BLASTING

process of reducing a solid body, such as rock, to fragments. An explosive or, occasionally, compressed air may be used. Conventional blasting operations include (1) drilling holes in a converging pattern, (2) placing a charge and detonator in each hole, (3) tamping and stemming the charge to compact the explosive, and filling the remainder of the hole with clay or rock, (4) igniting or detonating the charge, and (5) clearing away the broken material. In primary blasting (breaking rocks from the solid), an explosive is usually confined in holes to make more efficient use of the explosive energy. In secondary blasting (breaking rocks into smaller pieces), the charge is usually placed in a hole or on the surface with a cover of mud, called a mudcap. The shock of the blast is most intense near the charge. Compressed air, because it is not a fire hazard, is usually used in bituminous coal mines to break the coal into large fragments. Black powder and high explosives may cause crushing immediately around the hole, but at the limit of the crushed zone the strain wave proceeds radially, diminishing rapidly in intensity. The shock from nuclear explosives produces melting and vaporization of rock in the immediate vicinity, then a crushed zone, and, finally, if the charge is near enough to a free surface, a fractured zone. In surface blasting, large-diameter holes (over 7.5 cm [3 inches]) are used, with an average diameter of about 15 cm (6 inches) and an average depth of about 9 m (30 feet). In underground blasting, shorter, small-diameter holes are used. 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 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. The detonation of each charge of explosives is initiated by a blasting cap. Timing of the blasts with fuses is fixed by the length of the fuses, while timing with electric blasting caps is done with timing elements in the caps themselves.