Any explosive engineer, blaster, shot-firer or hole-loader will tell you that the shot only can be as good as the drilling allows. It is extremely important that the holes are drilled where they will do the most good, not just for the convenience of the driller. The best explosive engineer cannot make up for improperly drilled holes.
During drilling operations, the rock fails in three ways – crushing, chipping and spalling. The blaster must analyze the mechanics of a drilling system to reveal the limitations and advantages for each type of rock. For example, a rock with a high compressive strength is likely to respond well to the crushing and chipping action of a percussive bit. On the other hand, a relatively weakly bonded rock may not respond to percussive action, but will give good performance for a wear-resistant rotary drag bit.
The best planning, figuring, calculations and explosives are worthless if the area to be shot is not drilled properly and responsibly. Basically, if the drilling goes bad and is off pattern, the entire blasting program will fail. If the driller is informed to remain on a specific pattern, he must stay on the pattern and not alter it unless he consults with the blaster-in-charge.
The driller also must keep the blaster informed of any changes in the rock that he is drilling or any deviations he makes so the blaster may make adjustments to the shot. The driller informs the blaster about cracks and shifts in the rocks, changes in the strata and sand, or mud seams in the rock, so that explosives can be loaded in the hole with these factors taken into consideration. The driller also must inform the blaster of any short holes – any holes that are not the expected or planned depth. In other words, the driller serves as the eyes of the blaster. Consequently, the drill and driller can make or break a blasting operation.
The most common method of production blasting in quarrying, strip-mining and construction excavation is bench blasting. This method involves inclined, vertical or horizontal blast holes drilled in single- or multiple-row patterns to depths ranging from a few feet to 100 feet or more, depending on the desired bench height. Where the excavation is shallow (< 20 ft.), one level may suffice. In deep excavations, a series of low benches – offset from level to level – are recommended for operational convenience.
Bench blasting ideally reduces all rock to a desired rubble size range. This is basic in order to facilitate handling of rubble or muck to meet limitations imposed by equipment – such as bucket size – or to produce a usable material.
Actually, even a satisfactory blast may leave a few oversized blocks that must be broken by blasting with a light charge placed in small drill holes in the boulder, a technique known as blockholing. A quick method for smaller boulders, called mudcapping, involves blasting with a part of a stick of powder or a small, bagged binary charge placed against the boulder and covered with mud or a bag of sand. Plastic bags filled with water also can be used. Note: Mudcapping and blockholing may produce objectionable air blasts.
Boulder outcroppings in fields under preparation for farming or on road right-of-ways also may require blasting. Besides mudcapping and blockholing, there are two other methods commonly used in blasting rock boulders and outcroppings. Snakeholing includes the placement of explosives under the rock. Seam blasting is used when the blaster is lucky enough to find a crack or seam, and can load the explosive charge into it. The method selected will depend upon a number of factors, including the equipment at hand and the depth of the rock in the earth. Secondary blasting is noisy and generally produces many flying fragments. Accordingly, it is seldom suitable for use in residential areas.
Rough terrain or loose overburden may prohibit drilling the bench from the top. In such cases, lifters (nearly horizontal blasthole charges) may be used instead. Snakeholes are similar to lifters, except that they always are located at the toe of the slope. They should be inclined slightly downward. Snakeholes also may be supplemented above with rows of lifters inclined 20 degrees to 30 degrees upward from horizontal. The pattern is commonly fired in sequence, starting at the top. High quarry faces (>75 ft.) have been successfully blasted using a combination of snakeholes and vertical holes. Lifters and snakeholes usually are not employed in structural excavation. Their use generally requires that previously blasted rock is excavated before drilling can commence for the following rounds. Snakeholes may produce excessive flyrock, and if they are drilled on an incline to below the final gradeline tolerance, the final rock surface is damaged.
This article is provided through the courtesy of the National Park Service. It is excerpted from its “Handbook for the Transportation and Use of Explosives.”