Introduction to
Soil Stabilization
Understanding the Basics Basics of of Soil Stabilization: An Overview of Materials and Techniques Techniques
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Table of Contents Introduction to Soil Stabilization ............ ......................... .......................... .......................... .......................... .......................... ................3 ...3 Brief Histor History y ............ ......................... .......................... .......................... .......................... .......................... .......................... .......................... ........................3 ...........3 Document Docume nt Purpos Purpose e ............. .......................... .......................... ......................... ......................... .......................... .......................... .........................3 ............3 Defining Soil Stabilization ........... ........................ .......................... .......................... .......................... .......................... .......................... ................4 ...4 What is Soil Stabil Stabilization ization? ? ........... ........................ .......................... .......................... .......................... .......................... .......................... ................4 ...4 Why and when is it used? ............................................................................................5 Applications ..................................................................................................................6 About Soil ............ ......................... .......................... .......................... .......................... .......................... ......................... ......................... .......................... ................7 ...7 What kind of soil is that, anywa anyway? y? ............ ......................... .......................... .......................... .......................... .......................... ................7 ...7 Standards help identify soil types ................................................................................7 USDA ............ ......................... ......................... ......................... .......................... .......................... .......................... .......................... .......................... ..................8 .....8 USCS ............ ......................... ......................... ......................... .......................... .......................... .......................... .......................... .......................... ..................8 .....8 AASHTO.................... AASHT O................................. .......................... .......................... .......................... .......................... .......................... .......................... ..................9 .....9 Chemical Soil Stabilization ........... ........................ .......................... .......................... .......................... .......................... .......................... ..............10 .10 Additives Additiv es ............ ......................... .......................... ......................... ......................... .......................... .......................... .......................... .......................... ................1 ...11 1 Portland Portlan d Cement ............. .......................... ......................... ......................... .......................... .......................... .......................... .........................1 ............11 1 Quicklime/Hy Quickl ime/Hydrated drated Lime ............. .......................... .......................... .......................... .......................... .......................... .........................1 ............12 2 Fly Ash ......................................................................................................................12 Calcium Calci um Chlori Chloride de ............. .......................... ......................... ......................... .......................... .......................... .......................... .........................1 ............12 2 Bitumen ............ ......................... .......................... .......................... .......................... ......................... ......................... .......................... .......................... ..............12 .12 Chemical and Bio Remediation ................................................................................13 Mechanical Soil Stabilization ............ ......................... .......................... ......................... ......................... .......................... .......................14 ..........14 Compaction Compac tion ............ ......................... .......................... .......................... .......................... .......................... ......................... ......................... .....................14 ........14 Soil Reinfor Reinforcement..................... cement.................................. .......................... .......................... .......................... .......................... .......................... ..............14 .14 Addition of Graded Aggregate Materials ..................................................................15 Mechanical Remediation ..........................................................................................15 The Basic Soil Stabilization Process ............ ......................... .......................... .......................... .......................... .......................16 ..........16 Assessment and Testing ..........................................................................................16 Site Preparation ........................................................................................................16 Introduce Introduc e Additi Additives ves............. .......................... .......................... .......................... ......................... ......................... .......................... .......................17 ..........17 Mixing ............ ......................... ......................... ......................... .......................... .......................... .......................... .......................... .......................... ..................17 .....17 Compaction and Shaping/Trimming ........... ...................... ...................... ...................... ....................... ....................... ..................17 .......17 Curing............................. Curing................ .......................... .......................... .......................... .......................... ......................... ......................... .......................... ..............17 .17 Glossary ............ ......................... .......................... ......................... ......................... .......................... .......................... .......................... .......................... ..................18-21 .....18-21 Appendix ............ ......................... .......................... ......................... ......................... .......................... .......................... .......................... .......................... ..................22 .....22 Acknowledgements/References Acknowledgements/R eferences ............ ......................... .......................... ......................... ......................... .......................... ...................23 ......23 2
Introduction
Introduction to Soil Stabilization A land-based structure of any type is only as strong as its foundation. For that reason, soil is a critical element influencing the success of a construction project. Soil is either part of the foundation or one of the raw materials used in the construction process. Therefore, understanding the engineering properties of soil is crucial to obtain strength and economic permanence. Soil stabilization is the process of maximizing the suitability of soil for a given construction purpose.
