PLOSIVES.
Modern blasting operations may he divided into three classes: (1) small-shot blasting. in which comparatively small volumes of rock are moved at a single blast ; (2) blasting by mines, in which large masses of rock are broken up by a single heavy blast; and (3) surface-blasting, in which the explosive is placed on or against, or simply near to, the rock to be broken up, and which is possible only with very high explosives. Small-shot blasting is employed in the great majority of quarrying, mining, and engineering operations. It consists in piercing the rock with a comparatively small number of drill-holes from 1 inches to 3 inches in diameter and from IS inches to several feet in depth; charging these holes with explosives, generally blasting powder or dynamite, with the proper fuse or electric-wire connections; tamping the space above the explosive with earth. sand. clay, or water, and finally firing these charges by means of a time-fuse or wires from an electric battery or magneto-machine. The relative location of the drill-holes, their size and depth, and the amount of explosive used vary according to the object which it is sought to accomplish by the blast. Where the purpose is merely to break up the rock in the most efficient manner for its removal, as in excavating a foundation, the holes will be placed quite close together and heavily charged, so as to shatter the rock thoroughly. In quarrying, where the object is to loosen the rock in large and regularly shaped masses, the holes are arranged in rows and lightly charged, so that the explosion will split the rock along ap proximately definite lines without shattering it. In tunnel-work, the object is to take out a cylindrical mass of rock the size of the tunnel cross-section. This is usually accomplished by drilling and firing a small number of converging holes so as to take out a eone-shaped or wedge shaped centre core, and then to enlarge this opening by drilling and blasting successive rings of holes surrounding this centre opening. In excavating a canal through rock, a row of holes is usually drilled transversely across the canal so as to throw down and break up a slice or sec tion of rock extending from one side of the channel to the other. See TUNNELS; QUARRY. QUARRYING; and CANALS.
Blasting by mines is employed when great ledges of rock have to be removed at one opera tion, or where a large quantity of broken stone is required. Mining is usually accomplished by sinking vertical shafts into the top of a cliff, or by driving similar shafts horizontally into its face. or by a combination of the two methods. The methods of blasting by mines will he better explained by describing a number of typical blasts of this sort than by a general discussion of processes. In the work of removing the rock
obstruction of the Danube River known as the 'Iron Gates' a verticaZ cliff, Groben Point, was removed by a succession of notable mine-blasts. The largest of these blasts was made in May, 1894. A tunnel 3 X 4 feet in cross-section and SO feet long was driven into the face of the cliff and enlarged at its inner end into a 6 X 6 X6 foot chamber. This chamber was filled with 12 tons of second-grade dynamite, the tunnel packed with loose rock, and closed with a brick masonry wall. The firing of the blast removed about 78,000 cubic yards of rock. To break down the face of a quarry at South Bethlehem, N. Y.. in 1889, 50 vertical holes 30 feet deep and 7 feet apart were driven near the edge of the cliff, and charged with 40 pounds of dynamite each. This blast displaced between 20,000 and 25,000 tons of rock. In a similar blast at Crarae Quarry in Argyllshire, Scotland, in 1886, a blast of 14.000 pounds of powder dislodged 60,000 tons of gran ite rock. To secure a large quantity of rock for building a dam near Teller. Colo., a granite mound known as Vesuvius Butte was blown up on December 18, 1899. A horizontal tunnel, with several angles to prevent the blowing out of the blast. was driven into the side of the butte near its bottom until the inner end was about at the centre of the butte. From the end of this tun nel a horizontal tunnel 70 feet long was driven to the right and to the left, so that the whole excavation formed a T-shaped tunnel. The ends of the cross-arm of the T were packed with some 32,000 pounds of black powder and the remaining space filled with rock and earth. The main tun nel was also packed with earth, with green tim ber bulkheads placed at each angle. This blast was fired by electricity, and its explosion opened up a crater 72 feet deep and 150 feet in diameter, and broke up 110,000 cubic yards of rock. By far the largest blast ever made was fired in con nection with the removal of Hell Gate Rock in the East River at New York in 1576. This rock had an area of about 9 acres, and to remove it 24 longitudinal and 46 transverse galleries or tun nels, with their faces pierced with 12,561 holes 3 inches in diameter and 9 feet deep, were exca vated in its interior. Into each drill-hole was inserted a rack-a-rock cartridge, and then a dy namite cartridge, in all 240.399 pounds of rack a-rock and 42.331 pounds of dynamite, and then water was admitted to the mine. The blast was fired by electricity, and removed 270.717 cubic yards of rock, while about 80,000 cubic yards of rock had been taken out in excavating the gal leries.