CHAPTER XX. SUBMARINE TUNNELING (Continued). THE SHIELD AND COMPRESSED AIR METHOD. THE HUDSON RIVER TUNNEL OF THE PENNSYLVANIA RAILROAD.

The shield and compressed air method of excavating subaqueous tunnels is used when the distance is small between the roof of the tunnel and the bed of the river. These tunnels are usually driven from the shafts sunk from each shore. It is very seldom they can be driven also by an intermediate shaft. This, however, was done in the case of the Belmont tunnel under the East River. Here the tunnels passed under the man-of-war reef where a working shaft was sunk.

The plant is located at some convenient point near the head shaft. It consists of a set of boilers to provide the power for the different machines. They are low and high pressure compressors, the former supply the air through the tunnel; the latter, the air for working the drills, in case rock is encountered, and power for hauling and hoisting purposes. The various pumps force the water for the hydraulic rams that drive the shield and work the erector. They also remove the water from the tunnel which always collects in variable quantities at the bottom of the excavation. Besides the machines for light and ventilation purposes, the head shaft is provided with an overhead construction where are housed the hoisting machines, the telephone and other means of communication with the work at the front. Usually a long trestle is built in connection with the head shaft, leading to the dumping place and yard. On this inclined elevated structure are located, also, the tracks upon which will run the small cars used inside the tunnel for hauling purposes.

The shafts are excavated on a square, rectangular or circular plan and are usually lined with masonry. It is only recently that shafts excavated through loose soils have been lined with the same cast-iron lining used in the tunnels, the only difference being that the rings were laid flat on the ground and attached to those already sunk.

After the shaft has been sunk to the required level, the tunnel is driven toward the river by any one of the methods used for land work. At some convenient distance from the shaft, the dimensions of the tunnel are enlarged for a length of 20 or 30 ft. In this larger space, called the shield chamber, the shield is assembled, mounted, and, when completed, it is slowly pushed toward the river. The tunnel is excavated from the shield chamber on, with dimensions equal to the exterior shell of the shield.

The construction of the shield and the hydraulic jacks used for its advance are explained in a preceding chapter.

In very loose soils, a solid bulkhead of masonry is built across the tunnel, after the shield has advanced to a certain distance and some rings of the cast-iron lining have been erected. The bulkhead is provided with three air locks—two near the floor of the tunnel, for working purposes, and one near the roof, called the emergency lock, which, as the name suggests, is used only in case of danger. The air locks are steel cylinders from 10 to 15 ft. long and 6 ft. in diameter, made up of boiler plates. They are provided with doors at each end, besides the pipes for the admission and exit of compressed air. The working locks also have narrow-gauge tracks for hauling purposes. In rock or more consistent soil the bulkhead is constructed after the shield is far ahead, since there is no immediate necessity, under these conditions, to use the compressed air. In both the loose and good soils, when the shield has been advanced over 500 ft. from the bulkhead, a second bulkhead, with air locks, is erected in the tunnel. The first is left in place but used only in case of emergency.

To direct the shield along the center line and through curves and grades, accurate measurements are taken, and the distance between the shield and the last ring inserted in the iron lining is regulated accordingly. The alignment inside the tunnel is maintained in a very simple way. For this purpose, points corresponding to the center line are marked on the roof at distances of 100 ft. Nearly 100 ft. from the shield, a transit is set up on a strong scaffold spanning the tunnel, and it is supported by the flanges of the iron lining. A plumb-line is hung from one of the points of the roof already determined, as indicating the center line; and the transit man aligns his instrument with this plumb-line; after this he “plunges” his telescope. A rodman next places a horizontal rod of special construction between the flanges of the last ring of the lining. This rod has in the center an open slot which carries a glass with a black vertical line. The slot is graduated, the zero of graduation remains in the center while the vertical line is moved right and left. The rodman places a lamp behind the slot and the transit-man tells him how to move the dark line until it coincides with the axis of the tunnel. If the ring, just erected, be a little out of alignment, it is readjusted by pushing the shield a little more on the side that has swerved from the axis of the tunnel. As the shield is pushed forward, it is kept in place by four men with graduated rods, one man on each side of the shield, one on top and the other on the floor. As the shield progresses, they repeat aloud in succession, the distance indicated on the rods, which is the distance from the shield to the outer circumferential flange of the last ring of the lining. When an advance of one foot has been made, readings are taken at every inch; and when very near the required distance, they are taken at every quarter of an inch. In this way it is not difficult to bring the shield back into line, in case it may have shifted a little to the right or left. When curves are met, the rings are no longer cylindrical segments but tores, so that the segments at one side are longer than those on the other. In this case, the shield is advanced more on one side by a quantity equal to the difference of the two sides of the ring to be erected. At each advance the shield is moved 2 ft. or 2¹/₂ ft. ahead, the distance corresponding to the length of the cast-iron rings of the lining. Within the space now open between the shield and the lining another ring is inserted. The ring is composed of different segments provided with flanges and holes bored so they can be bolted together. The segments of the lining are very heavy and difficult to handle but they are easily set by means of the erector.