Brief History The necessity of improving the engineering properties of soil has been recognized for as long as construction has existed. Many ancient cultures, including the Chinese, Romans, and Incas, utilized various techniques to improve soil stability, some of which were so effective that many of the buildings and roadways they constructed still exist today. Some are still in use. In the United States, the modern era of soil stabilization began during the 1960s and ’70s, when general shortages of aggregates and petroleum resources forced engineers to consider alternatives to the conventional technique of replacing poor soils at building sites with shipped-in aggregates that possessed more favorable engineering characteristics. Soil stabilization then fell out of favor, mainly due to faulty application techniques and misunderstanding. More recently, soil stabilization has once again become a popular trend as global demand for raw materials, fuel, and infrastructure has increased. This time, however, soil stabilization is benefiting from better research, materials and equipment.
Document Purpose The purpose of this manual is to provide a general overview of soil stabilization practices used in the construction and maintenance of structures designed for supporting motor vehicle use. It is not meant as a guidebook or to provide application advice. Only a qualified geotechnical engineer can make recommendations on the techniques and materials required for suitable sub-base design.
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Defining Soil Stabilization
ABOVE: Rotary mixer mixing dry cement into the soil
Defining Soil Stabilization What is Soil Stabilization? Soil is one of nature’s most abundant construction materials. Almost all construction is built with or upon soil. When unsuitable construction conditions are encountered, a contractor has four options: (1) (2) (3) (4)
Find a new construction site Redesign the structure so it can be constructed on the poor soil Remove the poor soil and replace it with good soil Improve the engineering properties of the site soils
In general, Options 1 and 2 tend to be impractical today, while in the past, Option 3 has been the most commonly used method. However, due to improvement in technology coupled with increased transportation costs, Option 4 is being used more often today and is expected to dramatically increase in the future. Improving an on-site (in situ) soil’s engineering properties is referred to as either “soil modification” or “soil stabilization.” The term “modification” implies a minor change in the properties of a soil, while stabilization means that the engineering properties of the soil have been changed enough to allow field construction to take place. There are two primary methods of soil stabilization used today:
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• mechanical • chemical or additive
Defining Soil Stabilization Nearly every road construction project will utilize one or both of these stabilization techniques. The most common form of “mechanical” soil stabilization is compaction of the soil, while the addition of cement, lime, bituminous, or other agents is referred to as a “chemical” or “additive” method of soil stabilization. There are two basic types of additives used during chemical soil stabilization: mechanical additives and chemical additives. Mechanical additives, such as soil cement, mechanically alter the soil by adding a quantity of a material that has the engineering characteristics to upgrade the load-bearing capacity of the existing soil. Chemical additives, such as lime, chemically alter the soil itself, thereby improving the load-bearing capacity of the soil.
Why and when is it used?
ABOVE: Compaction is a form of mechanical soil stabilization
Traditionally, stable sub-grades, sub-bases and/or bases have been constructed by using selected, well-graded aggregates, making it fairly easy to predict the load-bearing capacity of the constructed layers. By using select material, the engineer knows that the foundation will be able to support the design loading. Gradation is an important soil characteristic to understand. A soil is considered either “well-graded” or “uniformly-graded” (also referred to as “poorly-graded”). This is a reference to the sizes of the particles in the materials. Uniformly-graded materials are made up of individual particles of roughly the same size. Well-graded materials are made up of an optimal range of different sized particles. It is desirable from an engineering standpoint to build upon a foundation of ideal and consistent density. Thus, the goal of soil stabilization is to provide a solid, stable foundation. “Density” is the measure of weight by volume of a material, and is one of the relied-upon measures of the suitability of a material for construction purposes. The more density a material possesses, the fewer voids are present. Voids are the enemy of road construction; voids provide a place for moisture to go, and make the material less stable by allowing it to shift under changing pressure, temperature and moisture conditions.
