As the cost of drilling is a significant proportion of the total drilling and blasting operation the use of as large a drill hole as possible will result in greater hole spacings and reduced footage. However, the choice of hole size and drilling pattern is frequently restricted because of the depth of face possible and the degree of fragmentation required.
Material should be produced in a manageable size without the need for additional treatment. In a cut to fill operation, the material size is restricted by the maximum thickness of rock fill that can be compacted by the rollers available. This then places a limit on the drill hole size and spacing. Alternatively, it may be more economical to hire larger rollers capable of compacting deeper layers.
Deep faces (where possible) offer economy in the use of explosives but, for safety reasons, when using front end loaders for loading of the broken rock, a maximum face of 10 meters is recommended.
A decision to drill and blast need not slow down production. The blasting operation must be matched to the capabilities of the earth moving fleet and the total operation should be planned so that intermittent unscheduled blasting does not interfere with the project’s continuity.
Productively can be maintained by working several faces and arranging blasting to be undertaken at times when there is the least likelihood of interruption to other operations.
A blast should be designed to be as large as possible within circumstantial restraints, to ensure that the overall percentage of “Low productivity” material is kept to a minimum and that a minimum of productive time is lost in preparing and moving equipment.
In addition to producing the necessary quantities, the designed blast must achieve a degree of fragmentation which still not adversely effect the cost of handling during succeeding phases of earth moving. A well designed blast should allow loading to be undertaken without any dozer stockpiling being required.
When blasting adjacent to batters, holes should be more closely spaced and more lightly charged. Unless per-splitting is adopted, all batter holes should be shot on the last delay or shot separately after the main charge had been set off (perimeter blasting). Shooting should always be to a near vertical free face parallel to the holes; and the blast should be designed so that back break and damage to the sub graded and batters is minimized. To avoid such damage, “pin cushion” blasting (no free face) in conjunction with ripping should not be used