When the erector is not mounted on the shield, it is located in the middle of a girder placed across the iron rings of the lining and just at the rear end of the shield. The girder, at both extremities, has flanged wheels resting on rails which are placed on brackets. These brackets are attached temporarily to the flanges of the iron lining. The erector is provided with an arm capable to swing in a full circle. Its movements are regulated by two hydraulic jacks, located horizontally on the spanning girder. On the extreme end of the revolving arm are projections with holes for the bolts. Each segmental plate of the lining has a kind of plug in the center which is cast together with the plate and is provided with holes for the bolt. In placing the segmental plates of the lining, the arm of the erector is swung over the plate to be lifted, then two bolts are passed through the holes in the projection of the erector, and through those in the plug. The arm of the erector is then moved upwards until the plate, free from all obstacles, is swung very near its intended position. There it is adjusted and held until bolts are inserted to fix it to the plates of the preceding ring.

In connection with the method of excavating submarine tunnels by means of shield and compressed air, the excavation varies with the quality of soil encountered. In compact rock the usual heading and bench method, so common in land tunnels, is also employed in this case. The shield is left behind in presence of good rock.

The men at the front attack the rock with air drilling machines and charges of dynamite. The holes are driven at a smaller depth than in land work; very light charges of dynamite are used and only a few holes fired at each round. Every precaution is taken in order not to disturb the shield and the bed of the river any more than is possible, because at a shallow depth the blast would tend to widen the existing crevices in the rock and thus permit an inflow of water. When the rock is fissured or disintegrated and the roof of the excavation at the front requires timbering, the shield should be kept closer to the front. In this way the quantity of timber for strutting is greatly reduced, so lessening the probabilities of fires. It is very difficult, in compressed air, to extinguish fires and in almost every instance the only way is to flood the tunnel. This was done at the Manhattan end of the tunnel under the East River for the extension to Brooklyn of the New York Subway.

The excavation is made by hand in loose but compact soils such as clay. The men work on platforms located at the front of the shield and they are protected from the caving-in of the roof by a hood added for working through loose soils. The men excavate the material which is shoveled inside the tunnel and is carried away in small cars. The shield is very close to the front of the excavation in loose soil. The East Boston tunnel, under Boston Harbor, connecting with the Boston Subway, was excavated through blue clay. The minimum distance between the bottom of the water and the roof of the excavation was 18 ft. The tunnel was excavated by means of compressed air and the shield which was only used for the roof. It slid on top of concrete side walls built in two drifts which were excavated nearly 100 ft. ahead of the shield. The tunnel was lined with concrete, the arch being reinforced by longitudinal steel rods which received the thrust of jacks used for advancing the shield. The material in the drifts under the shield and the bench was removed by hand and carried away in small cars.

Subaqueous tunnels driven through very loose soils can be excavated by simply leaving the doors open while the shield is pushed ahead. The material, dislodged by the cutting edge of the shield, is forced through the doors and falls on the floor whence it is removed in small cars. In very loose soils the excavation has been made in a still more economic way; the shield with closed doors is simply squeezed through the soil. This method is financially convenient, because all the excavating and hauling operations are eliminated and the tunnel progresses from 40 to 50 ft. per day, but clearly indicates a lack of stability. In this manner, the Hudson River tunnel of the New York and New Jersey Railroad was constructed.

The pressure of the air in the tunnel depends upon the depth and as a rule it varies between 20 and 40 or even more pounds per square inch above atmospheric pressure. Working in compressed air causes a peculiar disease commonly known as “bends” or “caisson disease” often proving fatal. To prevent and remedy the disease, the engineers should order a set of rules to be strictly observed. The preventative measures should be, first, to employ only sober, strong and healthy men, never one who has not successfully passed the examination of the attending physician; second, to order the lock tenders never to allow any man in or out of the tunnel unless he has spent at least ten minutes within the locks. Both compression and decompression should be thorough and it cannot be in less than this time. A stop of only a few minutes in the locks is not sufficient and this incomplete compression or decompression is the real cause of the bends. The men become careless after they have been in the compressed air for some time, and they try to reduce this tiresome operation to a minimum, hence the duty of the engineer to strictly enforce this rule. The remedial measures should consist of constant medical attendance near the shafts and the erection of a compressed air hospital where the men affected by bends for lack of decompression may be attended and cured.

