Fig. 57.—General Details of the Brandt Rotary Drills Employed at the Simplon Tunnel.Larger illustration
Fig. 57.—General Details of the Brandt Rotary Drills Employed at the Simplon Tunnel.
Larger illustration
—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 aplane 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 27⁄8ins. 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 23⁄4ins. 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. 21⁄2ins., the tool is quickly withdrawn from thehole 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 113⁄4ins. 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 151⁄2sq. 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 21⁄2tons is available. In the rock found at Iselle, namely, antigorio gneiss, a hole 23⁄4ins. 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 21⁄2hrs.
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 rocksides; 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 13⁄4hr. to 21⁄2hrs. 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 13⁄4hr. and 21⁄2hrs.; charging and firing, 15 mins.;clearing away débris, 2 hrs.; or for one whole attack, between 41⁄2hrs. and 51⁄2hrs., 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, furnishing1⁄10gallon 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.
—Water is abundant at either end, and therefore hydraulic power is the motive force employed. On the Italian side, a dam 5 ft. high has been thrown across the Diveria at a point near the Swiss frontier, about 3 miles above the site of the installations. A portion of the water thus held back enters, through regulating doors and gratings, a masonry channel leading to two parallel settling tanks, each 111 ft. by 16 ft., whence, after dropping all its sand and solid matter, the nowpure water passes into the water-house, and, after flowing over a dam, through a grating and past the admission doors, enters a metallic conduit of 3-ft. pipes. Each of the settling tanks and the approach canal are provided with doors at the lower end leading direct to the river, through which all the sand and solid matter deposited can be scoured naturally by allowing the river-water to rush freely through. For this purpose the floor of the basins is on an average gradient of 1 in 30. For a similar reason the river-bed just outside the entrance to the approach canal is lined with wooden planks, from which the stones collecting behind the dam can be scoured by allowing an iron flap, hinged at the bottom, to change its position from the vertical to the horizontal in a gap left purposely in the dam, so causing a rushing torrent to sweep it clean.
The chief levels are:
giving a total fall of 175.20 ms. or 570 ft., and a pressure of 17.52 atmospheres.
The quantity of water capable of being taken from the Diveria in winter, when the rivers which are dependent upon the mountain snows for their supply are at their lowest, is calculated to be 352 gallons per second. Thus, taking the fall to be diminished by friction, etc., to 440 ft., and the useful effect at 70%, there is obtained 2000 H. P. on the turbine shaft.
The metallic conduit varies in material according to the pressure; thus cast-iron pipes 3 ft. in diameter and13⁄16in. thick are used up to a pressure of 2 atmospheres, from which point they are of wrought-iron. The cast-iron portion has of late caused a good deal of trouble, owing to settlement of the piers causing occasional bursts, consequently a masonry pier has been placed under each joint of this portion. The following table gives the thicknesses and diameters, varying with the pressure:
This pipe is supported every 30 ft. on small masonry piers, on the top of which is placed a block of wood hollowed out to receive the pipe, thus allowing any movement due to the contraction and expansion of the conduit. However, to prevent this movement becoming excessive, the pipe is passed at intervals of 300 yds. to 500 yds. through a cubical block of masonry of 13 ft. side, strengthened by longitudinal tie-bars. Five bands of angle-bar riveted round the pipe, with their flanges embedded in the masonry, constitute a rigid fixed point. Straw mats are thrown over the pipe where it is exposed to the sun. The temperature of the conduit is not, however, found to vary greatly, since the pipe is kept full of water. To supply the rock-drills with water at a maximum pressure of 100 atmospheres, or 1470 lbs. per sq. in., a plant of four pairs of high-pressure pumps has been laid down, and a still larger addition is in course of erection. At present, two Pelton turbines of 250 H.P. each, running at 170 revolutions per minute, drive the pumps, by means of toothed gearing, at 63 revolutions per minute. These pumps are of very simple but strong construction, single suction and double delivery, entailing one suction and one delivery-valve, both heavy and both of small lift. The larger portion of the plunger has exactly double the cross-sectional area of the smaller portion, so that in the forward stroke half of the water taken in at the last admission is pumped into the high-pressure mains, and at the same time a fresh supply of water is sucked in. During thebackward stroke half of this new supply is pumped into the mains, and the remainder enters the second chamber, to be pumped during the next forward stroke. Thus the work done in the two strokes is practically the same. The pumps are in pairs, and are set at an angle of 90°, to insure uniform pressure and uniform delivery in the mains. Their size varies; but at Iselle there are three pairs, with a stroke of 2 ft. 21⁄2ins., and the plungers of 211⁄16in. and 17⁄8ins. (approximately) in diameter, supplying 1.32 gallons per second.