ABOVE: The addition of lime slurry is a form of chemical soil stabilization
Uniformly-graded materials, because of their uniform size, are much less dense than well-graded materials. The high proportion of voids per volume of uniformly-graded material makes it unsuitable for construction purposes. In well-graded materials, smaller particles pack in to the voids between the larger particles, enabling the material to achieve high degrees of density. Therefore, well-graded materials offer higher stability, and are in high demand for construction. With the increased global demand for energy and increasing local demand for aggregates, it has become expensive from a material cost and energy use standpoint to remove inferior soils and replace them with choice, well-graded aggregates. One way to reduce the amount of select material needed for base construction is to improve the existing soil enough to provide strength and conform to engineering standards. This is where soil stabilization has become a cost-effective alternative.
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Defining Soil Stabilization Essentially, soil stabilization allows engineers to distribute a larger load with less material over a longer life cycle. There are many advantages to soil stabilization: • • • • • • • • • • • •
Stabilized soil functions as a working platform for the project Stabilization waterproofs the soil Stabilization improves soil strength Stabilization helps reduce soil volume change due to temperature or moisture Stabilization improves soil workability Stabilization reduces dust in work environment Stabilization upgrades marginal materials Stabilization improves durability Stabilization dries wet soils Stabilization conserves aggregate materials Stabilization reduces cost Stabilization conserves energy
Applications Soil stabilization is used in many sectors of the construction industry. Roads, parking lots, airport runways, building sites, landfills, and soil remediation all use some form of soil stabilization. Other applications include waterway management, mining, and agriculture.
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About Soil
ABOVE: Rotary mixer adds emulsion to soil while a road grader grades the treated and compacted material
About Soil Soil mechanics is a complicated subject, and for good reason: the methods that are used to improve the engineering characteristics of a soil will have a heavy influence on the success of the project. This publication, for the sake of brevity, only touches on the subject of soil. For in-depth information, it is recommended that you consult a geotechnical engineer.
What kind of soil is that, anyway? Ask a person from Malaysia and a person from Texas to describe soil, and you will get two completely different answers. And so it is in the world of soil: a soil’s characteristics can not be reliably understood based on its name. Soil types that may be similar often are referred to by different names depending on what region you are in. Some of the names are colloquial and only known locally. And, naturally, the opposite is true: similar names can mean different soil types depending on where you are.
Standards help identify soil types
ABOVE: the USDA Soil Texture Triangle is used to classify soils by weight of three unique particles: sand, clay and silt – see Appendix A
Because engineers rely on the predictability of soil behavior to accurately design and build their projects, various organizations worldwide have created standards to help engineers identify soil types scientifically. It would be impractical to show them all in this document, so here is a general look at how it is done in the United States.
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About Soil In the U.S., there are three standards that are generally the most accepted for road construction purposes. These standards do not classify soil in exactly the same way, but one or more of them may be used on the average project. The United States Department of Agriculture (USDA) soil classification system, typically used by farmers, classifies a soil by sieve analysis to find the percentage by weight of three unique particles: sand, clay, and silt. The percentages of the three particles are plotted on the USDA Soil Texture Triangle (Appendix A, page 22), which determines the classification. For example: a soil sample that contains 40% sand, 40% silt, and 20% clay is classified as “Loam.” Since the USDA system only considers the particle sizes of a soil sample, it does not evaluate the mineralogy of the clay and its potential water capacity or plasticity.
ABOVE: USCS sieve analysis of soil
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The Unified Soil Classification System (USCS), used by geo-technical engineers, applies to soil particle sizes of less than 3 inches in diameter. This system classifies soils based on their engineering properties, such as shear strength, permeability, and settlement potential. Utilizing a number of tests, such as sieve analysis and the Atterberg Limits, a soil sample is classified by particle or grain size (coarse, fine, organic, and peat), grainsize distribution (well-graded or uniformly-graded), and how the particles interact with moisture. Laboratory tests are also required for accurately determining soil type under the USCS classification system. These tests measure the plasticity, cohesiveness and bonding characteristics of a soil sample.
