = (D1/D2)

Head

H1/H2 = (N1/N2)2

H1/H2 = (D1/D2)2

Power

(N1/N2)3

(D1/D2)3

P1/P2 =

P1/P2 =

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from the system. All of the energy terms in Bernoulli’s equation have units of length, allowing modeling the system HGL as a cross-sectional elevation grade line. 2 HGL = V + P + Z + H f 2g ρg

(9.5-8)

where

V = fluid velocity g = gravitational acceleration P = internal gauge pressure ρ = fluid density Z = elevation or hydrostatic head Hf = frictional head losses

m3/h

and Q slightly less than 613 (2,000 gpm). The efficiency would be better than if the system was throttled using the control valve method because less energy is wasted. When considering a pump curve, the designer must be aware of several limitations that may preclude the system from achieving the desired operating point. Most pump curves show two additional curves that increase with higher Q: the hydraulic power required and the NPSHR (a characteristic of the pump from the manufacturer). Care must be taken when locating the pump to ensure that suction head requirements are met: The net positive suction head available (NPSHA) is above the NPSHR under all operating conditions. The pump driver must be able to supply sufficient power to overcome the pump inefficiency and the electrical losses of the motor and to supply the hydraulic power requirements. Affinity laws Manufacturers design pumps for particular operating ranges, but within these ranges the curves can be shifted. Speed changes are practical methods for modifying the operating characteristics of pumps when the applications change in the field. The effects of changing the speed ratio between the original operating point and the new pump speed (N1/N2) are shown in Table 9.5-1. As can be seen, the flow ratio changes are linear with the speed ratio; head is a squared function, and power requirement increases as the cube. Speed ratio changes are most useful when coupled to a variable-speed motor. Keep in mind the old rule of thumb for rotating machinery that if you double the speed, the maintenance costs go up eightfold. Impeller sizes/trimming ratios (D1/D2) have the same effects as changing speed ratios but are more difficult to change after manufacturing. This capability allows adjusting fixed-speed pumps to match the required operating point of a long-term installation without sacrificing the efficiency that a throttling valve would cause. Limited adjustments by reducing the impeller diameters can be done at the mine site machine shops, but this requires that the pumps be disassembled. Increasing impellor diameters is not possible without obtaining a new impeller from the manufacturer. Efficiencies suffer if the impeller diameter is significantly reduced from the original diameter (Casada 2006).

PiPelineS

Pipelines are the hydraulic analog of wires in an electrical system. Like the electrical system, the pipelines have carrying capacity (flow rate Q), voltage capacity (pressure ratings), and resistance to flow (Hf frictional losses). Bernoulli’s equation (Equation 9.5-8) shows the total mechanical energy in a fluid flowing in a pipeline as the HGL. The difference between the HGL and the Z elevation in the profile represents the gauge pressure inside the pipeline. If energy were conserved along the flow in a pipe, the HGL would be constant, and energy could be exchanged among the four components. The first three terms can be reversibly transformed one to another; however, the Hf is usually lost as heat

The contributions of the individual energy terms (all in length units) are the following: • • • • •

HGL Velocity head (V2/2g) Pressure head (P/rg) Elevation head (Z) Friction head (Hf) losses

Practical graphical Solution for Rapidly Designing Pump locations One of the most straightforward ways to lay out a pipeline and locate booster pumps along the route is a graphical cross section. This process works because all factors in the dimensional analysis of the Bernoulli equation reduce to length. The sum of these factors represents the total energy in the system or the HGL. The topography is drawn in cross section along the actual proposed route (Figure 9.5-3), scaled to compare the pit surface elevations against the HGL in the pipe system. When the HGL intersects the topography, the system has used all of the available energy, and as a result water will no longer flow. By using cross sections in this manner, it is readily apparent where to install booster stations along the pipeline route. System designs use the high-pressure pipe in the bottom and lower-pressure-rated pipe (less expensive) as the elevation increases. The required pressure ratings drop as the pipe elevation increases (internal pressure decreases) and the Hf slope in the HGL flattens. The first pipe connected to the discharge of Pump A has the highest HGL, is at the lowest elevation, has the highest internal gauge pressure, and thus has the thickest walls required to resist bursting. The highest-pressure-rated pipes have the smallest inside diameters and the greatest frictional head losses (Hf). Beginning at the sump, the HGL is at the water surface elevation. Entrance and suction side losses reduce the HGL further in the pump inlet piping (NPSHA). The pump adds energy to the system, raising the HGL by the TDH of Pump A. Continued plotting of the HGL in each pipe segment shows Hf as percentage slope related to the pipe friction losses until the HGL reaches the discharge location. Sumps, pumps, and discharge locations are included along the pipeline route. The point where the topography intersects the HGL is the maximum elevation that the available TDH energy from Pump A can push the water. The booster Pump B is placed at an accessible location along the pipeline below the elevation of the HGL/topography intersection. The additional TDH from Pump B raises the HGL sufficiently to arrive at the tank with no further booster pumps required. The residual pressure

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Hf

TDH B

HGL

TDH A

v2 2g Tank

Pump B

Pump A

Sump

figure 9.5-3 Pump station design cross section

in the line above the minimum necessary to reach the tank will accelerate the flow from the pipe end. At the discharge, the HGL consists entirely of velocity head as the pressure head drops to atmospheric and delta elevation term falls to zero. After the pump locations have been determined, it is possible to increase the safety factor by systematically placing the booster pumps at lower elevations than the profile indicates. Note that Pump B is placed on a bench well below the HGL/ topographic intersection from Pump A. This accounts for future wear in the pumps and scaling or system degradation, as well as for approximations in estimates of the minor head losses. This method assumes that the entrance, shock, and fitting losses are minor compared with the elevation change in the system. If this is not the case, more precision should be applied to the NPSHA analysis of the pump inlet structures. Pipeline Material Selection Selecting the appropriate pipeline materials for pumping projects involves balancing many competing characteristics. The factors that determine which material is best for any given project include the following. Pipe material characteristics: • • • • • •

Corrosion resistance Electrical conductivity Chemical compatibility Thermal expansion Cost Availability, lead time, logistics Pipe materials:

• Metal – Carbon steel – Stainless steels – Copper – Ductile iron bell and spigot – Corrugated steel pipe (CSP) • Plastic pipes – Polyvinyl chloride – HDPE • Masonry – Vitrified clay – Concrete precast

• Composite – HDPE-lined carbon steel – Grouted steel HDPE pipe: • • • • • • •

Ease of construction, joining Labor skills available Weight Ductility Strength Pressure ratings Temperature

high-Density Polyethylene The invention of HDPE pipe is among the most influential products to appear in mining since 1960. Its unmatched combination of low weight, strength, flexibility, ease of installation, and corrosion resistance revolutionized pumping processes in mining. HDPE pipe has many advantages over steel pipe for use in the harsh mine environment. It is much more flexible than steel, allowing installation in areas where it would not be practicable to install steel pipe. HDPE pipe is resilient and often more suitable in the mine pit environment than are metal or concrete alternatives. Occasionally, heavy equipment can be driven directly over HDPE pipes, although it is necessary to protect the pipe from damage with a thin layer of fine material or conveyor belting. Before driving the equipment across the pipe, the pipeline should be depressurized to limit the transfer of hydraulic pressure to other locations in the system. An HDPE pipe will crush flat and then begin to rebound immediately after the load is removed. Attempting this with any rigid pipe results in permanent deformation or crushing. HDPE is resistant to most chemicals and is not subject to corrosion—crucial characteristics when dealing with acidic or caustic leach solutions. The main alternative material to HDPE for these applications is expensive stainless steels. Cathodic protection systems are not required for buried installations because the HDPE pipelines are nonconductive and not subject to galvanic corrosion. These characteristics give HDPE pipelines a design life of 50 years’ service when buried. The low density of HDPE makes it an ideal material to connect floating barges to the shore, and its flexibility allows


it to follow the barge as the water level changes. HDPE is slightly less dense than water; trapped air gives it enough buoyancy to float the pipe out to barge pumps. HDPE pipe is the easiest material to work with in the field because of its low melting point. Joining HDPE requires specialized fusing machines that combine the three operations of the fusing cycle: (1) milling the pipe ends, (2) melting the surfaces, and (3) fusing the ends together. Typically, pipe larger than 6-in. diameter arrives at the job site as a truckload of 12.2- to 15.2-m- (40- to 50-ft-) long segments. The assembly process is considerably faster than welding steel pipes. Semiskilled workers with field-fusing machines can make three to four joints per hour. Because HDPE pipe is flexible and light, the pipe bed preparation requires little work, and small forklifts or backhoes are the largest pieces of equipment necessary to handle the pipe segments. When it becomes necessary to make repairs, a chain saw is usually sufficient to cut out the damaged section and replace it with a new one, using the fusing machine. When repairs are necessary in areas that are too tight to excavate fully, it is possible to slip an electrofusion coupler over the ends of the broken pipe. This sleeve system allows repairs in inaccessible locations and contains its own internal heating elements. A generator is connected to an electrofusion controller, which melts the collar onto the pipes, sealing the ends. Several characteristics limit the applicability of HDPE pipe in some situations. HDPE is limited to moderate pressures, with the upper end of its range limited to design pressures less than 2,068 kPa (300 psi) for SDR 6 pipe. The larger the SDR, the lower the pressure rating due to thinner pipe walls. Common working pressure ratings range from 207 kPa (30 psi) for SDR 40 pipe to 1,758 kPa (255 psi) for SDR 7.3 pipe. Steel pipe, on the other hand, has thinner walls and is useful for extremely high pressures. Additionally, the maximum depth of burial for HDPE is shallower than for that of steel. The pressure ratings of HDPE pipe are temperature dependent, because HDPE is a thermoplastic with a low melting point. The designed pressure rating temperature is for a pipe at 23°C (73°F) and has a service factor of 2. The working pressure needs to be derated 50% for pipes installed at 60°C (140°F) and increased to 200% working pressure rating for installation at 4°C (40°F). As the temperature of the installation diverges from 23°C (73°F), the allowable pressure ratings continue to move toward both extremes. Ultimately, the pipe becomes brittle at very cold temperatures, crystallizing at about –101°C (–150°F), and softens and loses its strength as it approaches its melting point. other Pipe Materials Other materials have characteristics that make them the choice for some circumstances. Composite grouted steel pipelines combine concrete, grout, or other masonry material as a liner inside steel pipe. These are useful in slurry pipelines, tailings, or culverts carrying large amount of debris where the main issue is erosion resistance. Galvanized CSP is one of the major culvert, manhole, and sewer construction materials. Zinc galvanizing offers corrosion protection, allowing longterm installations at competitive costs. Combining welded steel pipelines with a liner of thin HDPE has the advantages of corrosion resistance of the HDPE and the pressure ratings from the strength of the steel pipe. Pipe joints usually use bolted, raised-face steel flanges, and inner smooth HDPE flanges to prevent the corrosive fluids

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from contacting the steel. These double-layer pipelines allow for integral leak detection by installing sample ports in the outside steel line. Anchors, Point loading, and Thrust Blocks Anchors need to support the weight of the pipe, water, and live loads developed by the flow in the pipelines. These axial loads can be quite large where pipes hang from supports high on the side of pit walls. The pipe walls must also be strong enough to hold these loads between anchor points; the wall’s strength determines the maximum distance between anchors. Pipes need support at pump stations and wherever they are attached to pumps, valves, or other fittings. It is poor engineering practice to allow loads from the pipe’s weight to transfer through the flange bolts. Case deformation due to external loads may reduce internal clearances, causing unacceptable wear or binding in valves and pump impellers. Thrust blocks are designed to help pipes resist the forces developed when water flow changes direction or velocity; for example, the thrust from an uncontrolled fire hose nozzle. Calculations for the thrust block must counter the changes in the momentum of the water stream. The impulse/ momentum changes result from the forces applied to the water and resisted by the thrust block. Confined Stress Due to Temperature Changes Pipelines subject to changes in temperature will expand or contract proportional to the length of the pipe and the temperature change. HDPE pipe is particularly problematic in this regard, because it has a coefficient of expansion six times higher than steel, and its flexibility makes it less able to resist bending stresses. Warming pipelines can deflect into sinusoidal loops (snaking), and this may result in them moving off the designed right-of-way. Placing piles of earth on the pipe at regular intervals, called point loading, serves to control the lateral movement. If the temperature is high during installation, shrinkage occurs as the pipe cools. This can result in pulling the pipe completely out of fixed pump stations, tanks, or concrete anchors. As an example of these issues using PlexCalc II, a 41-cm (16-in.) diameter SDR 15.5 pipe can be installed without taking into account thermal expansion. HDPE has a thermal expansion coefficient of 9 # 10–5 in./in./°F (Performance Pipe 2003). (Note: For a 22°C [40°F] temperature swing, a 914-m [3,000-ft] pipe changes length by 3.3 m [10.8 ft]). Hot installation at 27°C (80°F) cooling to 4°C (40°F) is the short-term temperature change induced by pumping cold water into an empty pipe and will result in a tensile force of 107 kN (24,000 lb-force) trying to pull out the fittings. The change associated with cold installation at 4°C (40°F) warming to 27°C (80°F) will result in a compressive force of 160 kN (36,000 lb-force) if the pipe is constrained and must be guided at 49-m (160-ft) intervals to prevent buckling.

fielD floW MeASuReMenTS

The two necessary measurements that indicate the current operating conditions of a pump are the pressure (head) and the flow rate (Q). The intersection of these two parameters defines the operating point on the pump curve. Many types of pressure gauges are available to obtain the head, but measuring the flow is more difficult. For permanent installations, flow measurements use preinstalled devices in the pipeline at strategic points to monitor flow. These include mechanical rotating

