Blasting

blasting, process of reducing a solid body, such as rock, to fragments by using an explosive. Conventional blasting operations include (1) drilling holes, (2) placing a charge and detonator in each hole, (3) detonating the charge, and (4) clearing away the broken material.

Upon detonation, the chemical energy in the explosive is liberated, and the compact explosive becomes transformed into a glowing gas with an enormous pressure. In a densely packed hole this pressure can exceed 100,000 atmospheres. The high pressure shatters the area adjacent to the drill hole and exposes the rock beyond to very high stresses and strains that cause cracks to form. Under the influence of the gas pressure, the cracks extend, and the rock in front of the drill hole yields and moves forward. If the distance of the hole to the closest surface is not too great, the rock in front of the hole will break free.

Holes are so placed as to require a minimum quantity of explosive per volume of rock broken (called the powder factor). Most blast-hole patterns are based on the fact that fragmentation is most uniform if the exploding charge is within a particular distance from an exposed face of the rock. To break up a large body of rock, charges are placed in a series of holes drilled so that, as the holes nearest the exposed surface are fired, the blasts create new exposed faces at the proper distances from the next set of holes, in which firing of the charges is slightly delayed. The holes are fired in a predetermined order, at intervals of only thousandths of a second.

Blasting is commonly used to break materials such as coal, ore, stone, or other mined materials, to demolish buildings, and to excavate foundations for civil structures.