About Soil American Association of State Highway and Transportation Officials (AASHTO) classification system, used by transportation engineers, is a soil classification system specifically designed for the construction of roads and highways. The system uses the grain-size distribution and Atterberg Limits, such as Liquid Limit (LL) and Plasticity Index (PI), to classify soils. This classification system is defined by AASHTO standard M 145-91 (1995) and consists of a symbol and group number. The AASHTO classification system applies to soil particles smaller than 3 inches. Soils are classified into one of two general groups: (1) (2)
Granular materials (coarse-grained) with 35% or less by weight fines passing the #200 sieve Silts and clays (fine-grained) soils with more than 35% passing the #200 sieve
The soil is then further classified into one of 7 sub-classifications: A1, A2 and A3 for granular soils; A4, A5, A6, and A7 for silts and clays. These classifications provide the engineers with very accurate engineering property information.
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Chemical Soil Stabilization
ABOVE: Mixing lime slurry into the base soil of a future parking lot
Chemical Soil Stabilization One method of improving the engineering properties of soil is by adding chemicals or other materials to improve the existing soil. This technique is generally cost effective: for example, the cost, transportation, and processing of a stabilizing agent or additive such as soil cement or lime to treat an in-place soil material will probably be more economical than importing aggregate for the same thickness of base course. Additives can be mechanical, meaning that upon addition to the parent soil their own load-bearing properties bolster the engineering characteristics of the parent soil. Additives can also be chemical, meaning that the additive reacts with or changes the chemical properties of the soil, thereby upgrading its engineering properties. Placing the wrong kind or wrong amount of additive – or, improperly incorporating the additive into the soil – can have devastating results on the success of the project. So, in order to properly implement this technique, an engineer must have: 1) 2) 3)
4) 5)
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A clear idea of the desired result An understanding of the type(s) of soil and their characteristics on site An understanding of the use of the additive(s), how they react with the soil type and other additives, and how they interact with the surrounding environment An understanding of and means of incorporating (mixing) the additive An understanding of how the resulting engineered soil will perform
Chemical Soil Stabilization Combining the additives with the soil is typically done with various machines. The method used is usually based on three factors: what machines are available, the location (urban or rural), and the additives that are being used. The mixing should be as uniform as possible. The most economic and time-efficient method is to use a rotary mixer, a large machine that incorporates additives with the soil by tumbling them in a large mixing chamber equipped with a rotor designed to break up and mix the materials. It is capable of uniformly introducing additives and water while breaking up the soil into an optimal homogenous grade. The rotary mixer does all mixing in place, and is unrivaled in production by other methods. For some applications that require more precision, a pugmill is used. A pugmill is essentially a large mixing chamber that is similar to a cement mixer. Measured pre-graded aggregates, additives, and usually water are mixed in the pugmill and then applied to uniform thickness. Pugmills produce high quality stabilization, but at higher costs and slower production. Blade mixing is done with the use of a motor grader. Blade mixing is not nearly as efficient as the previously described systems, but it is far less complex. Essentially, the additive is placed in flat windrows and the blade of the grader mixes the additive with the soil in a series of turning and tumbling actions. Other machines are similarly used for mixing as well, including scarifiers, plows, and disks. It is very difficult to uniformly control mixing percentages and mixing depth using this technique.
ABOVE: Rotary mixer
ABOVE: Motor graders are often used for blade mixing additives
Additives There are many kinds of additives available. Not all additives work for all soil types, and a single additive will perform quite differently with different soil types. Generally, an additive may be used to act as a binder, alter the effect of moisture, increase the soil density or neutralize the harmful effects of a substance in the soil. Following are some of the most widely used additives and their applications: Portland Cement Portland cement is a mechanical additive that can be used for soil modification (to improve soil quality) or soil stabilization (to convert the soil to a solid cement mass). The amount of cement used will dictate whether modification or stabilization has occurred. Nearly all types of soil can benefit from the strength gained by cement stabilization. However, the best results have occurred when used with well-graded fines that possess enough fines to produce a floating aggregate matrix.