756


g = gravitational acceleration X

Y

figure 9.5-4 Pipe discharge drop method

vanes, hot wire transducers, differential pressure manometers with orifice plates, ultrasonic flowmeters, and numerous others. The common factor is that these devices (except the ultrasonic flowmeter) must be inside the flow to function. The ultrasonic flowmeters are portable and use transducers strapped to the outside of the pipe. All of these instruments measure the flow velocity (V): calibrating them for measuring Q requires knowledge of the inside diameter of the pipe to calculate the flow area (A). Q = VA is used to calculate the flow rate. Pipe Discharge Drop Method The simplest flow measurement method is the pipe discharge drop, which can be used any place where the pipe protrudes from the bank and the flow discharge drops freely by gravity (Figure 9.5-4). This measurement method uses projectile kinematics to calculate the velocity of the discharge stream. The only instrument necessary is a calibrated stick for measuring the horizontal distance (X) from the mouth of the pipe to the point where the stream drops a fixed distance (Y). A folding carpenter’s ruler is a perfect tool for this measurement. The ruler is moved until the distance from the top of pipe to water flow at the pipe end is Y: X, Y, and the flow depth are recorded as a fraction of the pipe diameter. Y is corrected for the wall thickness and compensated for the discharge running at less than full pipe flow to obtain the actual stream drop. If the operators take measurements at the same locations frequently, it is a simple procedure to precalculate the tables matching the distance dropped because the pipe diameter, wall thickness, and table of partial area flow corrections are constant. Even better is setting up a spreadsheet with the parameters and calculations prebuilt to calculate flows from any pipe discharge, using Equation 9.5-9 to calculate the discharge velocity of the flow. If the flow is not a full pipe discharge, it is necessary to estimate the area of pipe flow. Precalculating a table with this conversion only requires measuring the fraction of the pipe diameter that contains running water. Vo =

X 2Y g

(9.5-9)

where Vo = discharge velocity X = horizontal distance to measurement point Y = vertical drop of flow stream adjusted for wall thickness and partial flow

Open channel flows use similar schemes to determine the velocity of the water flow in natural channels. Various methods include measuring the time that wood chips take to traverse a known distance, allowing the observer to calculate the velocity of the surface flows. Measuring the channel cross section is necessary to estimate the flow area, allowing the flow rate to be calculated. This method neglects the vertical gradient in the velocity profile. Bubble streaming lines can give a visual representation of the velocity variations across the channel. Weirs and flumes are calibrated structures installed into open channels that are useful for creating fixed gauging stations to measure flow. These use characteristics such as specifically constructed long-throated flumes or broad-crested weirs that correlate the depth of flow to the quantity traveling through the section. Long-throated flumes are replacing the previously used Parshall flumes because their streamlines are more linear and do not have to be calibrated in the laboratory (Swanson and Baldwin 1965). Many computer programs are available for designing flumes for specific flow.

MoDeling, inSTRuMenTATion, ConTRolS, AnD PoWeR

Bearing in mind the limitations of the accuracy of the hydraulic predictions previously discussed, an understanding of these highly complex systems requires simulation and modeling. It can be difficult to calculate the complete state of even the simplified system used in the “Simplified Control System Example” section. Given the construction costs tied up in systems that perform poorly, simulation is essential to design an effective pit-dewatering system. Consider the piping network flow diagram as an interconnected series of arcs and nodes analogous to electrical networks. Many of the computer-based, pipe network modeling systems force designers to break the system down into basic components and assign characteristics to each before running iterative passes until the system operating point is found. Dewatering systems do not operate as disconnected discrete components (Wood and Lingireddy 2010). Hydraulic networks simulations are based on the same mathematical relationships that govern electrical networks, the hydraulic equivalent of Kirchhoff’s laws: • The sum of flows into and leaving any node equals zero. • The sum of pressure drops around any closed loop equals zero. • Hydraulic simulations use specialized boundary nodes that have fixed pressures or act as flow sources or sinks. Control System Components Like the electrical network, hydraulic networks contain other components besides pipes (wires) and nodes (connections) that transform the pressures and flows. Components are specialized arcs that connect one or more nodes and transform the pressure, accumulate flow, or perform other specific functions. Table 9.5-2 shows some of the common modeling components in the hydraulic system, their electrical analogs, the functions they perform, and the number of nodes they connect to other components. Because water is noncompressible under ordinary conditions, communication and feedback are necessary between different parts of the network. The problem is that unless


water accumulation components, such as tanks or reservoirs, exist somewhere in the system, the volume flowing into the system must be the same as that which leaves. To ensure this happens without incident, the components at the end must be able to communicate and control the components at the beginning of the pipeline. In order for this to happen, the system must contain the following components: • Hydraulic sensors detect fluid levels, pressures, and the presence or absence of flow. • Sensors detect the mode/state of pumps, valves, control positions, and system demands. • Actuators, motor controls, and relays control the active components in the system. • Communication links collect the inputs, transmit them to the command and control component, and send command signals to the actuators. • The controller (the brains of the system) executes command and control logic, evaluates inputs, and sends appropriate commands to the actuators over established communications links. Simplified Control System example Complex control relationships can be derived from the simplified network shown in Figure 9.5-3, which contains a pump drawing water from a sump and pumping it uphill through a second pump to a tank. For the following description, it can be assumed that pipeline from Pump A discharges directly into a tank feeding Pump Station B. Hydraulic sensors are necessary to measure the state of the hydraulic network and should be able to detect if the sump contains enough water to prime the pump, fill the pipe, and deliver sufficient water to the tank. The system should have a sensor to detect if the tank needs water or if it is too full and, therefore, about to overflow. When pumping, flow sensors would tell the controller if flow exists downstream from the pumps in order to avoid damage. In the simplified system, only one actuator is necessary to turn on each pump. The mode sensor to supply the feedback that this has occurred could come from an additional set of contacts to supply the binary state of the starter to the controller as an input, “on” or “off.” Communication links carry the inputs from the sensors and outputs from the controller. The controller is physically connected to these links and to the distant sensors and actuators through wires running to the controller. Other options require that radio links, wireless computer networks, or other telemetry hardware carry the signals. These have the advantage of not requiring physical cables, although they increase the complexity and potentially the cost of the communications network. The command and control system (controller) is the intelligence behind the pumping system. The controller takes the inputs from the hydraulic and state sensors and the feedback from the actuators and applies the preprogrammed control scheme to create the appropriate action commands to the actuators. A controller can be as simple as a manual switch operated by an employee; when activated, it sends power to the motor and an indicator light, showing the pump status. Remote operations controllers (ROCs) allow the system controller to be located almost anywhere as long as the radio communication link is reliable. ROCs sometimes supply both the

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Table 9.5-2 hydraulic modeling components and electrical analogs hydraulic Component

electrical Analog

Pump

Battery

Adds pressure (head) between nodes

2

Valve

Switch

Opens or closes arc to flow

2

Flow control valve—partly opened

Rheostat or Modulates or changes flow by potentiometer changing arc resistance

Tank or reservoir

Capacitor

Accumulates and releases water volume based on conditions (may also have fixed HGL elevation if used as boundary node)

1 or 2

Check valve

Diode

Allows flow in only one direction

2

hydraulic function

number of nodes

2

programmable controller and the radio hardware in the same box, simplifying system design. Most of the modern controllers use some type of a programmable computer-based system. Programmable logic controller (PLC) systems are the most commonly used type of industrial controller. Programmers program a PLC using a simplified language called “ladder logic” that uses simple constructs to simulate signals that enable contacts, timers, and sequences to create the appropriate output for each system operation. Systems that are more complex run on larger computers incorporating powerful programming languages that can create internal simulation models capable of controlling entire water-handling networks with many components and interrelationships. When the pump network has more than one stage in a series, it becomes necessary to interlock the pump stations and holding tanks. Intermediate tanks receiving water from below and supplying water to a local pump pushing up to the next stage must balance the flows to avoid overflowing the system. Normally, tanks in this sort of configuration use three level sensors to feed information to the controller about the tank level state: 1. High tank level sensor shuts down the lower stage pump—the tank is full. 2. Low tank level sensor shuts down the next stage pump— the tank is empty. 3. Mid-tank level sensor starts the next stage pump in the series when the tank has enough water to pump without quick cycling. This allows both stages to pump simultaneously for part of the cycle, improving the system’s efficiency.

PuMP STATion DeSignS

Design of pump stations must balance a number of competing constraints in the pumping system, especially the availability of electric power. The design life of the pump station at a particular location will determine the effort that goes into its layout and construction. The access quality and space availability for the station may also limit the selection. Deciding on the type of motor to drive the pump is among the primary design constraints. Selecting the appropriate drive system depends on several competing characteristics. Under normal circumstances, diesel is generally the more expensive power source than electricity. However, at remote sites where

758


utility company electric power is not available, it may be less expensive to run the pumps from diesel engines than to use the same fuel to generate electricity for electric-driven pumps. Diesel-Driven Pumps Diesel-driven pumps are convenient to use for self-contained trailer-mounted pumps that operators can deploy quickly. This makes diesel pumps the choice for pumping water out of the shovel-loading faces or other in-pit situations where the sump must move every time the pit advances. The most effective pit-dewatering crews have a number of these pumps covering a wide performance range. The variable-speed operation inherent in the design of the diesel power plants allows considerable flexibility in the varying performance of the head and flow rates at the operating point of the attached pump. The main disadvantage of diesel engines is that they require operator access for daily fueling and engine servicing. This requires maintaining an access road capable of carrying fuel trucks to the pump station as well as a turnaround area. The energy conversion efficiency of the best diesel engines falls far short of comparable electric drives. High-efficiency, low-speed diesel engines have thermal conversion efficiencies of more than 50% compared with electric motor efficiencies in the mid- to upper-90% range. When using diesel engines, automation is not as viable as when using electric motors. Some diesel engines can operate remotely with sensors controlling the starting and stopping, but these features do not account for the fine-tuning that diesel starting and shutdown procedures usually require. The engine speed must usually be set manually, although pneumatic dashpot controls may allow automatic operation requiring a complex feedback system if the operating point is not constant. electrical Motor-Driven Pumps One of the most critical parameters that determines pump performance is the rotational speed at which the pump operates. As discussed in the “Affinity Laws” section, changes in the pump’s discharge head are proportional to the speed ratio squared. Maintenance costs for rotating machinery are proportional to the speed ratio cubed; doubling the speed raises the maintenance costs eightfold. The lowest rpm that meets the required operating point is the most reliable. Low-speed pumps usually cost more and are proportionally heavier than the higher-speed pumps to achieve the same performance, so the initial costs versus operating costs need to account for these factors. Motors for common pumping applications are usually three-phase induction motors with power ratings ranging from 4 to 224 kW (5 to 300 hp). The rotational direction of the motors must match the designed rotation because centrifugal pumps can only run in one direction. When using a threephase powered system, reversing any two phases to the motor changes the motor’s rotation direction. The number of poles, p, in the stator windings and power line frequency, f, determine the motor’s synchronous speed: synchronous speed = 120 f/p

(9.5-10)

Induction motors produce torque at any speed less than synchronous, so the motor’s armature runs slower. Synchronous motor rpm ratings appear odd because of the armature lag; for example, a four-pole motor whose synchronous speed is 1,800 rpm may run at 1,750 rpm.

In synchronous motors, the more the motor speed lags behind the synchronous rate, the more current it consumes and the more torque it produces. This effect is highest during startup when the motor’s current can be up to 10 times larger than the normal running current. Starting requires special attention in order to prevent excessive current causing undesirable voltage fluctuations in the incoming power grid. Soft-start motor controllers are necessary on the larger motors, particularly if the installation is at the end of a long power line. Soft starters can also control the system’s ramp-up speed, thereby effectively acting to control the dynamic shock effects to the system (see the “Issue: Water Hammer” section). Electric-driven pumps are more suited for fixed installations that move infrequently, such as booster stations or floating barges. The selection of the electric pump operating point requires more engineering than does the variable-speed diesel-driven pump. The highest-efficiency installations couple the pump directly to its drive motor so the electric motor’s speed must match the pump at the desired operating point. Field speed adjustments are limited to changing drive pulley ratios if the pump is belt driven. Otherwise, adjustments to the pump operating point involve throttling valves in the discharge pipe network with a commensurate loss of efficiency. Ratios not obtainable by changing the number of poles are possible by using belt- or geared-drive systems. If variable speeds are necessary, variable-frequency drives (VFDs) can be economic for smaller pumps up to about 56 kW (75 hp). VFDs change motor drive speeds by electronically converting the line power frequency supplied to the motor, thus making a wide range of output speeds possible. However, this comes at a significant cost, both in acquisition and efficiency. Automatic operation is easy to design into these pumps because numerous types of level switches or instruments for switching electric signals are available. Start-up against an empty pipeline may present problems keeping the pump “on the curve” because there is little backpressure until the water rises enough to prevent cavitation (a damaging condition caused when internal pressure falls below the vaporization point of the fluid (see the “Issue: Cavitation” section). VFD motor controllers are available that can control the start-up speed profile, gradually increasing the pump output pressure as the pipeline fills. Pump stations with electric drives do not have to maintain regular access for daily operations, which makes electric pumps suitable for installation in difficult terrain and confined areas. The major constraint is that power lines or flexible trailing cables have to be run to the pump’s location from the motor control panel. The starter panels and other controls must be located where operators have access, but these can be some distance from the pump. Booster Stations Pumps capable of handling large variations in the suction lifts are well suited for pumping from sumps but often do not have high head discharge pressures. This is where the pump system frequently combines booster pumps in a series with sump pumps, fully utilizing the best characteristics of both. Fixed booster pumps maintain constant discharge heads because the hydrostatic lift to the discharge location does not change. Fixed boosters stations may be in-line or tank fed with level controls that cycle the pumps to match inflows. Tank-fed booster pumps are normally designed to maintain the water


level above the suction eye of the pump, ensuring a positive suction head. floating Barges The decision to use a floating barge system is usually a good fit when pumping the water from a long-term sump with fluctuating levels due to irregular or seasonal inflows. Water elevation changes result in changing pump static lift conditions that can affect the pump capacity. This can happen in very deep ponds where the surface elevation drops significantly during the life of the system to the point where the static head exceeds the dynamic head capacity of the pump. Booster stations may be required but can be added later if the barge pump’s initial capacity is adequate. When properly designed, barge pump installations do not suffer from suction head issues (NPSHA) because the pumps maintain a fixed distance between the pump inlet and the water surface. This is a dramatic improvement over trailermounted pumps that have to be moved down the ramp each time the water surface drops by between 3 and 4.6 m (10 and 15 ft). Increasing sump levels float the barge, whereas the trailer pump would risk inundation. Barge pumps are much less labor intensive to operate, but their installation and maintenance costs are likely to be higher. Minimum water depth must be maintained in the sump to account for the pump and barge draft requirements. To avoid grounding the pump or sucking debris and rocks into the system, minimum clearance must be maintained between the suction inlet and the sump bottom. The suction line must be located deep enough to avoid the formation of vortices that suck air into the inlet screen or pipe. This is detrimental to performance and can cause the pump to lose prime. In horizontal centrifugal pumps, the flexible suction lines must be weighted to prevent the screen from floating too close to the surface, but they must also be supported to keep it out of the bottom sediments. Bottom clearance issues can be a problem with vertical turbine-style pumps, because the intake depth is fixed by the design distance between the pump intake and the barge deck. This style of pump is more likely to ingest debris if the sump’s water depth drops to the minimum necessary to float the barge. With proper barge design, the pump intake can be kept at a safe distance from the bottom by keeping the pump intake pipe shorter than the bottom supports of the barge, thereby leaving sufficient clearance. The use of a vortex prevention collar around the suction intake can allow the suction to be closer to the surface than the flow would otherwise allow. This collar is essentially a fin that prevents the streamlines that are approaching the suction line from rotating. The depth required above the suction hose to prevent the formations of vortices is dependent on the pumping rate (McNally n.d.).

