The drilling pattern (drilling geometry) depends on certain parameters of which the blast-hole diameter is one of the most important.
The following is a simple guide for estimating a suitable drilling pattern for bench blasting. It includes the different variables that affect the blasting result.

d, blast hole diameter, mm
  • The selection of a blast hole diameter depends on the type of work and the desired fragmentation of the rock. Large diameter holes give better drilling economy but may at a later stage of the preparation of the rock lead to added costs. In large-scale mining operations it is possible to use blast hole diameters of up to 400 mm while in a small scale construction operations a blast hole diameter of 30 mm may be the best choice. Normally the rock calls for 0.4 kg of explosive per cubic meter to be well fragmented. With smaller blast holes the explosive is distributed more uniformly in the rock mass which normally gives better fragmentation.
  • B, burden, m
    The distance between the blast hole and the free surface is called “burden”.
    The amount of explosive that a blast hole can take depends on the blast hole diameter and the density of the explosive. What the hole takes per linear meter is called charge concentration and is expressed in kg/meter There is a relation between the charge concentration of a defined explosive and the distance to the free surface. Different explosives give different burden

    Distances, B.

    Dynamite in plastic hoses: B = 40xd mm
    Emulsion in plastic hoses: B = 38xd mm
    ANFO poured into the hole: B = 32xd mm

    Dynamite in a 76 mm blasthole should have a burden of 40x76 mm = 3040 mm ˜ 3.00 m.

S, spacing, m
  • Spacing is the distance between the individual holes in a row of holes. Experiments have shown that a spacing of 1.25xB gives the best fragmentation result.. Our example gives a spacing of 1.25x3.0 m = 3.75 m.
K, bench height, m
  • In quarry operations and open pit mining, the bench height is normally 15.0 – 18.0 m however, in construction work it may vary due to the topography of the surface of the rock.
  • U, sub-drilling, m
    In order to keep the bottom on the right level it is necessary to drill under the intended level. A sub-drilling of 0.3xB is sufficient in most cases.
I, hole inclination, degrees
Inclining the holes approx. 70O gives less over break to the next round and more stable slopes. In vertical holes much of the energy in the bottom part of the holes is transmitted into the rock as shock waves without breaking the rock. The most favorable breakage angle in the bottom of the hole is 45O but for practical reasons it is kept between 70O and vertical. When blasting with a 70O angle, the hole depth has to be adjusted accordingly with a factor of 1.05. H, hole depth, m
The actual hole depth can then be calculated:H = 1.05(K + 0.3xB) m for 70 O hole.b, specific drilling (drill factor) m/m3
The drill factor is important to know when estimating the drilling needed for excavation of a certain rock volume. The drill factor is calculated by dividing the
hole depth with the volume of blasted rock the hole produces.b = H/(BxExK) m/m³.

Summary of drill-blast parameters.
Hole diameter, d, mm
Burden, B = 38xd for emulsion explosives, mm B = 32xd for ANFO
Spacing, S = 1.25xB m
Bench height, K, m
Subdrilling, U = 0.3xK, m
Hole inclination, I = 70O factor 1.05
Hole depth, H = 1.05(K + U) m
Drill factor b = H/(BxSxK) m/m³