In both the mining and construction industries, blasting is the predominant method for fragmentation of consolidated mineral deposits and rocks. The public relation problems of users of explosive...
The fundamental concepts of an ideal blast design should be adopted for carrying out any blast, which are then modified as per geologic conditions
Blasting Course
Designing Blasts
Two important aims in blasting.
To fragment the rock to a desirable size and To displace the fragments a desirable distance
At the best overall cost 6
5
4 Cost/m3
Explosive Drilling
3
Load & Haul Crushing & M illing Overall Cost
2
1
0 0.5
0.7
0.9
1.1
Powder Factor (kg/m3)
Three things that affect outcome
Explosive Rock Blast Geometry
Rock
Rock Strength Rock Density Rock Structure
Rock Strength and elasticity UCS and Tensile Strength Has an influence on creation of cracks during blasting Measured in MPa Tensile strength about 1/10th UCS
Rock Density Influences the displacement distance during a blast Influences fragmentation Higher density = higher energy for same end result
Rock Structure Rock Structure has largest influence on fragmentation results. Sometimes fragmentation completely controlled by structure
Rock Structure
Rock Structure
Rock Blastability Index Rock UCS Rock density Joint Spacing Joint Orientation
Spacing = 1.15 x Burden (in plan, energy distributed evenly.) Red circles represent contours of equal energy around each borehole. Spacing > 1.15 x Burden (in plan, energy not distributed evenly and more overlap can be seen in the energy contours in one direction.)
Staggered Pattern
Wherever possible use a staggered pattern. A square pattern is only suitable for narrow boxcut blasts
Deciding on Burden - Scaled Burden
Burden Scaled Burden = Mc
Burden = Scaled Burden × M c
Scaled Burden
For Reduced Heave or coarser fragmentation Shale/Mudstone Sandstone Limestone/Dolomite Granite Dolerite
RBI = 20 to 40
RBI = 40 to 60
RBI = 60 to 80
1.3 1.2 1.2 1.2 1.1
1.2 1.1 1 1 1
1.1 1 1 0.9 0.9
For increased heave or finer fragmentation Shale/Mudstone Sandstone Limestone/Dolomite Granite Dolerite
RBI = 20 to 40
RBI = 40 to 60
RBI = 60 to 80
1 1 0.9 0.9 0.8
0.8 0.8 0.75 0.7 0.7
0.7 0.7 0.65 0.6 0.6
Burden Stiffness
bench height Sb = ≥ 2.5 burden
Burden Stiffness - Matching bench height to hole diameter Poor explosive distribution. Only the bottom half of the bench contains explosive. Tendency to crater because the surface is closer than the vertical free face. This results in A tendency for uncontrolled fly rock Poor fragmentation in top half of bench Uneven floors Very little muckpile movement
Explosives Energy
kg per hole Design Powder Factor = Burden × Spacing × Bench Height
total kg Actual Powder Factor = block volume
Energy Factor = Powder Factor × RWS
Stemming Stemming affects the following results in a blast: – Fly rock – Air blast – Fragmentation – Heave All blasts require stemming