TRouBleShooTing PuMPing SySTeMS

Because pumps are simple machines that are good at pushing water, a system that suddenly fails is most likely because of operator error. Upon pump failure, mechanics need to (1) check for improperly set control valves or broken valve shafts, (2) ensure that the source has water and the motor has power before blaming the pump, and (3) determine whether the discharge pressures are as expected and that the discharge pipelines have had time to fill with water. Depending on the diameter and length of the pipe, it takes a surprisingly long time for an empty pipe to fill enough to begin discharging

759

(Equation 9.5-11). A 1.6-km (1-mi) pipe with a cross-sectional area of 0.1 m2 (1 ft2) will take almost 40 minutes to fill from a pump pushing 227 m3/h (1,000 gpm). time to fill pipe, min = c

area ^lengthh k m Q

(9.5-11)

where

area = pipeline cross-sectional area, m2 (ft2) length = pipeline length, m (ft) k = unit conversion factor, 60 min/h (7.48 gal/ft3) Q = flow rate, m3/h (gpm)

If the causes are not immediately apparent after checking the obvious, the second thing to do is look for changes in the system that may or may not have been intended. Changes in piping during normal maintenance are a common reason that pumping networks that have been running successfully for some time suddenly appear to fail. These include changes in length, pipe diameter, new valves, or other physical changes that could result in the pump capacity being no longer sufficient. Maintenance on the pipe may have created an air-locked condition in the pipeline or the pipe may be empty. issue: Air locking Pipelines Air in the pipeline can consume all available pump energy trying to push the air against the hydrostatic lift. An example is the force needed to push a balloon down to the bottom of a swimming pool. The air’s buoyancy resists the flow in the pipe when flowing water tries to push the entrained air bubbles down from a local topographic high to a lower elevation. Under less extreme air conditions, the fluid in the pipe can flow under a relatively stable air pocket. Even in the flat-lying areas of the pipe, air bubbles restrict the cross-sectional area, increasing the flow velocity and friction losses in the pipe. Eliminating air from all overland pipelines prevents loss of pumping efficiency. Turbulence increases the friction losses, and gas compressibility robs the system of energy, converting it to heat. Air release/vacuum breaker combination valves should be used at all significant topographic high points on the pipeline profile. These valves are usually large ball-check valves with a floating ball and must be installed vertically. Air release function allows the air bubbles out of the pipe where they naturally accumulate. The ball falls from the seat while air is present, allowing the trapped air to escape from the pipeline. When the water rises into the valve body, the ball moves up into the seat to seal the valve. If the flow conditions drop the internal gauge pressure into vacuum, the ball check falls back, allowing air into the pipe and preventing suction collapse. This is necessary to prevent vacuum conditions where the flow is intermittent or where the pipeline profile significantly declines in elevation, particularly when using HDPE or other flexible pipelines. Pumping over a ridge frequently results in the hydraulic flow regime changing from pressurized flow (Hazen– Williams equation) to open channel flow in a partly filled pipe (Manning’s equation). The pipeline flows with the pipe full until reaching the maximum elevation of the section. Depending on the flow rate, either the pipeline will develop a steady open channel flow or will begin cycling the trapped air bubble, as previously described. An air release/vacuum breaker valve will allow air into the pipe, thus smoothing the

760


flow, and should create a stable open channel flow. The pipeline may convert back to full pipe pressure flow near the bottom of the hill. Computations of the parameters in this sort of mixed-flow regime are complex. It is usually sufficient to be aware of the potential issues when evaluating a pipeline in rolling topography. issue: Cavitation Cavitation occurs wherever the local flow conditions reduce the absolute pressure below the fluid’s vapor pressure. Rapidly moving water creates low-pressure areas, particularly when passing through structures that result in flow area reduction, resulting in formation of bubbles. Velocity increases at flow-control valves, restrictions in the cross-sectional area, or around the ends of rapidly moving components as they reduce the pressure, causing the fluid to begin vaporizing. The bubbles themselves are not damaging, but, after the restriction, the fluid slows and the pressure rises above the vapor pressure. This causes the bubbles to collapse suddenly, resulting in localized shock waves and pitting. Cavitation occurs most frequently at the suction eye of the pump impeller, behind the impeller tip, and on the downstream side of valves where this collapse zone exists. It occurs on pumps when NPSHA is less than NPSHR due to the design of the inlet piping failing to maintain a proper positive suction head. Fittings and elbows too close to the pump inlet can cause the restrictions and turbulence, leading to excessive friction losses. Cavitating pumps frequently sound as if they are pumping gravel even though no sediment is present in the fluid. issue: Priming Priming the pump is the operation that removes the air from the pump body and fills it with fluid. Centrifugal pumps cannot handle much air in the pump chamber; below 0.5% is usually acceptable, but above 6% by volume, the pump’s capacity is seriously degraded. Removing air from the pump chamber is necessary to allow positive suction lift to develop. Selfpriming pumps usually have a vacuum pump to remove air from the pump case, allowing external atmospheric pressure to push the fluid into the intake. Foot valves (check valves at the end of the suction pipe) hold the prime by keeping water from draining back into the sump when the pump is turned off. issue: insufficient Suction head Many pumping problems occur when the pump is located too far above the water surface of the feed source. The elevation difference between the suction eye of the pump and the water surface varies as the sump level changes. The pressure of the fluid must be higher than the vapor pressure at the suction eye of the pump at the operating temperature, or cavitation can result. Pumps are efficient at pushing water along pipes, which is the main reason why they are useful in mining. There is a common misconception that pumps suck water out of sumps with equal efficiency, but this is not the case. Pumps must maintain a positive pressure at the inlet of the pump in order for water to flow (Equation 9.5-13). A pump’s NPSHA is limited by available pressure Po, the vapor pressure of the fluid Pv, the static head assisting (resisting), and minor losses hL (Figure 9.5-5). The gauge pressure inside a pressurized tank, Po would be equal to atmospheric pressure, Pa on an open sump. When calculating minor losses with Equation 9.5-12,

Po

ΔZ

figure 9.5-5 Simplified net positive suction head

the loss coefficient (k) must be used; this is a characteristic of the particular transition geometry. h L = kc V m 2g 2

(9.5-12)

The pump’s inlet is frequently located above the water surface, requiring additional lift. The maximum NPSHA from an open sump is limited to the elevation-adjusted atmospheric pressure (Pa) less the vapor pressure of the fluid less the elevation head below the suction eye. The pump manufacturer determines the NPSHR while testing the pump during design and manufacturing. The operating point of the pump affects the NPSHR because minor losses in the entrance piping increase with higher velocity heads. The NPSHA must be higher than the NPSHR throughout the entire operating point range. NPSH =

Po + Pv + ΔZ − h L pg

(9.5-13)

where NPSH = net positive suction head Po = pressure on the water surface (Pa if tank is not pressurized) Pv = vapor pressure of fluid pg = specific weight and gravitational acceleration ∆Z = elevation head (lift if negative) hL = head losses due to transitions and fittings Methods to solve problems on the suction side of the pump include the following: • Lowering the pump closer to the water surface of the sump, reducing the static lift. • Ensuring that the pipe is clear and the suction screen is not blocked. • Reducing the length of the suction hose and increasing the diameter of the suction piping—both of which reduce friction losses in the intake piping. • Ensuring that the suction side fittings are not leaking; air ingestion can break the priming. • Ensuring that the pump is not cavitating; cavitation can have the same effects as air ingestion, limiting the pump efficiency. • Ensuring that the foot valve is holding to maintain prime.


• Reducing the number of fittings and keeping them as far from the pump as practicable to limit the friction losses in the suction line. • Ensuring that the pump is operating near the BEP. • Splitting the load by using several smaller pumps in parallel instead of a single large pump. issue: Minimum flow Centrifugal pumps must have sufficient fluid flow to avoid overheating. Pumps convert the mechanical energy from the drive shaft into various forms, including • Hydrostatic pressure (head), • Fluid kinetic energy (velocity head), and • Heat (friction losses). Fluid flowing through the pump carries the heat from the mechanical and hydraulic losses in the system. If the system is prevented from flowing by closed valves, insufficient suction head, or insufficient discharge head pressure to overcome the system’s static head, most of these losses are converted into heat, eventually causing the water inside the pump casing to boil. Explosions are possible if the pressure due to vapor expansion exceeds the maximum casing pressure rating. Significant damage will occur to the pump impeller from cavitation at temperatures well below the normal boiling point. As the temperature increases, the vapor pressure of the fluid increases, resulting in more cavitation. This has the same effect of operating the pump with insufficient NPSH by increasing the necessary static pressure required to prevent vaporization. issue: Sediment, Debris, and Screens The primary source of debris entering the pumping system is through inadequately maintained intake piping. The first line of defense is to install an appropriate screen over the suction pipe to prevent large solids from entering the pump suction. The suction hose should be kept above the sump bottom to prevent the flow from sucking debris onto the screen. During construction, pipe segments must be inspected and cleaned before they are joined. When dragging pipes into place, the ends act as a scoop and can pick up a surprising amount of rocks and debris, which require flushing before closing the section. Large rocks will migrate along the flow and jam against valves, preventing them from properly actuating. Pipe that has been stored outside should be inspected prior to construction for animals that have moved in. In-line screens, traps, or cleanout fittings should be installed periodically in the pipe and located to protect valves, pumps, regulators, or other sensitive fittings. Drains and cleanout fittings should be placed where the discharge during cleaning does not cause damage. When the pipeline runs through hilly country, it is important to ensure that the pipeline has sufficient drain valves installed at logical topographic low points to allow flushing of the system. This allows removal of sediment buildup in pipe systems before blockages occur and is particularly necessary if the pump system handles turbid runoff or slurry, because the solids will pass through the normal intake screen but may settle in the pipe. issue: Pressure Bleed Through Check valves In a high-lift multistage system, pressure can bleed through check valves, causing excessive hydrostatic pressure in the

761

lower stages and resulting in pipeline ruptures when the pump system starts. Systems can experience problems with pipe segments splitting in the bottom stage of a series of pumps because of starting pumps against excessive hydrostatic head. This can be a problem when using pumps in a series without an atmospheric pressure break between stages. Check valves are designed to allow flow in only one direction. When holding pressure against the upstream face, the valve closes, preventing backflow and holding the differential pressure across the valve. Pitted faces or improperly sealed valves allow small water seeps that can equalize pressure on both sides. Because water is incompressible, this can happen with little seepage volume over time in systems that have almost no trapped air. The lower stages of the pipeline are the most likely to experience pipe rupture because the pressure cascades through multiple stages above. The dynamic head from starting the lower pump is added to an already overpressurized pipe segment. Indications of this problem show up in HDPE pipe sections that have been repaired numerous times. The solution is to design multistage pumping systems with transfer tanks between the stages at atmospheric pressure. More area is necessary for the pump stations, and the system will be less efficient, but the starting sequence and dynamic interactions are easier to control. An alternative would be to install accumulator tanks to allow expansion against a compressible air bladder. issue: Water hammer Water hammer is a dynamic shock/pressure wave that travels through the fluid in a pipeline. The primary cause is a shock wave created when the flow’s velocity or direction changes rapidly. Water hammer shock waves can travel at the speed of sound in the fluid, not at the fluid flow velocity. The speed of sound in water can exceed 1,219 m/s (4,000 ft/s). Equation 9.5-14 is a simplified peak pressure formula empirically relating pipeline pressure spikes and the transitions caused by rapidly closing the valves (Plast-O-Matic Valves 2002). ΔP = ;

^0.07 h VL

t

E+ P

(9.5-14)

where ∆P = pressure spike from water hammer V = flow velocity in ft/s L = pipeline length in feet t = time in seconds P = pipeline pressure in psi For example, consider the magnitude of the pressure spike from an ordinary action: An operator closes a butterfly valve in a 4.8-km- (3-mi-) long pipeline flowing at 1.5 m/s (5 ft/s), thereby stopping the flow. The operator takes 5 seconds to close the valve on a pipe that is carrying 690 kPa (100 psi). What is the pressure spike from this action?

8^0.07h 5 s ` 5, 280 mi j 3 miB ft

ΔP =

ft

5 seconds = 8, 270 kPa ^1, 200 psi)h

+ 100 psi

Added to the hydrostatic pressure, spikes of this magnitude can overstress pipes, valves, and pumps, causing significant structural failures. Water hammer can cause oscillations in

762


both positive and negative (limited by vapor pressure and kinetics) excursions in a system, resulting in damage. Valves should operate by controlling opening and closing times using automated valve actuators at programmable rates. Pump stations should have flywheels on the pumps to slow the velocity change rate, dV/dt during start-up, and unplanned power interruptions. Electrical motor soft starters should be used to ramp pump motor speeds when starting and stopping pumps. In some installations, the time constants for flow changes may be as long as an hour to limit system pressure damage. Water has inertia and carries considerable momentum when moving. The longer the pipeline and the higher the flow rate, the larger the mass that is moving. Check valves that slap closed rapidly when flow direction reverses are a common cause of water hammer. Long pipelines with high flow rates need to have shockabsorbing structures designed into the system. Hydropneumatic dampeners, accumulator tanks, air traps, or air bladder tanks absorb and dampen shocks by using compressible gas and must be sufficiently large to absorb the shock energy expected from sudden transients in the pipe. Air release/vacuum breaker valves can be useful for allowing air into the pipe to cushion the shock of a water slug moving away from a closing valve. The air acts as a shock absorber, thereby limiting damage.

