27. Blasting

Blasting. In many cases, when rock is reached, blasting must be resorted to; in cities, this is a separate branch of contracting, and the blasting is usually let as a subcontract.

28. When a blast is to be made, a hole to receive the powder is first drilled in the rock by hand or power drills; these holes vary in diameter from 1/2 inch to 2 1/2 inches, and in depth from a few inches to many feet, while the direction varies according to the dip, or slope of the rock.

The borer or jumper used in drilling is a steel-pointed bar. Generally one man, in a sitting position, directs the drill, turning it after each blow to keep the hole cylindrical, occasionally pouring in water, and cleaning out the powdered stone with a scraper. A small rope of straw or hemp is twisted around the drill or jumper at the top of the hole. One or two men strike the jumper with sledge hammers.

When a sufficient depth is reached, the hole is dried by means of a rag on the end of a wire, and the charge of powder put in; a small rod of copper, called the needle, or nail, is inserted so as to reach the bottom of the charge. The remainder of the hole is then filled up with dry sand or tough clay, which is called tamping; if wadding is used, it is firmly rammed in by means of the tamping bar, which is a copper-faced punch of such size that it nearly fills the hole, and has a grove in it to receive the nail. This operation requires great care because of the danger of producing sparks by striking the rock with the tamping punch. Powder is now poured in the hole, and the blast is exploded by a slow match connected with it.

An improvement on this method of firing, consists in the use of a fuse; this may be described as a rope or hose, containing an inflammable composition. A suitable length of the fuse is placed in contact with the charge before tamping, and carried up to the mouth of the hole. On being lighted, it burns at the rate of 2 or 3 feet a minute, giving time for the blasters to get to a safe place before the explosion occurs.

Electricity is now used very extensively for the purpose of firing the charge, especially if the explosive is dynamite. A dynamite cartridge is placed in the hole, but no tamping is required; the cartridge is connected by two insulated wires with an electric battery, and on making the connection, by pressing a button on the outside of the box containing the battery, the charge is exploded.

In order to confine the pieces of rock that would otherwise be shot into the air after an explosion, and prevent damage to adjoining property, or possible loss of life, a number of heavy logs are placed over the rock to be blasted, and sometimes bound together by a heavy chain. The weight of the logs keeps the fragments of stone, etc. from flying when the charge is exploded.

29. Where extensive blasting operations are going on, rock-boring or drilling machines are used, insuring considerable saving in time and labor over the old method of hand drilling.

In these machines, the drill is repeatedly driven against the rock, either by compressed air or steam, the drill also being made to rotate slightly at each blow. The work of drilling can be done by the use of rock drills at least one-third cheaper than by hand power. There are several different machines; the Rand, Ingersoll-Sargeant, Burleigh, and Diamond are considered the best.

30. Wedging

Wedging. Where rock must be taken out so close to existing walls that injury might result from blasting, the operation of wedging is resorted to. This consists of breaking up the rock with wedges, which are of steel, about 8 inches long, with wire wound about them so as to form a handle. A workman holds the thin edge against the rock, and another man strikes repeated blows on the wedge with a hammer until the rock is split or broken.

When the rock breaks easily and runs in layers with defined 2-2 seams between them, large quantities may be cheaply gotten out by this means; but it is a slow and expensive process if the rock is hard and lies in large compact masses.