Method of Working.
—The drilling-machines employed are of the Brandt type, [Fig. 57], and are mounted in the following manner: A small four-wheeled carriage supports at its center a beam, the shorter arm of which carries the boring mechanism and the longer a counterpoise; near its center is the distributor. In the short arm is a clamp holding the rack-bar or butting column, which is a wrought-iron cylinder with a plunger constituting a ram, and is jammed by hydraulic pressure between the walls of the heading, thus forming a rigid support for the boring-machine, and an efficient abutment against the reaction of the drill. This rack-bar can be rotated on its clamp in a plane parallel to the axis of the beam. Three or four separate boring-machines can be mounted on the rack-bar, and can be adjusted in any reasonable position.
The boring-machine performs the double function of continually pressing the drill into the rock by means of a hollow ram (I) and of imparting to the drill and ram a uniform rotary motion. This rotary motion is given by a twin cylinder single-acting hydraulic motor (E), the two pistons, of 27⁄8 ins. stroke, acting reciprocally as valves. The cranks are fixed at an angle of 90° to each other on the shaft, which carries a worm, gearing with a worm-wheel (Q) mounted upon the shell (R) of the hollow ram (I), and this shell in turn engages the ram by a long feather, leaving it free to slide axially to or from the face of the rock. The average speed of the motor is 150 revolutions to 200 revolutions per minute, the maximum speed being 300 revolutions per minute. The loss of power between the worm and worm-wheel is only 15% at the most; the worm being of hardened steel and the wheel of gun-metal, the two surfaces in contact acquire a high degree of polish, resulting in little wearing or heating. Taking into consideration all other sources of loss, 70% of the total power is utilized. The pressure on the drill is exerted by a cylinder and hollow ram (I), which revolves about the differential piston (S), which is fixed to the envelope holding the shell (R). This envelope is rigidly connected to the bed-plate of the motor, and, by means of the vertical hinge and pin (T), is held by the clamp (V) embracing the rack-bar. When water is admitted to the space in front of the differential piston the ram carrying the drilling-tool is thrust forward, and when admitted to the annular space behind the piston, the ram recedes, withdrawing the tool from the blast-hole. The drill proper is a hollow tube of tough steel 23⁄4 ins. in external diameter, armed with three or four sharp and hardened teeth, and makes from five to ten revolutions per minute, according to the nature of the rock. When the ram has reached the end of its stroke of 2 ft. 21⁄2 ins., the tool is quickly withdrawn from the hole and unscrewed from the ram; an extension rod is then screwed into the tool and into the ram, and the boring is continued, additional lengths being added as the tool grinds forward; each change of tool or rod takes about 15 secs. to 25 secs. to perform. The extension rods are forged steel tubes, fitted with four-threaded screws, and having the same external diameter as the drill. They are made in standard lengths of 2 ft. 8 ins., 1 ft. 10 ins., and 113⁄4 ins. The total weight of the drilling-machine is 264 lbs., and that of the rack-bar when full of water is 308 lbs. The exhaust water from the two motor cylinders escapes through a tube in the center of the ram and along the bore of the extension rods and drill, thereby scouring away the débris and keeping the drill cool; any superfluous water finds an exit through a hose below the motors and thence away down the heading. The distributor, already mentioned, supplies each boring-machine and the rack-bar with hydraulic pressure from the mains, with which connection is effected by means of flexible or articulated pipe connections, allowing freedom in all directions. The area of the piston for advancing the tool is 151⁄2 sq. ins., which, under a pressure of 1470 lbs. per sq. in., gives a pressure of over 10 tons on the tool, while for withdrawing the tool 21⁄2 tons is available. In the rock found at Iselle, namely, antigorio gneiss, a hole 23⁄4 ins. in diameter and 3 ft. 3 ins. in length is drilled, normally, in 12 mins. to 25 mins.; a daily rate of advance of 18 ft. to 19 ft. 6 ins. is made in a heading having a minimum cross-section of 59 sq. ft.; the time taken to drill ten to twelve holes, 4 ft. 7 ins. deep, is 21⁄2 hrs.
When the débris resulting from one operation has been sufficiently cleared away, a steel flooring, which is provided near the face to enable shoveling to be more easily done, and to give an even floor for the wheels of the drilling-carriage, is laid bare at the head of the line of rails, and the drilling-machines are brought up on their carriage by eight or ten men. When advanced sufficiently close to the face, the rack-bar is slewed round across the gallery and is wedged up against the rock sides; connection is made between the distributor and the hydraulic main, by means of the flexible pipe, and pressure is supplied by a small copper tube to the rack-bar ram, thereby rigidly holding the machine. Next, connections are made between the three drilling-machines and the distributor, and in 20 mins. from the time the machine was brought up all three drills are hard at work, water pouring from the holes.