CoSTS

Due to the cost of designing and building pit-dewatering systems, efforts to keep water outside the pit are usually a good investment. Intercepting and diverting water at the pit rim is less expensive than pumping from the bottom of the pit. The direct pumping costs include power, infrastructure, and required space designated for the pump stations and pipeline rights-of-way. Pit water may also require treatment before discharging off-site. Indirect costs can be significantly higher because of the effects that water has on productivity, reducing highwall stability, and safety hazards that the operation must mitigate. Direct Cost example for Pit Pumping If a mine allows water to flow into the pit because of the lack of an effective diversion, at an average inflow rate of 6.3 L/s (100 gpm), this accumulates 200,000 m3 (52.6 million gal) annually. Water has to be pumped vertically 305 m (1,000 ft) from the pit floor. If the pumping rate is 63 L/s (1,000 gpm), the pumps would need to run 10% of the time at 60% hydraulic efficiency of the pump and a power cost of $0.05/kW·h. The annual costs in terms of power and the infrastructure to set up the dewatering system can be estimated using the applicable power equations in Equations 9.5-15 through 9.5-17: hydraulic hp = [(H)(Q)(SG)]/3,960

(9.5-15)

hydraulic kW = [9.81(H)Q(SG)]/1,000

(9.5-16)

hydraulic hp pump efficiency

(9.5-17)

brake hp =

where H = total dynamic head, m (ft) Q = flow rate, L/s (gpm) SG = fluid specific gravity

Results: • Motor power required = 314 kW (421 hp) (without pipe friction losses) • Annual power cost = 8,760 h/yr (10% duty cycle and 314 kW at 0.05 $/kW·h) • Annual power cost = $13,770/yr in electricity Infrastructure cost estimate: • $250,000 for 3,048 m (10,000 ft) of 8-in. pipe at $25/ft installed (along 10% road) • $75,000 for three pumps with head capacities of 107 m (350 ft) TDH per stage, including motors at $25,000 each $200, 000 power to pit floor $525, 000 total infrastructure The annual cost for power in this example just to pump the water was $13,770, or $2,180 per L/s ($138/gpm) allowed into the pit. These costs are avoidable (or at least reduced) by intercepting the water higher up the wall or diverting it completely. Direct Cost example of hauling Wet ore Increasing moisture content increases the density of the material, adding to the costs of haulage and conveying. If the dewatering crew can lower the moisture content of the ore from 4% to 2% by improving water control at the shovelloading face, a mine processing 89,290 t (100,000 st) per day saves 1,786 t (2,000 st) of excess water previously hauled to the crusher daily. This is the equivalent of pumping 21 L/s (333 gpm) of water out of the pit using the most inefficient of pumping systems—the haul truck. If haulage costs $0.75/t, on an annualized basis, this reduces annual operating costs by $548,000.

RefeRenCeS

BBC (British Broadcasting Corporation). 2000. Megatsunami: Wave of destruction. www.bbc.co.uk/science/ horizon/2000/mega_tsunami_transcript.shtml. Accessed January 2010. Casada, D. 2006. Pump System Assessment Tool (PSAT), Diagnostic Solutions, LLC. Presented at Salt River Project, Phoenix, Arizona. HEC (Hydrologic Engineering Center). 2009. HEC-HMS Hydrologic Management System, Version 3.4. U.S. Army Corps of Engineers. www.hec.usace.army.mil. Accessed January 2010. Loofbourow, R.L. 1973. Ground water and ground-water control. In SME Mining Engineering Handbook, Vol. 2. Edited by A.B. Cummings and I.A. Given. New York: SME-AIME. McNally Institute. n.d. Cavitation 1-3. www.mcnallyinstitute .com. Accessed January 2010. MSHA (Mine Safety and Health Administration). 1977. Federal Mine Safety and Health Act of 1977, Title 30 CFR (Code of Federal Regulations), 30 CFR 77.216-3(1), and 30 CFR 77.216(a). www.msha.gov/30cfr/77.216-3 .htm. Accessed May 2010. NOAA (National Oceanic and Atmospheric Administration). 1980. A Methodology for Point-to-Area Rainfall Frequency Ratios. NOAA Technical Report NWS 24. Washington, DC: NOAA.


Performance Pipe. 2003. The Performance Pipe Field Handbook, 1st ed. www.performancepipe.com/bl/ performancepipe/en-us/Documents/Field Handbook revision 8-09 web version.pdf. Accessed January 2010. Plast-O-Matic Valves. 2002. The effects of water hammer and pulsations. www.plastomatic.com/water-hammer.html. Accessed January 2010. Spindler, W.H., ed., 1971. Handbook of Steel Drainage and Highway Construction Products, 2nd ed. Washington, DC: American Iron and Steel Institute.

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Swanson, H.A., and Baldwin, H.L. 1965. Common water measurements. In A Primer on Water Quality. Washington, DC: U.S. Geological Survey. http://ga.water.usgs.gov/ edu/characteristics.html. Accessed February 2010. U.S. Army. 1971. Drainage control. In Engineers Reference and Logistical Guide Field Manual FM 5-35. Washington, DC: U.S. Army. Wood, D.J, and Lingireddy, S. 2010. KYPipe, University of Kentucky. www.kypipe.com. Accessed January 2010.

CHAPTER 9.6

Dewatering underground operations Peter L. McCarthy and Malcolm G. Dorricott

can be difficult or impossible to predict until mining takes place. Nevertheless, experienced engineers, geologists, and hydrologists are able to make satisfactory predictions in many cases, so that appropriate control measures can be put in place. Sudden inrushes of water or mud can arise from

The inflow of water can affect the cost and progress of underground mining, limiting the mining methods used and presenting hazards. Mine dewatering has effects both on the groundwater table, which is usually a shared resource, and on the environment receiving any discharge. Although many mines have been worked successfully beneath perched underground reservoirs, streams, lakes, rivers, and seas, there are also many examples of catastrophic inrushes. Experts such as hydrogeologists, hydrologists, and pumping engineers can contribute to the success of mining projects and should be consulted when appropriate. The general objective in the control of water in underground mining is to permit safe, efficient work with acceptable consequences. The most common method of control is pumping. Where an impervious cover above a mineral deposit can be maintained, it may not be necessary to draw down the groundwater aquifer, provided accesses, including shafts, can be developed by grouting, ground freezing, or casing. Many salt, potash, and coal mines are of this type. Pregrouting with cement slurries can reduce the hazards of serious water inrushes and minimize delays during shaft boring or sinking, whereas cover grouting using chemical grouts can reduce inflows to underground development. As part of any mine plan, an overall water balance is essential to ensure that enough water is available and that surplus water is managed and disposed of in an acceptable manner.

• Heavy rainfall events via mine openings or surface subsidence and caving zones; • Unplanned connections to the sea, lakes, rivers, swamps, clay deposits, wet cover, tailings dams, and water dams; • Connections to water pockets such as caverns in carbonate rocks and fault conduits; • Magmatic water, if mining near a volcanic caldera; and • Connections to adjacent flooded mines.

iMPACT of WATeR on oPeRATionS

The presence of water in underground mines impacts virtually all operations and activities. Its main beneficial effect is in the control of dust in drilling and during rock transportation. It also serves as a transport medium for hydraulically placed backfills and sometimes, as at the McArthur River uranium mine in Saskatchewan, Canada, as a medium for transporting ore. The negative effects of water include the following:

• Inflows of surface water through natural (geological) conduits, mine openings, and boreholes; • Inflows of groundwater through the natural permeability of the rock and from secondary (fracture) permeability; • Mine service water, such as water used for drilling and dust control sprays; and • Drainage from hydraulic backfill.

• Increased heat transfer from the rock in hot mines • Freezing in very cold mines • Increased humidity, causing less comfortable working conditions and reduced labor productivity • Reduced ground stability due to water pressure • Corrosion of plant, equipment, and ground support elements • Requirement for more expensive explosives • Spillage from trucks and conveyors • Water and mud rushes from orepasses and other sources • Erosion of roadways, particularly ramps • The need for special precautions for mining in the presence of water under pressure • Increased power consumption for pumping and ventilation

Although prediction of the volume of mine service water is straightforward, the magnitude of natural inflows sometimes

All of these impacts add to the cost of mining, and in extreme cases may be the difference between profit and loss.

SouRCeS of WATeR

The main sources of water encountered in underground mining are

Peter L. McCarthy, Chairman and Principal Mining Consultant, AMC Consultants Pty Ltd., Melbourne, Victoria, Australia Malcolm G. Dorricott, Principal Consultant, AMC Consultants Pty Ltd., Melbourne, Victoria, Australia

765

766


PReDiCTing WATeR infloWS

Prediction of water inflows requires an understanding of the climatic and hydrogeological factors plus any existing humancaused conditions, such as abandoned boreholes and nearby flooded mines, and the effects of planned mining work. Many sites present no significant water problem, but this should never be assumed. It is difficult to predict groundwater flows into underground mines, as they are dominated by structural controls that usually cannot be identified from exploration drilling. Hydrogeologists have access to sophisticated numerical modeling tools, but experience shows that these models often fail to accurately predict inflows to an underground mine because of the lack of data, particularly in the early stages of a project. To maximize the reliability of predictions, data should be collected at all stages of a project or mine development. Each mine should have an efficient data management system, which covers the collection, storage, and distribution of the data to the relevant people. The data to be collected at each stage of a project or mine should be determined by a group of competent persons prior to work commencing. For example, the following is a checklist for the collection of information required for the prefeasibility stage of a mine project: • • • • • • • • • •

Topographical information Rainfall quantity and intensity data River flow and flood level history Tidal levels and flows Earthquake frequency and strengths Climatic conditions including temperature, humidity, and evaporation rates Surface water runoff characteristics Groundwater characteristics including levels, salinity, temperature, recharge, discharge, and general flow directions Hydrogeological conditions, including geological units and their permeabilities and storage, and general geological features (faults) Information on soil and rock types, their characteristics, depth of weathering, and so forth

Precipitation and Runoff To predict the likely inflow from a single precipitation event is quite straightforward and can be calculated from the following equation (rational method): V=C#i#t#K where V = volume of potential inflow C = catchment area i = intensity of precipitation t = duration of event K = runoff coefficient It may be useful to consider the rate (Q = C # i # A) as i is a time-dependent variable (and A = area). The catchment area can be estimated from topographic data and an understanding of the regional hydrogeological conditions. It may be feasible to reduce the catchment area by diversion of water courses or containment. Intensity is derived from historical records. However, a choice must be made as to which event should be the basis

for design. Typically, a 1-in-100-year event is considered reasonable, assuming such records exist for the location, but the expected life of the operation needs to be taken into consideration. A short-duration mine is less likely to receive such an event than one that has operated for decades, but many mines have much longer lives than initially expected. The decision should be based on a risk assessment and a realistic consideration of the consequences of the scenario selected. The runoff coefficient must be selected carefully, because high-intensity events are more likely to occur when the ground is already wet from normal precipitation, thus initial abstraction is small. There is also a trade-off between storage capacity and pumping rate. Once the inflow volume of the worst-case scenario has been estimated, the option exists to divert the inflow to available storage and pump it out slowly after the event or to cope by matching the pumping rate to the expected inflow rate. The best option is usually somewhere between these two extremes. Of course, storage capacity must be empty if it is to be relied on in an emergency. An example of such a calculation is provided for the Mount Lyell mine in Tasmania, Australia (Atkinson 1982). At the time, the open-pit mine, located in an area of high rainfall, was transitioning to an underground mine, with breakthroughs from the underground mine to the base of the pit. The emergency storage capacity required for a 1-in-50-year storm event was calculated from the following equation: required storage capacity V = C # i # t # K – (pumped volume – other inflows) V = 45 # 104 # (1–0.25) # 1.1 # e(3.2 – 0.58 Ln t) # K – (600 # 0.75 – 60) # 3,600 # t # 10–6 where • The catchment area C is 45 ha or 45 # 104 m2, and the proportion of runoff intercepted and diverted away from the mine is 25% • 10% of the rainwater flow returns to the mine from the mud displacement pump system • The 1-in-50-year rainfall intensity-duration relationship is approximated by Ln i = 3.2 – 0.58 Ln t (Ln = log normal) • The runoff coefficient (K) increases from 0.5 to 0.9 during the event • Pumping capacity is 600 L/s with an availability of 75% • The water inflow from mining operations and groundwater is 60 L/s • The event duration (t) ranges from 1 to 22 hours The results of this analysis (Table 9.6-1) show that the maximum storage required is 7.5 ML (megaliters) after 10 hours. A lower pumping rate would require more storage and vice versa. A cost–benefit analysis is required to determine the optimal balance, taking into account the level of risk. hydrological Studies and Modeling Hydrogeologists, petroleum engineers, and civil engineers all start with Darcy’s 1856 observation that velocity of laminar fluid flow through sand is directly proportional to the permeability of the medium and the hydraulic gradient. Hydrogeologists find pumping tests useful in predicting the performance of water wells. Petroleum (reservoir) engineers