ABOVE: Truck spreading dry cement
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Chemical Soil Stabilization Quicklime/Hydrated Lime Lime is a chemical additive that has been utilized as a stabilizing agent in soils for centuries. Experience has shown that lime will react well with medium, moderately fine, and fine-grained clay soils. In clay soils, the main benefit from lime stabilization is the reduction of the soil’s plasticity: by reducing the soil’s water content, it becomes more rigid. It also increases the strength and workability of the soil, and reduces the soil’s ability to swell. It is very important to achieve proper gradation when applying lime to clay soils. By breaking up the clay into smallsized particles, you allow the lime to introduce homogeneously and properly react with the clay. ABOVE: Tanker spreading lime slurry
Lime can be applied dry to the soil, but if blowing dust is of concern or the work is being done in a populated area, the lime can be mixed with water to form slurry. A curing time of 3 to 7 days is normal to allow the lime to react with the soil, during which the surface of the stabilized soil should be wetted periodically.
ABOVE: Fly ash adds strength by reducing air voids, increasing the density of soil.
Fly Ash Fly ash, a chemical additive consisting mainly of silicon and aluminum compounds, is a by-product of the combustion of coal. Fly ash can be mixed with lime and water to stabilize granular materials with few fines, producing a hard, cement-like mass. Its role in the stabilization process is to act as a pozzolan and/or as a filler product to reduce air voids. A common application is as part of a lime/cement/fly ash mixture (LCF) to stabilize coarse-grained soils that possess little or no fine grains. Because it is essentially a waste product, it can be obtained rather inexpensively. Calcium Chloride Calcium chloride is a chemical additive that has the ability to absorb moisture from the air until it liquifies into a solution. The presence of calcium chloride in the moisture of a soil lowers the freezing temperature of that moisture. For this reason, calcium chloride is a proven stabilizing additive for cold-climate applications. If the water in the soil can’t freeze, there is less soil movement (i.e., frost heaves), making it much more stable. Calcium chloride also works well as a binder, making the soil easier to compact and reducing dust.
ABOVE: Rotary mixer adds bituminous emulsion to reclaimed roadbed.
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Bitumen Bitumen is a mechanical additive that occurs naturally or as a by-product of petroleum distillation. It is the black pitch used to make asphalt. Asphalt cement, cutback asphalt, tar, and asphalt emulsions are all used to achieve bituminous soil stabilization. Soil type, construction method and weather are all factors in choosing which bitumen to use. Bitumen makes soil stronger and resistant to water and frost. The use of bitumen can lead to fewer weather-related delays during construction, and makes compaction easier and more consistent.
Chemical Soil Stabilization Chemical or Bio Remediation Our industrial society produces many benefits, but occasionally there are unintentional, accidental, or criminal problems that occur. Petroleum hydrocarbons, lead, PCBs, solvents, pesticides, and other hazardous natural and man-made substances often contaminate soil. Because even contaminated real estate can be valuable – and because pollution is undesirable to begin with – efforts are made to return contaminated soil to an acceptable condition for human habitation. The goal of chemical or bio remediation is to convert hazardous substances into inert ones and to prevent hazardous substances from spreading or leaching. The type of additive depends on the contaminant(s) and the environment. Chemical additives are often proprietary chemical cocktails, but the science is well understood and they are quite effective at neutralizing hazardous substances. Bio remediation is typically done by the introduction of natural means: bacteria or insects that eat contaminants and convert them to inert waste, or plants that filter out contaminants and convert them to natural substances.
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Mechanical Soil Stabilization
ABOVE: Soil compactors mechanically stabilize soil by increasing density
Mechanical Soil Stabilization Mechanical soil stabilization refers to either compaction or the introduction of fibrous and other non-biodegradable reinforcement to the soil. This practice does not require chemical change of the soil, although it is common to use both mechanical and chemical means to achieve specified stabilization. There are several methods used to achieve mechanical stabilization: Compaction Compaction typically employs a heavy weight to increase soil density by applying pressure from above. Machines are often used for this purpose; large soil compactors with vibrating steel drums efficiently apply pressure to the soil, increasing its density to meet engineering requirements. Operators of the machines must be careful not to overcompact the soil, for too much pressure can result in crushed aggregates that lose their engineering properties.