The noise of the motors and grinding-tools is sufficient to drown all but shouts; and where the extension rods do not fit tightly, small jets of water play in all directions, necessitating the wearing of tarpaulins by the men directing the tools. Lighting is done wholly by small oil-lamps, provided with a hook to facilitate fixing in any crack in the rock; electricity will probably be used to light that portion of the tunnel which is completed.
Two men are allotted to each drill, one to drive the motor, the other to direct and replenish the tool, one foreman and two men in reserve completing the gang. A small hammer is freely used to loosen the screw joints of the extension rods and drill. A hole is usually commenced by a two-edged flat-pointed tool, until a sufficient depth is reached to prevent the circular tool from wandering over the face of the rock, but in many instances the hole is commenced with a circular tool. The exhaust water during this period flows away by the hose underneath the motor. In the antigorio gneiss, ten to twelve holes are drilled for each attack, three to four in the center to a depth of 3 ft. 3 ins., the remainder, disposed round the outside of the face, having a depth of 4 ft. 7 ins. The average time taken to complete the holes is 13⁄4 hr. to 21⁄2 hrs. Instead of pulverizing the rock, as do the diamond drills, it is found that the rock is crushed, and that headway is gained somewhat in the manner of a circular saw through wood. The core of rock inside the tool breaks up into small pieces, and can be taken out if necessary when the drill requires lengthening.
The lowest holes, inclined downwards, are full of water; consequently two detonators and two fuses are inserted, but apart from this, water has little effect on the charge. The fuses of the central holes are brought together and cut off shorter than those of the outer holes, in order that they may explode first to increase the effect of the outer charges. All portable objects, such as drills, pipe connections, tools, etc., have meanwhile been carried back; the steel flooring is covered over with a layer of débris to prevent injury from falling rock, and to the end of the hydraulic main is screwed a brass plug pierced by five holes; and immediately the explosions occur a valve is opened in the tunnel, and five jets of water play upon the rock, laying the dust and clearing the air. The necessity for this was shown on one occasion when this nozzle was broken by the explosion and the water had to be turned off immediately to avoid useless waste; on reaching the face, the atmosphere was found to be so highly charged with dust and smoke that it was impossible to distinguish the stones at the feet, although a lamp had been placed on the ground; and despite the fact that the air tube was in full blast, the men experienced great difficulty in breathing. A truck is now brought up, and four men clear a passage in front, through the heap of débris, two with picks and two with shovels, while on either side and behind are as many men as space will permit. The stone is thrown either to the sides of the heading or into the wagon, shoveling being greatly aided by the steel flooring, which, before the explosion, had been laid over the rails for nearly 10 yds. down the tunnel to receive the falling rock. These steel plates are taken up when cleared, and the wagon is pushed forward until the drilling-machine can be brought up again, leaving the remaining débris at the sides to be handled at leisure during the next attack. The roof and side walls are, of course, carefully examined with the pick, to discover and detach any loose or hanging rock. The times taken for each portion of the attack in this particular antigorio gneiss are as follows: Bringing up and adjustment of drills, 20 mins.; drilling, between 13⁄4 hr. and 21⁄2 hrs.; charging and firing, 15 mins.; clearing away débris, 2 hrs.; or for one whole attack, between 41⁄2 hrs. and 51⁄2 hrs., resulting in an advance of 3 ft. 9 in., or a daily advance of nearly 18 ft.
From this it appears that the time spent in clearing away the débris equals that taken up in drilling, and it is in this clearing that a saving of time is likely to be effected rather than in the process of drilling. Many schemes have been tried, such as a mechanical plow for making a passage; at Brigue, “marinage,” or clearing by means of powerful high-pressure water-jets, directed down the tunnel, was tried, but the idea is not yet sufficiently developed.
Another series of experiments has been tried at Brigue with regard to the utilization of liquid air as an explosive agent instead of dynamite; and for this purpose a plant has been laid down, consisting of one ammonia-compressor, two air-compressors, and two refrigerators, furnishing 1⁄10 gallon of liquid air per hour at an expenditure of 17 H. P. The system used is that of Professor Linde, who himself directs the experiments. The great difficulty experienced is that of shortening the interval of time that must elapse between the manufacture of the cartridge and its explosion. The liquid oxygen, with which the cartridge, containing kieselguhr (silicious earth) and paraffin, is saturated, evaporates very readily, losing power every moment; hence the effect of each cartridge cannot be guaranteed, and though it is an exceedingly powerful explosive when used immediately after manufacture, no practical result has yet been obtained.