Dewatering underground operations

The depression is an inverted cone, symmetrical around the well. Its slope decreases logarithmically to the radius of effect where the depression is not measurable. If permeability is not uniform, the cone is distorted. Where water is only in fractures, it is discontinuous. As a well is pumped, the drawdown increases until the well is dewatered or until, because of the steepening gradient, the rate of inflow balances the pumping, and flow becomes steady (Figure 9.6-1). As the cone expands, its shape may be changed by masses of higher or lower permeability or by recharge or barrier. Schematic profiles in Figure 9.6-2A show the extreme effect of a barrier surrounding a shaft being deepened. Those in Figure 9.6-2B show the effect of an irreducible recharge (e.g., on a shaft being sunk in the center of an island). The cone around such a shaft expands normally in Stages 1 and 2, but its shape is changed by a barrier or continuing recharge. The void space within a soil or rock is referred to as its porosity. The amount of water that can be stored within the porosity and subsequently released is referred to as effective porosity or sometimes coefficient of storage. The total porosity of shales and clays is very high, generally greater than 25% and in some cases 50%, but the water is absorbed in the crystal structure and cannot be removed easily, whereas the intergranular spaces are typically so small that movement of water under usual conditions is negligible unless huge areas are considered. Most of the porosity of coarse sandstones is effective as storage. Close-spaced open fracturing can give dense rock important storage capacity, but it is rare. Solution cavities can provide storage in otherwise dense limestone and dolomite. Some lava contains ash, erosion surfaces, and tubes with high permeability and more or less storage. Weathered surfaces of most strong rocks are likely to carry water. Induration (hardening) closes pores and fractures. The amount of water that can actually flow through and from the rock mass and into the mine depends on the

Table 9.6-1 emergency storage required for a 50-year storm Duration, h

Runoff Coefficient

intensity, mm/h

Storage, Ml

1

0.5

24.5

3.1

2

0.6

16.4

4.5

4

0.7

11.0

5.8

8

0.8

7.3

6.2

10

0.9

6.5

7.5

12

0.9

5.8

6.4

16

0.9

4.9

22

0.9

4.1

3.8 0

test within much deeper wells and on cores taken from them to forecast production of oil and gas. Civil engineers concentrate on soil moisture and the effects of pore pressure on stability. Efforts to calculate flow to tunnels, shafts, and underground mines are made difficult by complex geology and lack of data. In simplest form, the water table, below which the ground is saturated, is nearly a horizontal plane. In a succession of layers of diverse permeability, there may be multiple water levels. Artesian water is trapped under a layer of comparatively low permeability so that water rises above the ground surface. Where permeability is irregular or discontinuous, the water level is similarly erratic. Where water feeds into the ground, the water table is higher, and groundwater flows from such points to lower points of discharge. The slope of most natural water tables is gentle, which usually mimics land surface, and the motion of flow is slow. The water table, or piezometric surface, is depressed by pumping a well. The resulting gradient causes flow toward the well. The simplest analysis, such as the Theis solution (Theis 1935), is based on an ideal aquifer—homogeneous, isotropic, and horizontally infinite—and a perfect well, which is open to the full thickness of the aquifer or is very long compared to its diameter and thus receives water by horizontal flow.

r

r

R

R

Well A

s2 = 0 No Drawdown

Saturated Aquifer

767

Dewatered Aquifer m

Well B

Piezometric Surface to Which Water Would Rise in Observation Wells

s1

K2πm (s1 –s2) Q(for steady flow) = Cμlog10(R/r)

Steady Flow to Uncased Well A with Water Table Aquifer Fully Dewatered

m

s2 = 0 No Drawdown

Fully Saturated Aquifer

Steady Flow to Uncased Well B with Artesian Aquifer Completely Saturated

Note: In Well A, 100% dewatering is, in fact, impossible; if water is to continue flowing to the well, there must be some saturated wall through which it passes.

Source: Lucas and Adler 1973.

figure 9.6-1 Comparison of flow to wells in dewatered and saturated aquifers

768


(A)

(B) Barrier

Lake

1 2

1

3

3

4

2

Gauge Aquifer

Bypass

4

Water Pump

Source: Adapted from Lucas and Adler 1973.

figure 9.6-2 extreme effects of a barrier and continuing recharge

permeability of the rock mass and, more importantly, the presence and permeability of flow paths such as faults, shears, and bedding plane breaks. These are generally not measureable before mining, hence the limited value of modeling. Pumping tests can sometimes yield limited guidance. Pumping tests are made by pumping water from a well at a sufficient rate long enough to lower the water level measurably in several observation wells, which should be at prescribed locations with respect to the pumped well. The pumped well should penetrate the entire aquifer and, in usual tests, receive water freely from all of it. In thick aquifers this may not be possible, but useful conductivity data could still be obtained. The time and cost of pumping tests generally limit the number that can be made on a project. Hence, it is difficult or impossible to determine properties of more than one part of a complex rock mass, indicate uniformity of any part of the mass, or establish averages that can be used with confidence. Where these points can be determined in other ways and where the test can be set up to provide the information needed, there is no better source of data. Pressure tests with packers are water injection tests, in some ways the reverse of pumping tests, but they can be made in holes of usual diamond-drilling size, and the packers can be set to test any interval of open hole and can be reset repeatedly (Figure 9.6-3). Generally the water pressure at the section under test can be between 1.1 and 2.5# the hydrostatic head without danger of opening fractures. Where permeability is high, there is an advantage in testing at a low pressure difference to limit pipe friction and the likelihood of turbulence. Where it is low, a higher pressure difference provides a measurable inflow more quickly. Packers may fail to seat where erosion of weak rock has enlarged the hole and water can leak around them through fractures or connected pores. Such leakage can be reduced by using long packers, by making successive tests below a single packer near the bottom of the hole as it is deepened, and by testing below double packers. A series of tests can be checked by a single test of the same section of the hole. Leakage around packers or through pipe joints leads to overestimating inflow. However, any mud or grease caked on the walls of the hole would have the opposite effect. Borehole flowmeter surveys may be used to measure water flow patterns within a well. The flow log reveals zones of water entry and exit and allows flow contributions from individual zones to be measured. Spinner flowmeter logs are used during well pump tests to measure hydraulic conductivity. With very low flow rates, the spinner flowmeter may be insufficiently sensitive. The heat-pulse flowmeter may be used

High-pressure air or nitrogen to inflate packers, carried through small pipe or tube

Tight pipe string supporting packers

Water for testing can be supplied from • Variable-speed pump and bypass (indicated) • Constant-speed pump and bypass • High-pressure water line • Pressure tank (air or gas pressure)

Packer inflated by gas Interval between packets subjected to test pressure

Inflow can be measured by • Change of level in tank (indicated) • Counting strokes of plunger pump in tight ground • Meter or orifice in open ground and indicated by drop in test pressure when test is valved off

Its length can be varied by changing pipe between packers

Source: Lucas and Adler 1973.

figure 9.6-3 Pressure tests with pneumatic packers in straddle configuration

in a stationary mode at selected depths to detect linear flows down to 1 mm/s. Holes can be cleaned by swabbing (i.e., pumping by repeatedly lifting a column of water above plastic cups on a wire line inside a string of casing or tubing which may extend 100 m or more below the water level in the hole). Measurement of the rate of water level recovery after swabbing is a negativepressure test. The swab can be run inside tubing on which packers are set to determine inflow from the interval between them and to sample the inflow. A series of tests between packers efficiently expanded and contracted from the surface can be made at the rate of one to three tests per hour. Packer tests take time for lowering and recovering the packers and setting up for each test. Drill-stem tests are made with a special tool lowered into the hole on a string of drill pipe. Above the tool is a packer, which can be expanded to close off the bottom of the hole. The tool can be placed between two packers to test the section enclosed. A pressure transducer and data logger can record the pressure increase as the tool is lowered, the shut-in pressure in the test section, pressure changes during the test, and pressure decrease as the tool is being removed. To start the test,


a valve is opened, permitting fluid to flow from the ground through the tool into the empty pipe, and a sample is recovered. Potential production can be calculated from the rate of flow and the recorded bottom hole pressures. Each normal drill-stem test calls for a trip into and out of the hole with a string of tight pipe. use of Tracers Tracers are put into a groundwater system at some point and used to indicate the direction of water movement and, in some cases, its approximate rate, by being recognized at a point or points downstream. None of the many tracers is ideal under all conditions. A tracer should • Be recognizable after dilution, generally with portable equipment, in some cases in test holes; • Be unimpaired by physical, chemical, or bacteriological reaction and adsorption with the water being tested or rock in contact with it—at least until recognized; • Move with the water; • Be convenient to use, reasonably available at moderate or low cost, easily soluble in water, and require no elaborate equipment or procedure; and • Present no environmental hazard or cause anxiety to anyone. Fluorescein, an extraordinarily intense coal-tar dye, usually is purchased as a red-orange powder. When dissolved in water it is a brilliant green. One part in 40 million ordinarily is recognized by sight. One part of good quality fluorescein in 5 to 10 billion parts of clear water can be recognized in a colorless tube about 1 cm or less in diameter by 1 m long, with a black rubber stopper or other black bottom. A number of tubes can be mounted side by side in a rack. Examination should be made by good white light in front of a white reflecting surface. Tubes containing 0 to 0.002 ppm of fluorescein can be used comparatively. Fluorescein is not affected by carbonic acid but is made colorless by contact with peat, acetic acid, and mineral acids. It is unaffected even by long contact with limestone, sand, silt, montmorillonite, and other common clays. Its vivid color gives fluorescein a special advantage where it is desired to make results evident to all observers. Other dyes—fast crimson, congo red, methylene blue, and so forth—may be used similarly. Chloride ion or salt is recognized in test holes by decreased resistance to electric current or chemically, providing dilution is not too great. Dense solution may be trapped in low spots. Otherwise, salt solution seems to move at the same rate as the water. It is obscured by any natural brine and changes the permeability of some clays. Bromide, nitrates, and other ions are also used. Dextrose, recognized chemically, is not adsorbed and moves at the same rate as water but is attacked by soil bacteria and is more easily lost in dilution than radioactive isotopes. Radioactive isotopes are said to be recognizable in concentrations of 10–18, but are generally precluded because of public sensitivity to their use. The normal application is to place the tracer in a possible water source and watch for it to appear in the mine. For example, a tracer may be added to mine water discharge to show whether it is returning into the mine. This may be impossible if the suspected source is a very large body of water. If the mine is flooded, the tracer could be put in the mine and water pumped behind it to cause the tracer to be observed at the surface, although the results could be inconclusive.

769

ConTRol AnD ColleCTion of WATeR

Planning for the control and collection of water must consider all possible sources under normal conditions as well as the possibility of extreme events. Control of Surface Water The following measures can contribute to management of inflows to an underground mine: • Shaft collars and ramp portals can be built on elevated ground or surrounded by earth or rock bunds. Bunds should have a uniform level with no low points that could lead to failure. • Where a portal is built within an open pit, it should be elevated above the floor of the pit. • Surface earthworks should not unintentionally impede or divert natural stream and sheet flows. • Shafts should be pregrouted and imperviously lined. • Rivers can be diverted, lakes and swamps drained, and streams cleared and straightened to reduce direct inflow and recharge. • Intakes (including stream beds) can be covered with rolled clay or concrete. • Water can be intercepted in shallow wells or caught in water rings in shafts. • Slopes should be cleared and drains built. • Trees should be planted in low, flat areas to increase evapotranspiration. The mine should have a procedure that will be followed to manage the effects of extreme weather events. For example, with normal rain, work should be continued but awareness maintained. If cyclonic rains are in the area, the situation should be monitored continuously on weather radar and crews withdrawn from high-risk areas. If there is flooding potential, work should be stopped, all crews withdrawn, and the situation monitored until danger has passed. Management of Drilling In a situation that could pose a threat to future mining operations (e.g., holes from a dry lake bed or frozen lake), all drill holes should be fully or partially grouted. During exploration, hydrogeology data should be recorded for future planning needs. When working underground in areas suspected of containing high-pressure underground water, the hole should be drilled using appropriate equipment and procedures. Exploration drill holes must be cemented in some areas to prevent migration of water from different aquifers. If the hole is left unsupported, any soil or weak rock soon caves, partly plugging the hole and hiding it, after which treatment from the surface can be difficult. If an underground working is connected to a hole to which water has access, it may enter, perhaps with gas, at such volume and pressure that sealing from below is difficult. The practice at one salt mine is to leave a 45,000-t pillar centered on the mapped location of each hole. The capacity of high-pressure water to ravel weak rock and, if it carries grit, to erode strong rock and metal must not be overlooked. An extreme example of the problem was the inundation and loss of the Diamond Crystal salt mine at Lake Peigneur in Louisiana (United States) in 1980, when an exploration drill hole broke into the workings. Even without being connected to workings, wet holes can decrease stability by permitting water to saturate and weaken clay shales and raise pore pressure.

770


Some holes, if kept open, could be used for observation of water levels, treatment, geological testing, telephone or electric lines, driving raises, and so forth. Exploration holes generally should be plugged securely before abandonment, or, if needed for communication or observation, pipe or casing should be set through all weak or wet ground. It is sometimes useful to know the maximum flow expected if a borehole is encountered, so that emergency drainage and pumping capacity can be provided. Water freely supplied to vertical boreholes falls through them with head loss of 100%, meaning that the head loss per length of hole is unity. Flow can be estimated from the empirical Hazen–Williams equation, which in metric units can be expressed (Larock et al. 1999) as S = (10.67 Q1.85)/(C1.85d 4.87) where S = head loss (in meters of water) per meter of pipeline or borehole Q = volumetric flow rate, m3/s C = roughness coefficient d = inside diameter, m Flow through a clean diamond drill hole can be estimated by substituting S = 1 and a typical value of C = 115 into the previous equation, then rearranging and solving. For an AQ drill hole (1.89 in. diameter), d = 0.048 m, and hence Q = 0.0108 m3/s. Flows through clean holes of standard diamond drill sizes are shown in Table 9.6-2. Prevention of inrushes A risk assessment is required to identify the necessary precautions to manage potential sources of inrush. Prior to mining beneath surface water, near underground water sources or old workings, water should be drained or pumped to below the current working level. If this is not practical, a minimum safe approach distance should be determined and maintained. The maximum flood levels should be determined for all mining areas. Access to an underground working should be constructed above the expected maximum manageable flood level in that area. In flat land, sheet flooding to a depth of only 100–200 mm can rapidly erode a channel into an underground mine and flood it. Surface water should not be allowed to pond or accumulate near mines and should be drained or pumped away. Where this is not possible, adequately designed and constructed bunding must be in place and the freeboard monitored. The migration of water under the bund should also be monitored. No structures, dams, tailings dams, storage facilities, mine infrastructure, roads, or rock dumps should be built over the footprint of any underground workings if any potential exists for caving to the surface. Adequate crown and shaft pillars should be provided and monitored. Notably, rock dumps can be a significant water source for deep infiltration. Underground dams, water storage facilities, slimes dams, or any other storage facility that might be damaged and cause an inrush should be properly designed and located so they are not influenced by mining-induced ground movement. Where mining activity is close to the base of oxidation or the surface, the area should be fenced or barricaded and bunded. Allowance should be made for the angle of repose of the possible breakthrough area.

