A Treatise On The Principles And Practice Of Harbour Engineering

STONE: NATURAL AND ARTIFICIAL. 95 upon. A complété installation of coinpressed air drilling-plant was put down, and the work of boring was let to a firm of oontractors. Under the new system, both single firing and mine blasting operations were carried on. In the first case, holes 20 feet deep and 2| inches in diameter were charged with 20 to 50 Ibs. of gelignite. In the second case, the method adopted where the cliff was lofty and the rock exceptionally hard, a tunnel some 40 feet long was driven square into the face of the cliff, with two branches, eaeh also about 40 feet in length, right and left of it, the com- bined galleries taking the shape of the letter T. At the ends of the cross tunnels small chambers were formed, within which were placed a charge of, usually, 7 tons of gunpowder in two boxes; the tunnels were then built up and the charges fired by electricity. Very little noise or shock to the neighbourhood is said to have been caused by the firing, although as much as 113,000 tons of rock have been dislocated by a 7-ton charge. The yield, however, varied considerably, according to the nature of the rock at various places along the half mile of quarry face, and in some cases 9 tons of gun- powder were required to produce a fall of 70,000 tons of rock. Generally speaking, it was found that where a drill could penetrate 15 feet per day—the average depth of the holes,—the cost of single hole firing was equal to the cost of mine-firing with a working face of 120 feet. In other words, when the height of quarry face exceeded 120 feet, or when the drills failed to accomplish 15 feet per drill per day, mine-firing proved the more economical method. The rock, having been blasted, was loaded into waggons. Stones from 3 to 15 tons weight were tipped on the sea side of the breakwater ; those from 1 cwt. to 3 tons on the harbour side. Stones of 1 cwt. and less were sent to a ballast crusher for use in the concrete blockwork. The rock-getting and depositing plant consisted of one 120 H.-P. air-com- pressor engine, nine 8 H.-P. Ingersoll rock drills, five locomotives, fifteen steam cranes of powers ranging from 1J to 15 tons, and 175 waggons. The quantity of rock dealt with amounted in all to about two million tons. Quarrying for Alderney Breakwater.1—The stone of which the greater part of the breakwater is built is a local stone obtained from the Mannes quarries, and, although a sandstone grit, considerably harder than granite. The quarries were situated a couple of miles away from the site of the breakwater, and had a working face 75 feet high. The hardness of the stone may be gauged from the fact that where one jumper sufficed to bore a hole in granite, two were required for the Mannez stone. The mode of quarrying was as follows :—Shot holes from 6 to 8 feet deep were drilled at the toe of the quarry face, charged and exploded. When the rock was sufficiently undermined in this manner, a deep hole was drilled down from the top of the quarry to a slielf or bed, of which there were several, inclined at an angle of from 25° to 45° to the face. This hole was charged 1 Vide Min, Proc, Inst. C.E., vol. xxxvii. p. 86.