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Soil Reinforcement Soil problems are sometimes remedied by utilizing engineered or nonengineered mechanical solutions. Geo-textiles and engineered plastic mesh are designed to trap soils and help control erosion, moisture conditions and soil permeability. Larger aggregates, such as gravel, stones, and boulders, are often employed where additional mass and rigidity can prevent unwanted soil migration or improve load-bearing properties.
Mechanical Soil Stabilization Addition of Graded Aggregate Materials A common method of improving the engineering characteristics of a soil is to add certain aggregates that lend desirable attributes to the soil, such as increased strength or decreased plasticity. This method provides material economy, improves support capabilities of the subgrade, and furnishes a working platform for the remaining structure. Mechanical Remediation Traditionally, mechanical remediation has been the accepted practice for dealing with soil contamination. This is a technique where contaminated soil is physically removed and relocated to a designated hazardous waste facility far from centers of human population. In recent times, however, chemical and bio remediation have proven to be a better solution, both economically and environmentally. It is often cheaper to solve the problem where it exists rather than relocate the problem somewhere else and possibly need to deal with it again in the future.
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The Basic Soil Stabilization Process
ABOVE: Soil stabilization during the reclamation process can often occur without re-directing traffic
The Basic Soil Stabilization Process Both new construction and rehabilitation projects are candidates for soil stabilization. While the precise stabilization procedures will vary depending on many factors – including location, environment, time requirements, budget, available machinery, and weather – the following process is generally practiced: Assessment and Testing The soils of the site are thoroughly tested to determine the existing conditions. Based on analysis of existing conditions, additives are selected and specified. Generally, a target chemical percentage by weight and a design mix depth are defined for the sub-base contractor. The selected additives are subsequently mixed with soil samples and allowed to cure. The cured sample is then tested to ensure that the additives will produce the desired results.
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Site Preparation The existing materials on site, including existing pavement if it is being reclaimed, is pulverized utilizing a rotary mixer. Any additional aggregates or base materials are introduced at this time. The material is brought to the optimal moisture content by drying overly wet soil or adding water to overly dry soil. The grade is shaped if necessary to obtain the specified material depth.
The Basic Soil Stabilization Process Introduce Additives Cement, lime or fly ash can be applied dry or wet. When applied dry, it is typically spread at a required amount per square yard (meter) or station utilizing a cyclone spreader or other device. When lime is applied as slurry, it is either spread with a tanker truck or through the rotary mixer’s on-board water spray system. Calcium chloride is usually applied by a tanker truck equipped with a spray bar. Bituminous additives are typically added utilizing an on-board emulsion spray system on a rotary mixer. It can also be sprayed on the surface, but this method requires several applications and additional mixing.
ABOVE: Padfoot compactor follows rotary mixer
Mixing To fully incorporate the additives with the soil, a rotary mixer makes several mixing passes until the materials are homogenous and wellgraded. It is crucial that the rotary mixer maintains optimal mixing depth, as mixing too shallow or too deep will create undesirable proportions of soil and additive. Inappropriate proportions of soil and additive will decrease the load-bearing properties of the cured layer. Some projects require multiple layers of treated and compacted soil. When applying cement and fly ash, it is important to finish mixing as soon as possible due to the quick-setting characteristics of the additives. Compaction and Shaping/Trimming Compaction usually follows immediately after mixing, especially when the additive is cement or fly ash. Some bituminous additives require a delay between mixing and compaction to allow for certain chemical changes to occur. Compaction is accomplished through several passes using different machines. Initial compaction is begun utilizing a vibratory padfoot compactor. The surface is then shaped and trimmed to remove pad marks and provide a more suitable profile. Intermediate compaction follows utilizing a pneumatic compactor, which provides a certain kneading action that further increases soil density. A tandem drum roller is used on the finishing pass to provide a smooth surface. A final shaping gives the material a smooth finish and a proper crown and grade. Curing Sufficient curing will allow the additive to fully achieve its engineering potential. For cement, lime, and fly ash stabilization, weather and moisture are critical factors, as the curing can have a direct bearing on the strength of the stabilized base. Bituminous-stabilized bases often require a final membrane of medium-curing cutback asphalt or slowcuring emulsified asphalt as a moisture seal. Generally, a minimum of seven days are required to ensure proper curing. During the curing period, samples taken from the stabilized base will reveal when the moisture content is appropriate for surfacing.