Table 9.6-2 flows through holes of standard diamond drill sizes Size

Diameter, in.

flow, l/s

AQ

1.89

BQ

2.36

19

NQ

2.98

36

11

HQ

3.783

67

PQ

4.828

128

If tailings or surface water storage facilities overlie an underground mine, a risk minimization program should emphasize water diversion, groundwater drainage, and sound slope-stability practice. Monitoring should include checking the stability of structures on a regular basis, maintaining adequate storm freeboard, and having diversion structures to deal with an overflow. Tailings should not be used to fill surface subsidence above working mines because of the potential for liquefaction and inrush through a relatively small subsidence opening. This was the cause of the 1970 inrush at Mufulira mine in Zambia, which caused the death of 89 miners. Backfilling Backfill is used with many mining methods and may be used to reduce the risk of subsidence and some forms of inrushes. Planning should ensure that • Stope voids are sufficiently filled so that caving is minimized; • Controls are in place to prevent the fill material from liquefying during placement or remobilizing due to subsequent water inflows and ground vibration; • Fill strength is adequate to ensure stability during adjacent mining or pillar extraction; • Fill should be drained so that it cannot remobilize; and • Filling schedule limits the accumulated void space in the mine. Design of backfill may include tests for particle size, water retention characteristics, field capacity, volume, abrasiveness, transportation systems, fluidity, the possibility of liquefaction, and the need for additives such as fly ash, cement, and gypsum. During hydraulic filling of voids, monitoring should include pressures behind the barricades/bulkheads, ponding on the fill surface, the barricade/bulkhead integrity and drainage rates, and vertical filling rate. Draining underground Water In some cases, making initial openings in tight ground to drain conduits or masses of wet rock has advantages, but the procedure may be complicated by one or more of the following conditions: • High-pressure water, hot water, or dissolved gas • Weak ground, unstable in contact with water flowing under high pressure • Mud and grit causing or increasing erosion • Uncertain location of wet ground or conduits • Uncertain rate of inflow To indicate the water’s location, pressure, and perhaps something of the rate of flow, pilot holes usually are drilled ahead of development openings. Cover drilling, which is drilling in advance and to the sides of the heading with multiple holes, may be necessary for safety.


Tapping can be accomplished by developing an opening into the water source. Where water occurs in small conduits in a rock strong enough to resist erosion, the heading can be continued until the desired inflow is obtained. If water is carried by a well-defined clean conduit, this may still be a good approach, but drilling and blasting the last round is tricky, and, unless a pressure door has been built, only friction in the conduit and in the heading restrain the inflow. With more time and cost, tapping can be controlled by driving to a safe distance from the wet ground, cutting a drill station, and drilling many radiating holes. The safe approach distance depends on the geology, water pressure, and knowledge of location. Where high pressure is expected, and especially where it could erode the collars of the holes, work must be protected by drilling an oversize collar hole and cementing a collar pipe with a bypass tee and a full-opening valve. Drilling is completed through this valve. Water is drawn from many points throughout a sizable rock mass, which may be important to minimize erosion. Because these holes are short, head losses of 10 to 40 m per 100 m may be acceptable. In this range, each 50-mm-diameter hole may produce 4 to 8 L/s, and each 75-mm-diameter hole, 12 to 24 L/s. It may be worthwhile to utilize the pressure of tapped water to reduce the pump head. This can be done by piping water under pressure directly or through a settling tank, or kettle, to the pump suctions. If the concentration of sand and solids in the water is significant, then effective settling and regular cleaning of the pressurized settling tank can be a problem. grouting Grouting may be used to reduce the water flow into workings, but flow reduction is usually accompanied by a pressure buildup behind the grout, which can be dangerous. In drive and tunnel construction it is good practice to grout and then drill release holes after tunneling passes the area, arranging for the water to be drained toward sumps. Grouting can be used to • Control water in shaft sinking and tunneling, rarely stoping, in weak or fissured wet ground; • Reduce or stop flow past underground plugs and bulkheads; • Reduce leakage from reservoirs, especially under dams; • Consolidate and strengthen ground; • Plug a conduit through which work has been flooded; and • Make concrete for plugs and so forth with or without preplaced stone. Limitations are notable. The grout operator controls the nature of grout and the rate at which it is injected into the prepared hole. The operator can limit the pressure and, in some cases, influence the movement of water in the spaces to be filled. Generally, the operator’s picture of these voids and what happens in them is vague. After the grout has left the pipe into which it is pumped, it goes where it wants to, unseen. Although the existence of clay in voids interferes with cement grouting, appreciable water movement interferes with all kinds. Although new grout materials have increased the range of conditions in which grouting can be used, a degree of uncertainty persists. The usual procedure is to drill the ground through casing anchored sufficiently to withstand the pressure to be used, test permeability, and inject a grout that should be chosen and used in accordance with the conditions and objectives.

771

Conduits below the water table should be grouted before a heading reaches them. Thereafter, water movement is much more difficult to control. Deep holes usually are grouted in stages (i.e., the hole is drilled until a degree of permeability is found, then grouted). After the grout has set, the hole is drilled out, deepened, and again grouted. A method of stage grouting from the bottom up has been developed for grouting alluvium. Where walls are to be built, grouting should precede walling, if at all possible. If not, grout pressure must be kept as low as possible. Pressure against the ground must exceed hydrostatic; ordinarily, it is not allowed to exceed the calculated vertical stress. In special work it may be desirable to open fractures by pressure exceeding the in-situ stress. After grout pumping is begun, it usually is continued without interruption until the planned sealing-off pressure is reached. Dyes can be mixed with grout to tag various stages of injection. Grouting effectiveness is tested by drilling new holes between or near holes that have been grouted. The degree of confidence in the result depends partly on the uniformity of the ground and partly on the nature of subsequent work. Cement grouts do not enter the smallest fractures or pores finer than those of coarse sand. Prior treatment of the ground and admixture of sodium silicate and bentonite improve penetration, but, even with this help, pores of medium sand are a limit. Bentonite decreases strength but improves the pumpability of cement slurries and gives body. It also acts as a dispersant, reducing or preventing bleed, or separation of water. Neat cement slurries do not set unless the cement particles are brought together at a specific gravity of about 1.5. Setting time is reduced by use of high early-strength cement, by the addition of calcium chloride, and somewhat more by the use of special fast-set additives. High pressures and temperatures also shorten setting time. Sawdust, shredded plastic, and similar materials can be added to help plug large openings. As far as the delivery system and the size of the openings permit, fly ash, sand, and fine gravel can be added to reduce cost without sacrificing strength. Powdered aluminum reduces or counteracts shrinkage, but where the grout sets under high pressure its effectiveness is questioned. The water–cement ratio is important in controlling the behavior of cement slurries. In grouting deep holes that cut openings of various widths, the usual practice is to start with a thin slurry (e.g., 5:1 water to cement or even 10:1 by weight) in the expectation this will get into the smallest possible openings. Average slurry on one series of shaft pregrouts was 4:1, and water was reduced to 1:1 where possible. Cement slurries of about 0.5:1 can be pumped. Slurries can be made to stand under water at 20° to 30° from horizontal. Trial runs should be made with unusual mixtures. Acceptance of a large quantity of grout without pressure increase generally indicates that grout is running through a sizable conduit out of the ground that was intended to be grouted. Remedies may include thickening the slurry, adding bridging materials such as sand or chopped plastic, reducing the rate of pumping, and letting the hole stand for several hours. Where large quantities of cement grouts are to be used for an extended time, labor may be saved and slurry quality improved by mixing at a central plant with bins, weighing devices, water meters, agitators, and pumps. With pressure, slurry can be pumped through around 1.5 km of 25-to-50-mmdiameter pipe while the plant and the grout operator are connected by telephone. For short jobs, a batching plant and truck mixers are useful.

772


Reciprocating simplex or duplex grout-fitted slush pumps are usual. Compressed air drive is convenient because of its flexibility. For low pressure, centrifugal pumps can be used, which usually are fed by gravity from agitators.

In addition, the plan should be reviewed annually, when there is a change of scope in the mine, or following any major incident when the plan has been enacted.

ground freezing Ground freezing has been used extensively for groundwater control and excavation support in mining and underground construction since the late 19th century. The ground is frozen using coolant pipes to convert soil water into ice. The resulting frozen material is strong enough to allow a shaft to be sunk through deep saturated soils. The coolant, which flows through pipes installed in boreholes, can be brought to temperatures well below –150°C. Once the ground has been frozen, much less power is required to maintain it in a frozen condition.

Sumps are (1) accumulators to partially equalize the continuously changing rate of inflow and increase the regularity of pump operation, (2) reserve storage in which water can be held during short power interruptions and periods of suddenly increased inflow, and (3) settlers in which some suspended solids can be removed. Where pumping is substantial, the higher efficiency of separate settlers or clarifiers is advantageous.

Drainage System Design Mud and silt from blasthole drilling, fill decant water, road traffic in wet areas, and flow through caved or weak ground are problems in many mines, even where the natural permeability of rock is low. The drainage system must cope with the suspended solids load of the water. Drains alongside rail development at gradients of 0.5%–1.0% require frequent local sumps and associated ongoing cleaning, particularly if hydraulic fill is used. Rubber-tired equipment causes more mud to enter the drainage system and requires gradients of at least 2%, favoring a free-flowing system designed to minimize settlement before the main mine sump. Drainage should be via a dedicated system of side or center drains, boreholes, pipes, and designated storage areas. It should not be directed into old stopes, passes, or other workings unless a risk analysis has been conducted for the potential of an inrush. The benefits of controlling mud and water may include easier maintenance of track ballast or road surfaces; substantially improved tire life; neater, cleaner, and safer traveling ways; less water percolating to any lower work; and lower cost, less congestion, and less loss of time for cleaning. Pumping stations should be located to ensure sufficient emergency storage below the station, which gives time to carry out emergency pumping repairs at maximum expected flow rates. There should be a contingency plan for inrushes due to pump failure, including emergency evacuation procedures. Rising mains should be placed so that they can be regularly inspected to ensure no premature failures. The mine drainage system should be recorded on a suitable plan, such as a long section or isometric plan of the mine, so that it can be understood as a system. Contingency Planning Each site should have a contingency plan, which is enacted when a lead indicator exceeds its threshold value. The contingency plan, which is the responsibility of the mine or project manager, is a response plan that defines • Levels in the lead indicator that would trigger escalating responses, including possible damage reduction; • Those responses, including evacuation procedures; • Ongoing monitoring requirements; and • Types and sources of emergency support available.

ClARifiCATion AnD SeTTling SySTeMS

Clarification Removal of suspended solids from mine water may allow the use of more efficient clean water pumps, which is especially important in deep mines with large volumes of water to be removed. All clarification systems utilize gravity, sometimes assisted by inertial and/or mechanical means, to separate the solids from the liquid. Neutralization of acidic water can also be done at the settler. The most commonly used settling systems are described in the following paragraphs. Horizontal Sumps

Long development openings are used with horizontal sumps, relying on laminar flow and retention time to allow the solids to settle to the bottom, with clean water discharging over a wall at the end or into a launder (Figure 9.6-4). The sumps must be periodically cleaned, so multiple sumps in parallel are the usual layout, with one sump cleaned at a time, either by pumping out as a slurry or removal by a loader. Baffles have been used to control velocities and improve settling, but their value is questionable and cleaning is more difficult. The main advantage of these sumps is their simplicity and the absence of any mechanical devices. They are essentially labor free, except for cleaning. Their main disadvantage is low efficiency, requiring large excavation volumes, which makes them unsuitable for high water flows. To enable cleaning with mobile equipment, often the wall is destroyed and then rebuilt. Vertical Sumps

Vertical sumps also rely on gravity and retention time to settle out the solids. They are circular in shape, sometimes with a cone at the bottom to facilitate mud removal (Figure 9.6-5). Their height is typically the distance between two sublevels. The dirty water flows in via a central pipe, which discharges at about one-third of the cylinder’s depth, and clean water rises for removal via a ring launder around the top. Flocculants may be added to improve the settling characteristics of fine solids. Mud collects at the bottom and is removed at regular intervals, allowing continuous operation. These settlers are more compact than horizontal sumps, are much easier to clean, and do not have to be removed from service for cleaning. The large-diameter (5–10 m) vertical openings require reasonable ground conditions and are typically lined with Fibercrete or concrete. Multiple units can be accommodated in close proximity to the pumping station, and the settled mud can be gravity fed to the mud removal system.


Pickup Launders

Water Level

773

Feed

Local Flow Region

Fine Secondary Sludge

1

Coarse Primary Sludge Buildup

3

Source: Adapted from Vutukuri and Singh 1993.

10

figure 9.6-4 horizontal settler

4

Delivery Launder

Overflow Lip

5

2

Vi

Stilling Box

9 6

Vf 7

7,300 mm

8

Raw Water Channel

1. Feed Launder 2. Floc Bed 3. Control Valves 4. Feed Pipes—38 mm diameter 5. Brattice Wall 6. Quiescent Portion of the Settler 7. Mud Bung 8. Mud Drain Column 9. Bung Valve 10. Clear Water Overflow Launder Vi. Influent Velocity Vf. Free Settling Velocity of the Flocculated Particles

Source: Adapted from Atkinson 1982.

figure 9.6-6 Cross section of double-vee settler Clear Water Column

Sludge Drawoff

Source: Adapted from Hunter and Emere 1977.

figure 9.6-5 Cylindroconical settler Vee Settlers

Vee settlers evolved to maximize the benefit from addition of flocculants to achieve high throughputs in relatively small excavations. The settler has three internal compartments— two for initial settling and one for mud storage and thickening (Figure 9.6-6). The dirty water is introduced under a floc bed in a V-shaped compartment. The clear water rises and is removed via a launder, while the finer solids are captured by the floc bed, which overflows into the central compartment where it thickens for eventual removal. Coarser solids settle in the vee compartments and are periodically released into the central compartment via drain cocks, which may be automatically operated. The sludge in the central compartment thickens and is drawn off for separate disposal.