To avoid injury to the valves, the water to be pumped is taken from a stream up the mountain side, and is passed through filter screens. The high-pressure water, after passing an accumulator, enters the tunnel in solid drawn wrought-iron tubes, 31⁄8ins. in internal diameter,3⁄16in. thick, and in lengths of 26 ft. The diameter of these mains varies with their length, so as to avoid loss of pressure. With the 1250 yds. of tunnel now driven 10 atmospheres are lost.
At Brigue the installations are, as far as possible, identical. The Rhone water, however, before reaching the water-house, is carried from the filter basins, a distance of 2 miles, in an armored canal built upon the Hennebique system,[9]the walls and supporting beams, of cement concrete, being strengthened by internal tie-bars of steel. The concrete struts, resembling balks of timber at a distance, are occasionally 35 ft. high and 1 ft. 71⁄2ins. square. The metallic conduit is 5 ft. in diameter, with a minimum flow of 176 cu. ft. per second and a total fall of 185 ft. In case water-power should be unavailable, three semi-portable steam engines, two of 80 H.P. and one of 60 H.P., are always kept in readiness at each end of the tunnel, and are geared by belts to the turbine shaft.
[9]Network of steel rods embedded in concrete.
[9]Network of steel rods embedded in concrete.
—In tunneling, one of the most important problems to be solved is that of ventilation, and it is for this reason that the Simplon tunnel consists of two parallel headings with cross cuts at intervals of 220 yds. At Brigue, a shaft 164 ft.deep was sunk through the overlying rock until the “gallery of direction” was encountered. Up this chimney the foul air is drawn by wood fires, the fresh air—a volume of 19,000,000 cu. ft. per day, or 13,200 cu. ft. per minute—entering by heading No. 2, penetrating up to the last cross gallery, and returning by tunnel No. 1. The entrances of No. 1 and the “gallery of direction,” besides those of all the intermediate cross galleries, are closed by doors. By this arrangement, however, fresh air does not reach the working faces; therefore a pipe, 8 ins. in diameter, is led from the fresh air in No. 2 to within 15 yds. of the face of each heading, and up this pipe a draft of air is induced by means of a jet of water, the volume to each face being 800 cu. ft. per minute. One single jet of water from the high-pressure mains, with a diameter of1⁄16in., is capable of supplying over 1000 cu. ft. of air per minute at the end of 160 yds. of pipe, and during the attack the men at the drills are in a constant breeze with the thermometer standing at 70° F. At Iselle, air is blown into the entrance of heading No. 2 at the rate of 14,100 cu. ft. per minute by two fans driven from the turbine shaft. This air travels from the fans along a pipe 18 ins. in diameter, till a point 15 yds. up the tunnel is reached, where beyond a door the pipe narrows to form a nozzle 10 ins. in diameter. This door is kept open to allow the outside air to be induced up the tunnel, as the headings are at present only 2500 yds. long, giving a resistance of not quite sufficient power to cause the air to return. The fresh air then travels up No. 2, crossing over the top of the “gallery of direction,” from which it is shut off by doors, to the last cross gallery, returning by No. 1, and finally leaving either by the “gallery of direction” or by No. 1. A system of cooling the air and driving it on by means of a large number of water-jets will be installed in No. 2 where that heading crosses over the “gallery of direction,” but at present there is no need for it.
