Mine Development & Surveying

Explosives & Blasting

Powder factor, bench-blast geometry and charge per hole — the numbers that decide fragmentation, cost and safety.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Blasting converts the chemical energy of an explosive into the work of breaking and displacing rock. The single most important efficiency metric is the powder factor — the mass of explosive needed to break a unit volume (or tonne) of rock. Too low and the rock is poorly fragmented (oversize, hard digging); too high and explosive is wasted and flyrock/ground-vibration risk rises. The volume each blast breaks is set by the drill-hole geometry — the burden (distance to the free face), the spacing between holes, and the bench height — while the explosive each hole holds depends on its diameter, charge length and the explosive's density.

The Core Formula Matrix

Powder factor (mass per unit volume):

P F = \frac{E}{V} [kg/m^{3}]

where

E

= total explosive mass,

V

= rock volume broken.

Volume broken by a bench round of

N

holes:

V = B \times S \times H \times N

B

= burden,

S

= spacing,

H

= bench height

[m]

.

Charge mass per hole (continuous column):

E_{h} = ρ_{e} \frac{π d ^{2}}{4} ℓ

ρ_{e}

= explosive density

[kg/m^{3}]

d

= hole diameter

[m]

ℓ

= charge length

[m]

Bench-blast geometry: burden B to the free face, spacing S between holes, bench height H.

The ‘IIT Traps’

⚠
Powder factor, not its inverse. $P F = E / V$ (kg/m³). Computing $V / E$ gives the reciprocal (m³/kg) — a different quantity that flips the answer.
⚠
Diameter in metres before squaring. $\frac{π d ^{2}}{4}$ needs $d$ in metres; leaving 100 mm as '100' inflates the area by $1 0^{6}$ .
⚠
Burden vs spacing. Burden is measured *to the free face*; spacing is *between adjacent holes*. Swapping them changes the breakage volume and the blast result.
⚠
Volume vs tonnage powder factor. kg/m³ and kg/tonne differ by the rock density; don't mix the two definitions.

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

A blast uses

200 kg

of explosive to break

1000 m^{3}

of rock. The powder factor is:

Q2MEDIUM2 Marks · NAT

A bench round has

N = 20

holes with burden

B = 3 m

, spacing

S = 3.5 m

and bench height

H = 10 m

. If

420 kg

of explosive is used, the powder factor is ______ kg/m³. (Round off to two decimal places.)

kg/m³

Q3HARD2 Marks · NAT

A blasthole of diameter

d = 100 mm

is charged with ANFO (density

850 kg/m^{3}

) over a continuous column length of

6 m

. The explosive mass in the hole is ______ kg. (Round off to two decimal places.)