Maintenance of a stable floc bed is the key to successful performance of these settlers. This is facilitated by dividing the long settlers into shorter compartments. The overall capacity can be adjusted by varying the number of compartments in operation. Lamella Plate Settlers

In lamella plate settlers, the dirty water flows across a stack of parallel inclined plates. As the particles settle onto the plates, they slide down and out of the stream. Because the vertical distance between the plates is small, even small particles can settle out relatively quickly, provided the flow velocity is low. These devices are more commonly used in processing operations where space is at a premium. Mud-handling Systems Thickened solids from settlers can be removed from the mine in a variety of ways. This material (sludge) often contains valuable minerals, particularly in a gold mine with coarse free gold, and should be delivered into the processing stream. Typical removal systems include • Mixing with other rock: The sludge can simply be mixed with either ore or waste (depending on its value) and removed by the normal materials-handling system. This can be quite messy but may be satisfactory if the quantity of material is small. • Filtering: The sludge may be dewatered to produce a moist cake by a filter press or vacuum filter for removal by the normal materials-handling system or separately, if desired. • Injection: The sludge may be injected into the rising main, such that the clear water pumps remove the sludge

774


Header 7

Surface AL - 345

11 Level AL - 195

11

12

3 17 Level AL - 220

Settler

Mud Rising Main

Clear Water Rising Main

2

1 6

13

5

4

9 10

1. 2. 3. 8 4. 5. 6. 7. 8. 9. 18 Level 10. AL - 255 11. 12. 13.

Displacement Chamber Gate Valve Gate Valve Mud Charge Pump Mud Dam Clear Water Dams Header Dam Mud Rising Dam Nonreturn Valve Nonreturn Valve Small-Stage Pump Clear Water Rising Main Clear Water Pumps

Source: Adapted from Atkinson 1982.

figure 9.6-7 Mud displacement system at Mount lyell, Tasmania, Australia

without being damaged by it. The velocity in the rising main must be sufficient to prevent settling of the coarsest particles. • Pumping: If the solids are sufficiently fine, they may be pumped out by suitable slurry pumps. • Hydraulic displacement: This method uses the head of a column of clear water, supplemented by some additional pump pressure if required, to displace the thickened slurry to the surface. The system was developed in South Africa and is used at Mount Lyell (Atkinson 1982), where the discharge tunnel is sufficiently lower than the shaft collar to eliminate the need for pumping, except for the flushing cycle (Figure 9.6-7).

PuMPing SySTeMS

The design of pumping systems of even moderate capacity presents complications that require specialists but should be guided by those who are fully acquainted with broader plans and objectives of the work. The need to pump any inflow of water with complete reliability means at least one capable pumping unit plus one equal spare. In a three-unit system, any two should be able to pump the maximum inflow. The greater the number of units, the less the percentage of spare capacity, but beyond

some point a larger number of smaller units will cost more and will have lower efficiency. Where only brief pumping interruptions can be permitted, duplicates of all essential parts must be provided, including power supply, pipes, and valves. Positive displacement pumps are capable of handling dirty water and can greatly simplify the underground handling of water before it is pumped. In general, pumping for shaftaccess rail mines is optimized with centrifugal pumps. The steeper gradients and greater volume of mud generated by mobile equipment in trackless mines favor the use of positive displacement dirty water pumps. High-speed centrifugal pumps are capable of high efficiency provided close tolerances are maintained. They are vulnerable to wear by grit and essentially are clear water pumps. Therefore, means to remove suspended solids before pumping are an important consideration. Unlike most positive displacement pumps, centrifugal pumps can be rotated without fluid flow. Because the power demand is least when no water is pumped, large pumps generally are started with discharge valves closed. Flow also is limited or stopped if (1) discharge pressure is not sufficient to force water into discharge pipe (as from extreme impeller wear), (2) passages anywhere in the pump system are plugged, or (3) pressure in the suction is insufficient. Cavitation develops in the intake of impellers if pressure there drops below the vapor pressure of the fluid so that the fluid boils. The formation of bubbles reduces the capacity and efficiency of the pump, and their collapse damages it. To determine the potential for cavitation, it is necessary to calculate the difference between the total head on the suction side of the pump (close to the impeller) and the liquid vapor pressure at the operating temperature. The net positive suction head (NPSH) can be determined from NPSH = ps/γ + v2/2g – pv /γ where ps = static pressure in the fluid close to the impeller γ = specific weight of the fluid v = velocity of fluid g = acceleration due to gravity pv = vapor pressure The pump manufacturer will provide the required NPSH for a particular pump based on test work. The system should be designed so that the available NPSH is significantly higher than the required NPSH to allow for head losses in the suction pipe and in the pump casing, for local velocity accelerations, and for pressure variations. Typical relations of suction head, total dynamic head, and specific speed (revolutions per minute) for single- and doubleentry single-stage centrifugal pumps handling clear water at sea level are shown in Figure 9.6-8A. Figure 9.6-8B shows the various power losses typical of centrifugal pumps in relation to specific speed. The affinity laws for centrifugal pumps can be used to relate flow rate, head (pressure), and power consumption to changes in rotational speed or impeller diameter. The flow rate relationship is q1/q2 = (n1/n2)(d1/d2)


A. Relation of total dynamic head to specific speed at various suction heads or lifts

775

B. Typical losses in centrifugal pumps

4,000

3,500

3,000

2,500

1,600 1,800 2,000

1,400

100

Upper limit of specific speed for single-stage pumps, with clear water at sea level at 85°F.

700

2 3

95

5

4

600

5

10

Power, % Normal Input

500

ad He on ft i L cti Su on -ft cti Lift 15 Su t t f on ft 0-f t 5 cti -ft 51 55-f 1 Su -ft t 20 0-f

400 300

200

150

100 90 80

5a 10

90 5b 85

15

80

20

75

25

70 0

25

1

2

3

4

5

6

30

Specific Speed × 1,000

-ft Su

70

cti

1. 2. 3. 4. 5. 5a. 5b. 6.

on

60

Lift

H = Total Dynamic Head, ft

0

1

Losses, Percent of Brake Horsepower

900 800

1,200

1,000

Specific Speed for Single-Suction Pumps

50 40 30

Mechanical Losses 1% Impeller Losses 2.25% Disc Friction Losses Leakage Losses Casing Hydraulic Losses Casing Losses Vertical Pumps Losses Due to Suction Approach Double Suction Pump Output

20

Imperial GPM

7,000

6,000

5,000

4,000

H¾ 3,000

1,200

10

1,400 1,600 1,800 2,000

RPM Specific speed =

Specific Speed for Double-Suction Pumps

figure 9.6-8 Suction and losses, single-stage centrifugal pumps

where

q = volume flow rate, m3/s n = impeller velocity, rpm d = impeller diameter

The head or pressure relationship is p1/p2 = (n1/n2)2 (d1/d2)2

where p is the discharge head or pressure (m). The power relationship is P1/P2 = (n1/n2)3 (d1/d2)3 where P is power (kW). In other words, the capacity or rate of discharge varies with peripheral speed at exit of impeller, the maximum discharge pressure varies with the square of peripheral speed, and power input varies with cube of peripheral speed.

776


Design of Pump Stations Objectives for designing a pump station usually include the following: • Enough units of all essential components should be provided so that any one can be repaired while others handle greatest expected flow. Ordinarily, this requires spare units with appropriate valves and crossovers and a crane capable of moving the heaviest. • Pumps and all electrical equipment should be protected from being flooded. • Sufficient storage should be provided in sumps or otherwise. • Vertical or submersible pumps should be used to take water from sumps. • In severe conditions, the pump room should be protected with an adequate pressure door and a raise for access and ventilation. • Capacity should be added if needed. • Sufficient controls, usually at least automatic start-andstop, alarms for high water, and protection for pumps and motors, should be provided. • Positive suction head should be provided, if reasonably possible, by pumping from sump with vertical pumps or locating pump room lower than sump. • High-tension items should be protected from accidental jets and splashes. Floor should be sloped and a drain provided. • Ventilation should be used, as needed, for normal and emergency operation, discharging hot air to the mine or returning it to the surface or, if both are objectionable, cooling it mechanically, adding to the temperature of water discharged. System Design The first decision required in the design of a pumping system is whether to pump dirty water or to remove the solids to enable the use of clear water pumps. This will depend on a number of factors, including the volume of water to be pumped, the properties of the water, the depth of the workings, the expected life of the mine, and demands and constraints on disposal and/or reuse of the water. The second decision is to consider whether to pump in stages or in a single lift from the bottom of the mine. Some of the considerations include whether the water will be clarified or not before pumping, the depth and vertical and lateral extent of the workings, the expected life of the mine, the type of access available (shaft and/or decline), whether the overall mining sequence is top down or bottom up, and whether development of the mine is progressive or largely completed prior to production. Clearly, these two decisions are interdependent, and the overall system selection will consider all of these factors. Some general principles are listed here: • Deep mines with large water volumes and long life will favor clarification and clear water pumping (high capital cost but lower operating cost). • Shallow mines with low water volumes and shorter life will favor dirty water pumping. • A top-down mining sequence with progressive mine development will favor a staged pumping system. • A mine with large vertical and lateral extents will favor staged pumping.

figure 9.6-9 Crankshaft-driven piston diaphragm pump

• A shallow mine with limited vertical extent will favor single-lift pumping. Pump Selection Some of the types of pumps used in underground mining are described in this section. Small Portable or Semiportable Pumps

Sump pumps may be powered by compressed air or electricity. They are usually vertical centrifugal pumps with open impellers and abrasion resistance or, less commonly, diaphragm and displacement pumps. They are moved to and from sumps where they normally work submerged but can run dry without damage and must be self-priming. Compressed air pumps are limited to heads of about 100 m at flows less than 1 L/s but can deliver up to 10 L/s at low heads. Electric pumps exceed these limits and can be used to clear small inflows from shaft bottoms, clean sumps, and so forth, where high portability is desirable. Reciprocating Positive Displacement Pumps

These include dirty water pumps, mud pumps, slurry pumps, and grout pumps powered by compressed air or electric motors through belts or gears. Plunger pumps and diaphragm pumps have high abrasion resistance and low to moderate capacity but are capable of high discharge pressure. Valves are accessible for cleaning. Some positive displacement (PD) pumps displace the fluid directly, either via a piston working in a cylinder or a plunger working through a stuffing box. A supply of clear, high-pressure water may be required to lubricate glands and minimize wear. A diaphragm pump has a flexible diaphragm between the fluid being pumped and a chamber containing oil that is displaced by a piston, thus preventing any wear on the piston side (Figure 9.6-9). In large sizes, PD pumps are used together in main pump stations to deliver 100 L/s or more of dirty water directly to the surface from depths exceeding 1,000 m. Single-Stage Horizontally Split Centrifugal Pumps

These are usually directly connected to electric motors for compactness, dependability, and ease of control. Discharge sizes are up to about 300 mm. Generally furnished with close clearances for clear water, these pumps can work with high efficiency to capacities of about 200 L/s and heads to about 150 m. Pumps are made for larger capacity at somewhat lower


Discharge Rotor

777

Stator

Suction

Courtesy of Sundyne Corporation © 2010.

figure 9.6-11 Progressive cavity pump

heads. Installation and maintenance are simpler than for multistage pumps.

Vertical centrifugal pumps are made for capacities up to 950 L/s and heads to 250 m with motors as large as 1,000 kW. They are commonly used to raise water from a lower to an upper sump, from which it can flow with positive suction head to horizontal centrifugals. They also can pump to emergency storage.

figure 9.6-10 Multistage centrifugal pump

Horizontal Multistage Centrifugal Pumps

These are usually direct-connected to electric motors for compactness, dependability, and ease of control (Figure 9.6-10). Discharge sizes are from 75 to 250 mm with 2 to 10 stages. Although total efficiency is likely to be less than that of singlestage units, efficiency per stage may be higher. Heads typically range from about 150 to 700 m. Although the first cost of these pumps generally is higher than for single-stage pumps of similar power, their use may save the capital and operating costs of duplicate facilities. The close tolerances require clean water to prevent excessive wear, so the cost of building and maintaining settling dams is a consideration. Vertical Turbine (Deep Well) Pumps

These are essentially vertical centrifugal pumps made in comparatively small diameters to work in water wells and similar narrow but high spaces. Water ends are essentially similar whether close coupled to a vertical electric motor, connected by a long shaft to a motor at the top of a well casing, or connected to a submerged motor in the well. Vertical turbines are made with as many as 20 or more bowls (stages) for heads of 5 to 30 m per stage. Pumps are made in diameters from about 150 to 1,000 mm or more. Because pump intakes normally are submerged, priming is not a problem. Motors of close-coupled pumps can be well above normal water levels. Impellers can be removed if a pump is to work at less head, or more bowls and impellers can be added for greater head. Vertical Centrifugal Pumps

These are made in one and two stages with proportions like those of horizontal centrifugals. They are direct-connected below vertical motors by short shafts. Because the pump always works submerged, priming is not a problem. They have the simplicity and high capacity of horizontal centrifugals and in many applications approach or equal the convenience of vertical turbines.