The average temperature at the face is 73° F. during the drilling operation, 76° F. after firing the charges, and a maximumof 80° F., lately attaining to 86° F. on the south side, with 80° F. and 85° F. before and after firing. The temperature of the rock is taken at every 110 yds. in holes 5 ft. deep, and shows a gradual increase according to the depth of over-laying rock, to the conductivity of the rock, and to the form of the mountain surface. The maximum hitherto reached on the north side is 68° F., while on the south side, although a smaller distance has been traversed, it attains to 79° F., due to the more rapid increase in depth. Moreover, the temperature of the rock is observed at the permanent stations, 550 yds. from the entrances, in its relation to that of the tunnel and outside air, and though on the north side that of the rock varies almost as quickly as that of the tunnel air, on the south it is influenced very much less.
A few statistics may be of interest with regard to the progress of the last three months (taken from the trimestrial report of January, 1900). At Brigue, where there are three drilling-machines in No. 1 and two in the parallel heading, the total length excavated was 995 yds. or 6409 cu. yds. in 89 working days, the average cross-sectional area being 57 sq. ft. This required 507 attacks and 3066 holes, which had a total depth of 26,600 ft. and 14,700 re-sharpenings of the drilling-tool, with 44,000 lbs. of dynamite.
The average time occupied in drilling was 2 hrs. 45 mins., while charging, firing, and clearing away the débris took 6 hrs., 35 mins. At Brigue 648 men and 29 horses were employed at one time in the tunnel. At Iselle the numbers were 496 men and 16 horses, working in shifts of 8 hrs. Outside the tunnel, in the shops, forges, etc., the men work 8 hrs. to 11 hrs. per day, the total being 541 men at Brigue and 346 men at Iselle. On the Italian side, where the rock is very much harder, there were three drilling-machines in each heading; the total length excavated, with a cross-sectional area of 62 sq. ft., was 960 yds. or 6700 cu. yds. in 91 working days. This required 61,293 re-sharpened tools, 758 attacks, 7940 holes with a total depthof 33,000 ft., and 56,000 lbs. of dynamite. The average time spent in drilling was 2 hrs. 55 mins., and in charging and clearing 2 hrs. 36 mins. Thus, in the hard gneiss, to excavate 1 cu. yd. of rock required 81⁄2lbs. of dynamite, and each tool pierced 61⁄2ins. of rock before it required re-sharpening.
The drift method of excavating tunnels was followed in Section IV of the New York Subway, under Park Avenue between 33rd and 41st Streets. At this point the four tracks of the subway pass under a rocky elevation, known as Murray Hill, in two double track parallel tunnels, 43 ft. apart, center to center. Here already existed a double track tunnel which was built many years ago by the New York Central and Hudson River R.R., and is now used by the Madison Avenue surface cars. The two subway tunnels were driven close below the existing tunnel and also very near the foundations of expensive residences along Park Avenue, particularly on Murray Hill, one of the best residential sections of the city.
—The material penetrated by the excavation consisted chiefly of a surface outcrop of the mica-schist rock which underlies Manhattan Island. The rock was for the most part in compact strata, dipping at about 45° from East to West, but at intervals an unstable stratum was encountered which when free slid on the underlying stratum. Troubles from such slides were experienced during the construction of the tunnel.
—The cross-section selected for the tunnels had vertical side walls and a three-centered roof arch with the flattest curve at the crown. The interior dimensions were 25 ft. wide and 16 ft. high. The selected cross-section was not the best suited for a tunnel to be driven through rock, where the sharpest curve should be at the top, but in this case the flattened curve was chosen because of local conditions; chiefly, the presence of the existing tunnel and the consequent necessityof leaving a certain thickness of rock between it and the new tunnel, without depressing very much the grade of the subway.
Fig. 58.—Sequence of Excavation in the Murray Hill Tunnel.
Fig. 58.—Sequence of Excavation in the Murray Hill Tunnel.