ABOVE: Track-type tractor used for shaping
ABOVE: Pneumatic Compactor
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Glossary AASHTO (America Association of State Highway and Transportation Officials) Soil Classification System – a set of standards that helps transportation engineers determine soil characteristics for the purpose of designing and building roads, highways and associated traffic-supporting structures. It utilizes grain-size distribution and Atterberg Limits to define soils. Additive – a manufactured substance that is added to existing materials in order to improve engineering properties. Additive Stabilization – A method of improving the engineering properties of a material by adding chemical substances. Aggregate – the granular, load-bearing mineral component of a road structure, usually sand, gravel, shell, slag, crushed stone or fines. Atterberg Limits Method – a set of standards that describe seven stages of soil characteristics as it moves from a solid to a liquid. The most important stages are the Plastic Limit and the Liquid Limit. Base – also referred to as the “base course” – a layer of specified or selected material of planned thickness constructed on the sub-base for the purpose of serving one or more functions including distributing the load, providing drainage and minimizing frost action. Binder – a material used to bind the aggregate particles together, prevent the entrance of moisture, act as a cushioning agent, and, in some cases, waterproof the entire road surface. Chemical Additive – a substance utilized as an additive in chemical stabilization that chemically alters the parent soil to improve its load-bearing properties. Chemical Stabilization – A method of improving the engineering properties of a material by adding chemical substances or by altering the gradation of the particles in the material. Clay – a fine-grained mineral material (soil) that uses electro-chemical surface charges to bond well with water. Coarse-grained Soil – a USCS classification for soil comprised of particles (grains) that lack cohesion. Sand and gravel are considered coarse-grained soils. Coarse-grained soils are defined as well-graded or poorly-graded, which reflect the soils ability to be compacted. Cohesion – the ability of a material to maintain its strength when unconfined; i.e., cling together and maintain its form through changes in moisture content or submersion. Compaction – the process of reducing voids in a material through the use of mechanical manipulation; increasing the density. Cure, Curing – the process of allowing a chemical reaction to continue to completion. Cutback Asphalt – asphalt residues that are blended with distillates.
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Glossary Cyclone Spreader – a device that typically attaches to the rear of a hauling vehicle that distributes fine-grained materials in relatively uniform patterns by depositing specific volumes of the material on a spinning disc that disperses the material via centrifugal force. Density – the measure of mass per volume. Emulsion – in the context of soil stabilization, an emulsion is a mixture of water, a bitumen additive such as asphalt cement, and an emulsifier which enables the water and bitumen additive to combine. Emulsions act as a binder for the parent material, giving the parent material additional load-bearing strength as well as a degree of moisture resistance. Fines – generally, materials of small particle size; defined by USCS as those particles passing through a #200 sieve. Fine-grained Soils – soils composed predominantly of fines. Floating Aggregate Matrix – a desirable quality of a material to contain a homogenous distribution of particles of varying size throughout its mass. In terms of load-bearing materials, this quality produces the most strength. Geo-textiles – manufactured materials that are designed to lend reinforcement to geological materials; i.e., prevent erosion or provide a moisture barrier. Grade – the inclination of a surface. Gradation – a reference to the level of uniformity to which a soil has been pulverized. Grain – a mineral particle. Grain-size Distribution – the measure of the range and distribution of different particle sizes in a soil. Gravel – a coarse-grained mineral material; defined by the USCS as those particles less than 3 inches in diameter not passing through a #4 sieve.