Air Lifts

The efficiency of an air lift is low at best, yet there is no simpler pumping system. Typically two strings of concentric pipe are submerged, and compressed air is introduced through the internal pipe. Anything that can get into and through the larger pipe is pumped without doing damage. They are especially good in dewatering partially blocked shafts but require a minimum submergence of around 33% and so cannot completely dewater a shaft. Progressive Cavity Pumps

With abrasion-resistant rotors in rubber stators, progressive cavity pumps will pump any mud that can be drawn into them. They are widely used in mine dewatering (Figure 9.6-11). Multistage units are capable of heads up to 720 m at up to 60 L/s, or flows up to 150 L/s at lower heads. Testing Performance of Pumps Well-equipped manufacturers have extensive facilities for testing large pumps with precision. Comparable test facilities are uncommon in deep mines, yet records of performance are essential to evaluate the results of pumping and to guide maintenance. Modern pump stations are often well instrumented, but serviceable data can be obtained by such means as the following: • The total output by a weir on the surface can be measured, integrated, and recorded. In some cases, it is possible to measure rate by pumping into or out of a large sump or tank but usually with less accuracy. Inflow can be estimated by adding the water removed with ventilating air and subtracting any water piped into the mine for drilling and washing, and decant water from hydraulic fill. • The pumping rate of each pump can be measured by any of several means, including magnetic flowmeters,

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venturis, orifices, weirs, or pumping from or into a sump. Provision of these measuring devices in the discharge of each pump makes it possible to check each periodically with little labor and loss of pumping time. • Individual discharge and suction heads can be measured with calibrated Bourdon-type gauges or manometers placed appropriately. • Mechanical input can be determined from a calibrated watt-hour meter, subtracting the motor loss as taken from the manufacturer’s test data. Various torque dynamometers can be used for a more fundamental determination of power input. • Pump speed can be measured by a calibrated tachometer and stopwatch, and this is generally adequate. The rate of discharge of a centrifugal pump and its discharge pressure can decrease with time because of scale formed in the discharge or other change increasing the dynamic head; decrease in suction head due to scale, obstruction of intakes, and so forth; decrease in effective diameter of the impeller or other severe pump wear; blocking or other obstruction of the impeller passages; and wear causing excessive leakage between stages of multistage pumps and across wear rings in single-stage. All pumps discharging into a single line should be kept balanced to operate together efficiently. If unbalanced, the head of one pump could decrease until flow is reduced to the degree that the pump bearings seize. Cost of Pumps Usually other factors are likely to be more significant than the first cost of the pump. The cost of standard water ends of horizontal centrifugal mine pumps generally is 30% to 50% of the total cost of the pump-motor-starter unit. It may be only 5% to 15% of the cost of the pumping plant, including sumps, clarification, power supply, ventilation, suction, discharge, and required excavations. The construction of the pump influences the cost of the station (e.g., vertical pumps require less floor space and no suction line). Efficiency is important because power ordinarily is the largest part of operating cost. Where planning makes it practicable, a single high-head pumping installation can be more efficient, as well as simpler, than a series of pumping stations each working at a lower head. Initial duty calculations can be made assuming efficiency of 60% (motor # pump) for small units, 75% for larger pumps, and up to 80% for units of large capacity. Large units, in which both motor and pump are excellently matched to the pumping duty, may go higher (e.g., 90% pump # 93% motor). Pipework Normal working pressures may be increased considerably by the pulsations of reciprocating pumps or by surging and water hammer. Special steels or extra wall thickness may compensate for pressure or corrosion but with added cost or weight. Where significant ground movement is likely, the discharge pipe should be able to survive some misalignment. If it is likely that individual lengths or sections of a discharge line must be removed to clean scale or repair accidental damage, pipe should be coupled and supported accordingly. In some places, repair by cutting and welding is difficult. In some cases, reasons exist for not treating water underground, and protection of the outside of the steel pipe is likely to be incomplete. The angle of the pipe, its exposure to accidental

damage, working room, and service facilities also are factors in selection of pipe and couplings and how it is supported. Because many of these factors are difficult to evaluate, continuous dependable service is important, and repair can be highly inconvenient, innovation in high-pressure design pipe is uncommon. Preference for seamless steel pipe for high-pressure lines has decreased with better quality control in welding. Highpressure lines usually are no larger than 250 mm (10 in.) to limit the weight of long, steep columns, facilitate placement and repair in limited shaft compartments, reduce the number of pumps discharging to the same line, and make it easier to provide a spare line. Wall thickness needed for any pressure is calculated from the minimum ultimate tensile stress with a good factor of safety; 5 is frequently used in shaft rising mains, whereas 2 may be acceptable in more accessible areas. There is much more latitude in choice of pipe for lower pressures, such as discharge from single-stage pumps. Where volumes are large, 500-mm (20-in.) pipe may be used. On high-pressure lines, the practice is to join pipe with flanges. Simple flanges generally are preferred, but gasket material must resist extrusion by strength or by retention. These long strings generally are assembled from the bottom up by adding one or several lengths. With the use of a cage with extensible crawls to handle the pipe and decks where crews work, more than 100 m of pipe can be placed in a shift. Weight usually is carried on steel bearer beams or brackets concreted into the shaft wall at intervals of 120 m or less. The provision of an expansion joint below each bearer assists in equalizing weight, adds vertical flexibility, and makes it unnecessary to raise an entire string to replace a length. Continuous rising mains can be added in 20-m lengths, with each joint welded as it is added at the surface like well casing. Alternatively, screwed well casing may be used. Guides prevent side movement, but all weight bears either on a concrete bridge at the elevation of the pumps with the line in compression or is suspended from a bridge at the surface with the line in tension. Gate and check valves are placed in the discharge of each centrifugal pump for use in starting and for protection during the repair of a pump connected to an active discharge line.

BulkheADS AnD PlugS

Deep gold mines in South Africa have developed outstanding practice with plugs, demonstrated in tests and in both routine and emergency construction. Generally, these plugs were built in hard, strong quartzite, impermeable except on fractures. Culmination of this development came with the four plugs placed in emergency conditions in the West Driefontein mine in November 1968 at depths below 1,000 m. They were completed 20 days after the inrush, in 3.0 # 3.7 m crosscuts. Work was undertaken with water flowing more than 1 m deep on each level. Valves to stop bypassing water were closed on the 23rd day, and the lower plug withstood a head of 1,116 m of water on the 26th day (Cousens and Garrett 1969). Several conclusions are reached from South African practice (Garrett and Campbell Pitt 1961). Generally, it is more difficult to stop leakage past a bulkhead than to make it strong enough to resist thrust. Passage of water through rock fractures is related to the pressure gradient, which should be moderate. Many plugs are neither hitched, tapered, nor reinforced. Reliance is placed in the strength of the concrete bearing against the usual irregularities of the rock surface. Such


plugs have withstood pressures of more than 6.9 MPa, one at 15.5 MPa. No indication of structural failure resulting from thrust was noted in the examples reported. Leakage is likely along the floor and roof, even at low pressure, where mud and honeycomb, laitance (a weak layer due to excess water), and air pockets commonly weaken the rock–concrete contact. These leakages sometimes are sealed acceptably by one stage of grouting. At higher pressures, water is likely to break through rock fractures. This appears to result from rock movement induced by pressure on rock surfaces. Part of the water entering the fractures may not appear outside the bulkhead. Much of this leakage can be sealed by several stages of cement grouting at pressures to at least 2.5# the hydrostatic head, in holes drilled as far as 10 m into the rock. The effect of each stage of grouting seems to be to fill the fractures, perhaps poststressing the ground around the bulkhead, and increasing its resistance to the entrance of water. The loss of several bulkheads subjected to more than 6.9 MPa is attributed to failure of gaskets, threaded plugs, and other fittings. The possibility that even the smallest leakage through fractures may be enlarged by high-pressure erosion should not be underestimated. Preferred construction is by injecting cement–sand grout into clean, strong, angular rock previously packed between timber forms. This generally results in better concrete, easier logistics, and, in some circumstances, less time and cost than direct concrete placement. Ordinary portland cement usually is used but rapid-set cement can be required by urgency. Concrete should reach at least 17.2 MPa in 28 days. The four plugs at West Driefontein were made of cement–sand slurry only. Slurry can be mixed under good control at a central plant and pumped more than 1,000 m through small pipe. Horizontal cold joints should be avoided by all means. Recommendations from this work include seeking sites in tight sound rock. In good ground, at least, keyways are unnecessary, but note the length next recommended. The plug should be long enough that the pressure gradient is moderate. In one test, a gradient of 9 MPa/m was reached after several stages of rock grouting, but for working bulkheads, designed gradients of 900 to 1,400 kPa/m have proved effective (i.e., for each 6.9 MPa, allow between 8 and 12 m of plug). All mud and loose rock should be removed, the water flow across the floor should be stopped, and a ventilation hole provided for high spots in the roof. Pipe, gaskets, valves, and fittings should be tested at a pressure somewhat greater than that to be withstood. Several stages of grouting to reduce leaks should be planned. The first, through pipes at concrete– rock contacts, might be at around 2,000 KPa, but later stages through holes drilled successively deeper into the rock can be at successively higher pressures, up to at least 2.5# the expected hydrostatic head. Comparable experience in other rock is unknown, but several inferences seem to deserve consideration. In any strong rock that can be grouted effectively, similar practice seems applicable. In weaker rock, however, or one with seams that do not take grout well, it appears prudent to work to lower pressures and lower pressure gradients to reduce the risk of uncontrollable leakage through erosion and enlargement of rock permeability or defects. No large excavation should be subjected to pressure greater than the maximum hydrostatic head unless after

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full study and evaluation of rock stresses. Lastly, the use of expanding cement, using aluminum powder or similar, seems promising.

inRuSheS

An inrush is a low-probability but high-impact event, usually leading to major disruptions to production and multiple fatalities. For example, Vutukuri and Singh (1993) tabulate 33 mine inundations in England, Scotland, India, and the United States having an average of 31 and a maximum of 375 fatalities (at Chasnala colliery, Jharia, India, in 1975). One definition of an inrush is “the uncontrolled mass movement of material,” whereas subsidence is “a sudden collapse of a large volume of rock,” often referred to as plug or pipe subsidence (McCarthy and Harvey 1998). These definitions recognize that flows of mud, tailings, wet ore, or saturated surface material may also become inrushes, and that sudden subsidence, although a hazard in itself, can create unexpected connections to these materials. Causes of inrushes A review of 43 examples of inrushes, mainly in noncoal mines, shows the following causes (McCarthy and Harvey 1998); a further 18 examples gave insufficient details for classification: • • • • • •

Surface flooding enters mine: 33% Mining broke into old workings: 28% Strata water enters mine: 19% Accidental connection made with sea, river, lake, etc.: 14% Failure of a dam, seal, borehole, etc.: 5% Earthquake: 1%

It is interesting to compare this list with the next, which was prepared by Job (1987a, 1987b) based on 208 incidents in British collieries during the period 1851 to 1970. The much greater frequency of contact with abandoned workings is to be expected in intensively exploited bedded deposits. The lack of surface flooding may reflect a greater predictability and awareness of flood levels than has been the case in arid environments. This list does not include fill bulkhead failures, which are common but rarely become public knowledge unless a fatality is involved: • Contact with abandoned old workings: 78% • Clearing old shafts or shaft sinking: 9% • Failure of an underground dam or seal, or leakage of a borehole: 4% • Contact with surface water—pond, river, canal, or stream: 4% • Contact with surface unconsolidated deposits—glacial or organic: 4% • Strata water entering working: 1% frequency and location of inrushes A survey of mining periodicals for the years 1980–1996 identified 33 inrush incidents involving fatalities or loss of production. This is a rate of about two incidents per year worldwide, not including unreported incidents in China and the former USSR. These locations are for inrushes from 1980 to 1996: • • • •

Africa: 9 North America: 6 Southeast Asia: 5 Australasia: 5

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• Europe: 5 • India: 2 • South America: 1 The occurrence of inrushes seems to generally correlate with the level of underground mining activity. Without new preventive measures, a country such as Australia should experience one incident every 3.4 years. As about 70 significant underground mines exist in Australia, the return period for an inrush incident is 238 years, which means that personal experience and awareness of the hazard are likely to be low. The nature of low-probability, high-impact events means that a severe incident could happen at any time and that special management procedures are required to maintain awareness. More recent instances, particularly in coal mines in North America and China, suggest that this problem remains a significant one, is a serious threat to employee safety, and has a serious impact on the public perception of the mining industry.

SuRfACe WATeR DiSPoSAl

All of the water and associated sediments removed from the mine must be disposed on the surface or recycled. Some water is disposed into the atmosphere via the exhaust ventilation and evaporation from wet rock dumps and water storages. In many mines, the water from underground is an important source for processing operations and other surface operations. Today, even if the mine water is of better quality than the water in local streams, direct discharge into surface waterways is often not permitted or requires extensive and expensive purification. The presence of heavy metals or acidity will exacerbate this situation. Mines with superfluous water may construct evaporation ponds for water disposal. This is not particularly effective in cold climates with low evaporation rates. In some cases, properly treated mine water can be an important water source for the surrounding community. Any water discharged into the natural environment should meet internationally accepted water quality standards.

ACknoWleDgMenTS

This chapter was developed from “Ground Water and GroundWater Control” by R.L. Loofbourow, which appeared in the first edition of this handbook (Lucas and Adler 1973). The authors acknowledge the many contributors to that chapter.

RefeRenCeS

Atkinson, B.F. 1982. Mine dewatering at Mt. Lyell, the 1980s and beyond. In Underground Operators’ Conference. Melbourne, Australia: Australasian Institute of Mining and Metallurgy. Cousens, R.R.M., and Garrett, W.S. 1969. The flooding at the West Driefontein mine. J. S. Afr. Inst. Min. Metall. April: 421–463. Garrett, W.S., and Campbell Pitt, L.T. 1961. Design and construction of underground bulkheads and water barriers. In Transactions, 7th Commonwealth Mining and Metallurgical Congress. South African Institute of Mining and Metallurgy. pp. 1283–1299. Hunter, E.C., and Emere, G.T.C. 1977. The use of cylindroconical settlers for the clarification of underground water. J. S. Afr. Inst. Min. Metall. May. Job, B. 1987a. Inrushes at British collieries: 1851 to 1970 (Part 1). Colliery Guardian 235(5):192–199. Job, B. 1987b. Inrushes at British collieries: 1851 to 1970 (Part 2). Colliery Guardian 235(6):232–235. Larock, B.E., Jeppson, R.W., and Watters, G.Z. 1999. Hydraulics of Pipeline Systems. Boca Raton, FL: CRC Press. Lucas, J.R., and Adler, L. 1973. SME Mining Engineering Handbook, Vol. 2. New York: SME-AIME. McCarthy, P.L., and Harvey, S. 1998. Inrushes and subsidence. In Underground Operators’ Conference. Townsville, Queensland, Australia: Australasian Institute of Mining and Metallurgy. Theis, C.V. 1935. The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using ground-water storage. Trans. Am. Geophys. Union 16:519–524. Vutukuri, V.S., and Singh, R.N. 1993. Recent developments in pumping systems in underground metalliferous mines. Mine Water Environ. 12:71–94.

SME Mining Engineering Handbook 3ed Deawatering - Darling Peter

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