—The two parallel tunnels were driven exclusively from the ends reached by shafts; thus the tunnels were attacked at four parts. It was in these tunnels that a comparative test was made of the different methods of driving tunnels through rock. The contractor applied the heading and drift method at the southern ends of the tunnels, the eastern tunnel being driven by means of a drift while in the western tunnel the usual heading method was followed. This latter method is illustrated in thechapter followingand the eastern tunnel at 33rd Street, excavated by means of a drift, is here considered.
Fig. 58shows the sequence of cuts adopted for this tunnel. It was begun by a bottom drift, about 10 ft. high, 8 ft. wide and 7 ft. deep, which was located at one side of the axis of the tunnel, as indicated in the figure. This drift was immediately widened by removing the portions marked 2. About 50 ft. in the rear the part marked 3 was taken away, thus clearing the entire lower portion of the tunnel. Section 4, about 50 ft. to the rear of section 3, was then broken down and removed.
The methods of drilling and blasting were as follows: In taking out the original drift, a wedge-shaped center cut was made and then enlarged to the full size of the drift by drilling parallel holes. The succeeding sections, 2 and 3, were removed by driving parallel holes, while the top section, 4, was taken away by a center cut and parallel holes. The drills were mounted on columns, two drills to a column, and the holes were usually drilled about 7 ft. deep, starting with a diameter of 23⁄4in. and ending with a diameter of 13⁄4in. They were blasted with 40%dynamite in light charges, only a few holes being fired at a time, usually not more than three or four.
Fig. 59.—Traveling Platform for the Excavation of the Upper Side of the Murray Hill Tunnel.
Fig. 59.—Traveling Platform for the Excavation of the Upper Side of the Murray Hill Tunnel.
To remove section 4, a traveling platform 101⁄2ft. long and 25 ft. wide was used. This platform, as shown inFig. 59, consisted of two longitudinal beams mounted on four double flanged wheels which were running on tracks laid 23 ft. apart. Resting on top of these beams were four 12 in. × 12 in. uprights braced in every direction against the framework of the platform. This frame was built of 12 in. × 12 in. beams laid longitudinally, the transverse beams being 12 in. × 14 ins. The platform proper was made of 3 in. planks, and was set 9 ft. above the tunnel floor. The columns supporting the drills for the excavation of the upper section 4, were set up above the platform which was then reinforced by other vertical props, as indicated by the dotted lines in the figure. These props, however, were placed so as to leave a clearance beneath the platform for the cars to carry away the débris from the front. During the blasting the platform was moved back so that the blasted rock fell to the floor of the tunnel, whence it was loaded into boxes on the cars.
—When the rock was seamy and full of fissures, running in every direction, it was necessary to support the roof of the excavation. This was done in the following manner: After part 4 was removed the timbers supporting the roof of the excavation were set up. In this case, the polygonal strutting was used. This consisted of heavy timber frames placed transversely to the axis of the tunnel and supporting the planks or poling-boards which ran longitudinally against the roof of theexcavation. The seven-segment arch frame was used in the Murray Hill tunnel. At the bottom of part 4 were placed longitudinally 12 × 16 in. beams and upon them rested the inclined segments which, with a horizontal one, formed the arch frame as shown inFig. 60. When the pressures were too heavy the crown segment was reinforced by a 6 × 12 in. beam, kept in place by two 12 × 12 in. inclined props which rested on the templates. As the tunnel was lined with concrete, the timbering was left in place and it was built outside the line of the extrados of the concrete lining. Timbering was only used for a short distance but it necessitated a larger amount of rock excavation when it was required.
Fig. 60.—Timbering Used in the Murray Hill Tunnel.
Fig. 60.—Timbering Used in the Murray Hill Tunnel.
—Great efficiency was shown in the method of hauling away the excavated materials. Three narrow-gauge parallel tracks were laid on the floor of the tunnel and extended to the faces of the advance drifts. Small flat cars were run on these tracks. They carried steel boxes, 5 ft. square and 15 ins. deep, fitted with three lifting rings and chains. When filled, the cars were run to the bottom of the shaft, the boxes were hoisted by a stiff-legged derrick placed at the shaft head, and the débris was dumped into storage bins of 300 cu. yds. capacity. These bins were elevated 8 ft. above the street so that the wagons could be driven under it to take loads of spoil by means of chutes. The broken rock was loaded into the boxes by hand.
