Page:The American Cyclopædia (1879) Volume II.djvu/720

 700 BLASTING ing Pan, and Way's reef, were very consider- ably reduced by simply exploding large cani- sters of gunpowder, by means of a galvanic bat- tery, upon their surfaces. From Aug. 19, 1851, when the first blast was fired, to March 25, 1852, 28-4 charges, containing 34,231 Ibs. of powder, were exploded upon Pot rock, re- moving about 10 feet of its depth, as careful soundings have since shown, although it was asserted at the time that more had been re- moved. On Frying Pan and Way's reef 240 charges, containing about 28,000 Ibs., were ex- ploded, increasing the depth of water consider- ably. Since this pioneer work of Mr. Maillefert nitro-glycerine has been used in similar opera- tions with much greater and more satisfactory results. In fact, this compound, or some prep- aration of it, is now employed by the engineer as though it were a kind of chisel for chipping away projections of rock wherever they pre- sent themselves. Surface blasting has, how- ever, been abandoned, except for the removal of superficial or unimportant masses of rock. It has been found that when live rock, as firm, undetached, and undisintegrated rock is called, has been reached, the surface blast, even when made with nitro-glycerine, makes so little com- parative impression, that it is more expeditious and economical to drill and introduce the charge into the body of the rock. When, how- ever, it forms so much of an obstruction as to require several feet in depth and a considera- ble horizontal section to be removed, it has been found preferable to make large excava- tions into the body of the rock from beneath, proceeding according to the method of mining, and to remove the shell by the simultaneous explosion of charges introduced into it. Prac- tical applications of this method will be noticed further on. When it is designed to bore a tun- nel into a mountain, a heading, as it is called, is commenced at the floor of the tunnel and driven in the direction of its axis. If the plane of the floor is not beneath the plane upon which the work is begun, and the surface of the rock is sufficiently perpendicular, the work may be commenced by bringing a carriage, arm- ed with one or more Burleigh or other drills, to the face of the rock, drilling a horizontal line of perforations a short distance above the plane of the floor of the tunnel, driving the (See fig. 2.) If necessary, this operation is to be repeated until a step, facing downward and of sufficient depth, is formed to afford the most efficient displacement of rock by subsequent Fio. 2. Burleigh Drill at Work. drills in an obliquely downward direction, at an angle of about 45, charging the holes with gunpowder or nitro-glycerine, and firing them simultaneously by means of the galvanic battery. B Fio. 4. B' Section. Ftonl View. FIG. 3. Mode of Forming Steps ("Stoping"). blasts. Then another line of perforations is drilled in the step, in a plane parallel with its under surface, at a suitable distance above its edge, which are also charged with the ex- plosive and fired. (See fig. 3.) This process is to be repeated until the arch or crown of the tunnel is reached, and then a new bench is to be formed. This work can be advantageously performed by hand drilling, but when it is con- venient to work a power drill its employment will generally afford the greatest progress. When the tunnel is of sufficient height it is usual to drive the head- ing (H, fig. 4) forward beneath the crown, and to follow with one or more benches (B and B'). The work is al- ways driven against the perpendicular faces of the headings and benches, and in the direc- tion of the axis of the tunnel ; .but the lamina- tion of the rock may be such as to make it preferable to drill the holes in the upper surface of a bench, as at J, and throw the rock hori- zontally from the face, instead of commencing at J' and throwing it downward. Nitro-glycerine may be placed in the drill holes in cartridges, and fired without tamping or with water tamp- ing, its action being so instantaneous that a sep- aration is readily effected in the lateral direction, toward the under surface of the bench. When the floor of the tunnel lies beneath the surface and it cannot so readily be reached otherwise, or where counter tunnelling is desirable, a shaft is sunk to the required plane. The pro- cess of excavating a shaft is conducted upon principles similar to those which govern the driving of the tunnel, in so far as the forming of benches and the detaching of the rock in the di- rection of the line of least resistance is concern- ed, although a heading, from the nature of the case, could not be driven downward in advance of the rest of the shaft with any advantage. The working will of course be varied according to the structure and composition of the rock, and the position of its strata. It may happen at times that considerable portions can be removed with wedges and levers, and this may be the case in the tunnel as well as in the shaft, but not so frequently. In sinking a shaft a bench is formed, and successive portions are de-