2. Soft / Fractured rock – Low density explosive
3. Explosive with high gas production (such as ANFO) for displacement are appropriate for highly jointed or fractured rock.
4. Water resistance
5. Chemical stability
6. Fume characteristics
7. Bulk ANFO :
a. Zero Oxygen Balance = 94.3% AN + 5.7% FO
b. Over fuel mix, example: 92% AN + 8% FO, Prod.
6% less energy
CO
c. Under fuel mix, example: 96% AN + 4% FO
Prod. 18% less energy
NO2
Increase sensitivity
d. It’s generally better to over fuel ANFO rather than under fuel it.
8. Primers :
a. Primer diameter should closely match hole dia.
b. Two primers are recommended for blasthole over 15 meters deep [ANFO] and 10 meters deep [Emulsion Blend].
BASIC DRILL / BLAST DESIGN
1. BENCH HEIGHT
2. BLASTHOLE DIAMETER
3. BURDEN
4. BURDEN STIFFNESS RATIO
5. SPACING
6. SUBDRILLING
7. STEMMING
8. DECKING / AIR DECKING
9. ANGLE DRILLING
10. TIMING DESIGN / DELAY
BENCH HEIGHT
If the height is not predetermined :
BH (m) >> Blasthole Dia. (mm) / 15
BLASTHOLE DIAMETER
To achieve excellent energy distribution :
DIA (mm) = Bench Height (m) x 8
If charge diameter is less than the blast hole diameter, the “decoupling effect” must be taken into account.
As blasthole diameters increase the cost of drilling, loading & explosive generally decrease.
Smaller blast holes distribute the explosive energy better than large blast holes.
BURDEN
Burden (m) are normally equal the charge diameter (mm) x (20 – 35).
Initial Burden Estimation Guide (see table)
BURDEN STIFFNESS RATIO
Equal to the Bench height divided by burden
<< 2 : stiff and poor fragmentation. 2 – 3.5 : good fragmentation. >> 3.5 : excellent fragmentation.
BSR can be improved by using smaller hole diameter or greater bench height.
SPACING
Normally ranges from (1 to 1.8) x Burden
Optimum energy distribution:
S = 1.15 x B
Pattern is laid out in “Staggered”
SUBDRILLING
Normally ranges from ( 0.3 to 0.5 ) x Burden
or ranges from (8 – 12) x Hole diameter to much Sub drilling produces “Excessive
Ground Vibration”
Less Sub drilling produces “Excessive toe”
To improve fragmentation the blast hole primer should be placed at grade level.
DECKING / AIR DECKING
Minimum decking for dry holes:
Deck = hole diameter x 6
Minimum decking for wet holes:
Deck = hole diameter x 12
Air decking can reduce the amount of explosives to achieve good results by efficiently utilising the available explosive energy.
UNIFORM ENERGY DISTRIBUTION
Decking Alternatives
STEMMING
1. Normally ranges from (20 to 30) x Hole dia. or equal to 0.7 x Burden.
2. Crushed rock confine explosive energy
3. Better than drill cuttings.
4. Wet blast holes require more stemming for confinement than dry blast holes.
Relative Confinement (RC):
1.4 : Confine
1.4 : Fly rock & stemming ejection Vertical Energy Distribution (VED): Charge length divided by Bench height and 80% to produce uniform fragmentation
5. To improve VED : Reduce charge
6. Increase Bench height. Then recalculate
7. Burden and stemming dimensions.