—The tunnel was lined with concrete which was manufactured by a quite elaborate plant. A stone crushing plant, consisting of bins for raw and crushed stone, was erected at the shaft head and a mixing plant was suspended from the shaft. On the platform of the shaft head were two bins sideby side, one for crushed stone, the other for sand; both of which communicated, by means of trap doors, with a hopper chute. The materials from the hopper were delivered into a measuring box where cement was laid on top of the other ingredients by hand. They were then conveyed through a canvas chute into a cubical mixer operated by an engine. The mixer discharged its contents into skips set on cars at the bottom of the shaft and the concrete was hauled inside the tunnel ready for use.
The construction of the lining was accomplished by means of traveling platforms. The footing courses were laid first. Because these projected inward about 18 ins. from the faces of the finished sidewalks it was possible to lay a track rail on their top inner edges on each side of the tunnel. These track rails carried the traveling platforms. There were three of these platforms; the forward one was used for building the side walls; the center one, for carrying a derrick; the last one, for building the roof arch. The side wall platform was mounted on six wheels. On each side there was mounted an adjustable lagging which was curved to conform to the inside profile of the side wall. In operation this platform was run to the point where the side walls were to be constructed and the lagging was adjusted to position and fastened. Skips of concrete were then hoisted on its top, their contents were shoveled into the space between the lagging and the wall of the excavation and were there rammed into place until the finished concrete had reached the top of the lagging. When the concrete had set, the wedges holding the lagging in place were loosened and the platform was moved ahead and adjusted for building a new section of wall. The derrick platform was 231⁄2ft. wide and 18 ft. long. Transversely, it had three bays, two of which were floored over and one was left without flooring to allow passage for the concrete skips to and from the cars, on the tunnel floor beneath. At the center of the floored area was mounted a derrick to handle the skips. In operation, the derrick platform came between the side wallplatform ahead and the roof platform behind. The construction of the roof platform was practically the same as the side wall platform with the addition of roof arch centers at each bent on which lagging could be placed. The mode of procedure was to erect the form for a small space between the side walls already built and the haunches of the center, to shovel concrete from the skips and to run it into place. Then the roof lagging, a part at a time, was placed upward from the haunches and the concrete was filled and rammed behind it. The lining was built from the haunches upward until the two sides approached within a distance of about 5 ft. from each other at the crown. This 5 ft. crown strip or key was built by working from the rear toward the front end of the platform.
—The plant used by the contractors for Section IV. of the subway comprised a central power plant located about 4000 ft. from the work. This was on 42nd Street near the East River and furnished power for the work on both Sections IV. and V. The buildings consisted of an engine room 63 × 30 ft. and a boiler room, 42 × 28 ft. In the former room was located one Rand-Corliss air compressor, 22 × 40 × 48 ins., having a capacity of 5000 cu. ft. of free air per minute; in the latter room there were two 200 H.P. water tube boilers. There were also the necessary equipment of feed water pump, air condenser pump, etc. The compressors discharged into a 20 × 51⁄2ft. receiver of riveted steel through a 7 in. pipe. The air from the receiver was carried by a 10 in. pipe 3.277 ft. to the corner of Park Avenue and 41st Street, and was thence run south along Park Avenue in an 8 in. pipe, from which 3 in. branches led to the four headings of the work.
—The ventilation of the tunnel caused very little trouble. In cool weather the natural draft of the shafts and the air discharged from the drills served to keep the atmosphere wholesome. In warm weather, artificial means were necessary to clear the workings of foul air, particularly after blasting. They comprised at each end a 4 ft. American exhaustfan drawing air from a 12 in. riveted galvanized iron pipe, which extended to the working faces.
—The tunnel was lighted by electric lamps which extended even to the working face. During the blasting, however, all the lamps and wires within 100 ft. from the front were removed and gasoline torches were used; they were also employed before the electric lamps and wires could be replaced, to light the tunnel during the operation of clearing the débris.