ABOVE: Padfoot Compactors apply pressure to the soil with pads instead of a smooth drum
Liquid Limit – a highly significant Atterberg Limit; the point at which a soil contains so much water that it is considered a liquid. Mechanical Additive – a substance utilized as an additive in chemical stabilization that lends its own engineering properties to the parent soil, thereby upgrading the load-bearing properties of the soil. Mechanical Stabilization – any of several methods that employ mechanical means to improve the engineering properties of a soil. Moisture Content – the amount of liquid (water) per volume of a mass. Padfoot Compactor – A machine that applies compaction force with a vibrating steel roller that has small pads welded to the surface of the roller that are designed to deliver the load in small, patterned areas rather than as a static linear load.
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Glossary Permeability – a material’s ability to allow the passage of a gas or liquid. Plasticity – the property of a fine-grain soil that allows it to deform beyond the point of recovery without cracking or appreciable volume change. Plasticity is a critical factor during soil stabilization. Plasticity Index – the difference between the Liquid Limit and the Plastic Limit of a soil. This measure is used to determine the extent of soil stabilization required for fine-grained soils. Plastic Limit – a highly significant Atterberg Limit; the point at which a soil retains enough moisture to become plastic. ABOVE: Rotary Mixer
Poorly-graded – also referred to as uniformly-graded; the quality of coarsegrained soils to contain particles of relatively uniform size, making it difficult to compact. Pozzolan – any of various natural or man-made substances that possess characteristics similar to pozzolana, a volcanic ash used to make hydraulic cement. Pugmill – a mechanical device used for mixing materials, usually with paddles attached to rotating shafts. Rotary Mixer – a large machine, similar in operation to a garden tiller, used to pulverize materials in place. Usually self-propelled, but sometimes towed by a tractor. Sand – a mineral particle defined by USCS as a coarse-grained particle passing a #4 sieve but not passing a #200 sieve. ABOVE: Motor Grader with Scarifier
Scarifier – a ground-breaking device with claw-like tines that is attached to or towed by a heavy machine. Silt – a non-cohesive fine-grained mineral material (soil). Slurry – a semi-liquid mixture; typically, lime or cement suspended in water. Soil – unconsolidated material composed of mineral particles that may or may not contain organic substances. Soil Stabilization – the process of maximizing the suitability of soil for a given construction purpose. Soil Modification – the process of changing the characteristics of soil enough to provide a non-significant increase in soil strength and durability. Station – a non-standard area defined by engineers and marked by stakes at the site for the purpose of controlling preparation by manageable sections. Sub-base – a layer between the sub-grade and base. Sub-grade – the soil prepared to support a traffic structure. Essentially, it performs as the foundation of the structure, and is sometimes referred to as “basement soil” or “foundation soil.”
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Glossary Uniformly-graded – also referred to as poorly-graded; the quality of coarsegrained soils to contain particles of relatively uniform size, making it difficult to compact. USDA (United States Department of Agriculture) Soil Classification System – a soil classification system based on particle size utilized primarily for agricultural purposes but often used for construction purposes to define soils. USCS (Unified Soil Classification System) – widely utilized by engineers, this soil classification system categorizes soils into groups with distinct engineering properties, such as shear strength, permeability and settlement potential. Voids – the space in a mass not occupied by solid mineral material. Voids are bad for traffic structures because they allow the migration of mineral materials, moisture or gases that can disrupt the stability of the structure. Uniformlygraded soil has many voids; well-graded soil has few voids. Well-graded – the quality of coarse-grained soils to contain particles of many sizes, making it easier to compact. Windrow – a continuous uniform pile of material that allows for the even distribution of materials over the length of a site.
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Appendix Appendix A: USDA Soil Texture Triangle
Example: A soil sample that contains 40% sand, 40% silt, and 20% clay is classified as “Loam”.
Appendix B: Cutaway view of typical asphalt road construction project
Surface Course, or Pavement
Surface Base Course, or Binder Course Base
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Sub-Base
Acknowledgements Many thanks for your expertise and assistance to: Stan Vitton, Ph.D, P.E. Director, Institute for Aggregate Research Michigan Technological University
References: Soil Stabilization for Pavements: EM 1110-3-137 . Washington DC: US Army Corps of Engineers, 1984.
Huffman, John E. Base/Subgrade Stabilization. Salina, KS: The Asphalt Institute, Kansas State University at Salina, 1995
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