Contract Drilling

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, mThe 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

The use of primers with delays allows the direction of movement of rock material caused by the explosion to be controlled. In general, the primers will be set in the holes following the staked road course so that the explosives loaded in the front holes will go off first and blasted rock material will be directed by the subsequent exploding loads towards the already established road section.

In road sections where substantial rock blasting is needed, full bench construction of road with longitudinal transport of excess material is usually specified and rock blasting along the base of the fill will be required in order to establish the base for proper fill foundation.

Damage to adjacent stands at construction site by material escaping downhill during blasting operations can be reduced due to the “soft” character of this blasting technique. Its use is also highly recommended in the vicinity of springs as they might be affected by the disruptive effect on stratified bedrock of less sophisticated blasting. If buildings or infrastructure are at risk even by applying soft blasting techniques, hydraulic hammers are the only solution but their use has to be restricted to short road sections for cost reasons.