The more common method of tunneling through hard rock is to begin the work by a heading, instead of by a drift. This heading may be of small dimensions, and the remainder of the section may also be removed in successive small parts, or it may be the full width of the section, and the enlargement of the section be made in one other cut.
Fig. 61.—Diagram Showing Sequence of Excavation in Heading Method of Tunneling Rock.
Fig. 61.—Diagram Showing Sequence of Excavation in Heading Method of Tunneling Rock.
—When the tunnel is excavated by means of several cuts, which is the method usually employed in Europe, the sequence of work is as indicated byFig. 61. Work is begun by driving the center top heading No. 1, whose floor is at the level of the bottom of the roof arch, and which is usually excavated by the circular cut method. This heading is widened by removing parts Nos. 2 and 3 until the top part of the section is removed, then the roof arch is built with its feet resting on the unexcavated rock below. The lower portion of the section or bench is removed by first sinking the trench No. 4, after which part No. 5 is taken out, and then parts Nos. 6 and 7, and the side walls built. Part No. 8 for the culvert is finally opened. The heading is, as a rule, driven far in advance, but the excavation of each of the other parts follows the preceding one at a distance behind of about 300 ft.
The strutting, when any is required, is usually the typical radial strutting of the Belgian method of tunneling. The masonry lining is constructed practically the same as in tunnels excavated by a drift. The hauling is done on a single track laid in the heading No. 1, which separates into double trackswhere the full top section has been excavated by the removal of parts No. 2. These two tracks are again combined and form a single track along the top of part No. 5, which has been left wider than part No. 4 for this particular purpose. When part No. 3 is excavated a standard-gauge track is laid on its floor; and as the full section of the tunnel is completed by taking out parts Nos. 4 and 5, this single track is replaced by two standard-gauge tracks, into which it switches. Spoil is transferred from the narrow-gauge tracks on the upper level, to the standard-gauge tracks on the tunnel floor, by means of chutes, and building material is transferred in the opposite direction by means of hoisting apparatus.
When the excavation is made by a single wide heading, and a single other cut for removing the bench, which is the method preferred by American engineers, it is called the Heading and Bench method. The work begins by removing a top heading the full width of the section; this heading is usually made 7 ft. or 8 ft. high, and is excavated by the center cut method. The method of strutting usually employed is to erect successive three- or five-segment timber arches, whose feet rest on the top of the bench; when the bench is removed, posts are inserted under the feet of each arch. These arches are covered with a lagging of plank. In America it has often been the practice to let this strutting serve as a temporary lining, and to replace it only after some time, often after years, with a permanent lining of masonry. In a succeeding chapter, some of the methods adopted in relining timber-lined arches with masonry are described. The hauling is done by either narrow or broad gauge tracks laid on the floor of the completed section below. A device called a bench carriage is often employed to enable the cars running on the heading tracks to dump their loads into the cars below, without interfering with the work on the bench front. This device consists of a wide platform carried on trucks, running on rails at the sides of the tunnel floor, so that it is level with the floor of the heading. The front of this platform carries ahinged leaf which may be raised and lowered, and which forms a sort of gang-plank reaching to the floor of the heading. By running the heading cars out on to this traveling platform, they can be dumped into the cars below entirely clear of the work in progress on the bench front.
For the purpose of illustrating the two methods of driving tunnels by a heading, which have been briefly described, the St. Gothard and the Fort George tunnels have been selected. The St. Gothard tunnel is selected, as being one of the longest tunnels in the world, and because it was excavated by a number of small parts; and the Fort George tunnel, as being a double-track tunnel, driven by a heading, and bench, and having a concrete lining.
The St. Gothard tunnel penetrates the Alps between Italy and France, and is 91⁄4miles long. It was constructed in 1872-82.
—The St. Gothard tunnel was excavated through rock, consisting chiefly of gneiss, mica-schist, serpentine, and hornblende, the strata having an inclination of from 45° to 90°. At many points the rock was fissured, and disintegrated easily, and water was encountered in large quantities, causing much trouble.
—The sequence of excavation is shown byFig. 14,p. 36. First the top center heading, No. 1, whose dimensions varied from 8.25 × 8.6 ft. to 8.5 × 9 ft., according to the quality of the rock, was driven never less than 1000 ft. and sometimes over 3000 ft. in advance of parts No. 2. The excavation of parts No. 2 opened up the full top section, and parts Nos. 3, 4, 5, 6, and 7, were removed in the order numbered.
—Where regular strutting was required, the construction shown inFig. 62was adopted.
—The St. Gothard tunnel is lined throughout with masonry. After the upper portion of the section was fully excavated, the roof arch was built with its feet resting upon short planks on the top of the bench. Plank centers were used in constructing the arch. For the arch brick masonry was employed, but the side walls were built of rubble masonry. Shelter niches, about 3 ft. deep, were built into the side walls at intervals, and about every 3,000 ft. storage niches about 10 ft. deep, and closed with a door, were constructed. The culvert was of brick masonry.
—Water-power was used exclusively in driving the St. Gothard tunnel. At the north end, the Reuss, and at the south end, the Tessin and the Tremola, rivers or torrents were dammed, and their waters conducted to turbine plants at the opposite ends of the tunnel. The power thus furnished by the Reuss was about 1,500 H.P., and the power furnished by the combined supply of the Tessin and Tremola was 1,220 H.P. The turbine plant at both ends at first consisted of four horizontal impulse turbines, but later, two more turbines were added at the south end. Each of the two sets of four turbines first installed drove five groups of three compressors each, and the two supplementary turbines drove two groups of four compressors each. The compressors were of the Colladon type with water injection, and four groups of three compressors each were capable of furnishing 1,000 cu. yds. of air compressed to between seven and eight atmospheres every hour, or about 100 H.P. per hour, delivered to the drills at the front. This air when exhausted provided about 8,000 cu. yds. of fresh air per hour for ventilation.
The compressors at each entrance discharged into a group of four cylindrical receivers of wrought-iron each 5.3 ft. in diameter by 29.5 ft. long, and having a capacity of 593 cu. ft. The cylinders were placed horizontally, the first one receiving the air at one end and discharging it at the other end into the next cylinder, and so on. By this arrangement the air wasdrained of its moisture, and the discharge from the end receiver into the tunnel delivery pipes was not affected by the pulsations of the compressors. The delivery pipe decreased from 8 in. in diameter at the receiver to 4 ins. in diameter, and finally to 21⁄2ins. in diameter, at the front.
The drills employed were of various patterns. The first one employed was the Dubois & François “perforator,” in which the drill-bit was fed forward by hand. This was replaced by Ferroux drills having an automatic feed. Jules McKean’s “perforator” was employed at the north end of the tunnel. All of these drills were of the percussion type, and were mounted on carriages running on tracks. Their comparative efficiency was officially tested in drilling granitic gneiss with an operating air pressure of 5.5 atmospheres with the following results:
The heading was excavated by the circular cut method, the holes being driven as follows: Near the center of the heading three holes were first drilled, converging so as to inclose a pyramid with a triangular base. Around these center holes from 9 to 13 others were driven parallel to the tunnel axis. The center holes were blasted first, and then the surrounding holes. From 3 to 5 hours were required to drill the two sets of holes, and from three to four hours were required to remove the blasted rock. The number of holes drilled in removing each of the various parts was as follows:
—Two different systems were employed for hauling the spoil and construction material in the St. Gothard tunnel. To remove the spoil from parts Nos. 1 and 2 a narrow-gauge track was laid on the floor of the heading, and the cars were hauled by horses, the grade being descending from the fronts. These narrow-gauge cars were dumped into larger broad-gauge cars running on the track laid on the floor of the completed section and hauled by compressed air locomotives (Fig. 63). To raise the incoming structural material from the broad-gauge cars to the narrow-gauge cars running on the level above, hoisting devices were employed.