THE HUDSON RIVER TUNNELS OF THE PENNSYLVANIA RAILROAD.[13]

The tunnels constructed under the Hudson River for the Pennsylvania Railroad, consist of two parallel tubes driven side by side 14 ft. apart. The tubes are of circular cross-section, 23 ft. exterior diameter, and are lined with cast-iron rings. The tunnels were driven from two shafts, one on the eastern shore of the Hudson River near 32nd St. and 11th Ave., New York; the other at Weehawken, New Jersey, near the piers of the Erie Railroad. The horizontal distance between the shafts was 6550 ft. The permanent one at Weehawken was built on a square plan, 130 ft. to a side. It was lined with concrete masonry and the walls were battered in such a way as to become the shape of an inverted frustum of a pyramid. It was provided with five openings at the bottom, four of these are used by trains that run in the open, the fifth one leads to a power house near by. During the construction of the tunnels one-third of this shaft was used for the land portion of the tunnel under Bergen Hill, while the remaining two-thirds were devoted to the construction of the tunnel under the river. The working shaft on Manhattan Island was a side shaft of rectangular plan 30 ft. by 22 ft., the tunnel proper being connected by two drifts 10 ft. by 10 ft. each. The shield rooms 23 ft. long, were situated on both sides of the river just in front of the shafts. On the New York side, the shields, one for each tube, were built inside the iron lining of the shield chamber, and the hoisting tackle was slung from the iron lining. The erection on the Weehawken side was done in the bare rock excavation where timber falsework was used. After the shields were finished and in position, the first two rings of the lining were erected in the tail of the shield. These rings were firmly braced to the rock and the chamber lining; then the shields were shoved ahead by their own jacks, another ring was built and so on.

Shield.

—The shields used in these tunnels were designed by Mr. James Forgie, M. Inst. C. E. and M. Am. Soc. C. E., and were provided with three innovations: the segmental doors, the sliding platforms and the removable hood. The shields, Fig. 138, were circular, 23 ft. 6¹/₄ ins. in external diameter, and were 16 ft. long, exclusive of the hood. The tail of the shield overlapped the lining, the maximum being 6 ft. 4¹/₂ ins. during ordinary working; the minimum, 2 ft. during the operation of taking any ram out for repairing. The shields had only one transverse bulkhead made up of two continuous horizontal platforms and three vertical partitions stiffening angular web plates fore and aft the ram chambers. They were connected by angles and skin plates which formed a ring-shaped frame 25 ins. thick radially and nearly 5 ft. long. Between the vertical and horizontal partitions were left openings which either were partially or entirely closed by segmental doors pivoted on an axis parallel to the face of the shield bulkhead. There were nine of such openings on each shield, the clear width being 2 ft. 7 ins., the height varying from 2 ft. 2 ins. to 3 ft. 4 ins., according to the location. The hood at the front of the shield was designed so as to be detached underground and was made of complete segments to permit easy erection or detachment. The hood was extended as far as the upper platform, thus protecting only the roof of the excavation. It was attached to the shield by means of bolts, and, when removed, it was replaced by the cast-steel cutting-edge, built in 24 sections and placed all around the shield. The eight sliding platforms, another characteristic of this shield, could be extended 2 ft. 9 ins. in front of the shield by means of hydraulic rams, and, when so extended, were able to stand a pressure of 7900 lbs. per sq. ft. These sliding platforms were used as hoods for the protection of the men working through loose soils, while in rock they enabled the drilling and blasting to be carried on at three levels. A water trap or bird fountain was constructed, at the rear of the bulkhead of the shield, by means of angle irons to which steel plates were bolted. The opening to the face was so spacious that in an emergency the men could readily escape by getting over this trap into safety. Besides, with the assistance of compressed air, it was sufficient to perfectly trap the water-bearing ground, in case the face collapsed. Including rams and erector, the total weight of the shield was 193 tons.

Hydraulic Rams.

—The shield was operated by hydraulic pressure. The machines were designed for a maximum pressure of 5000 lbs., to a minimum of 2000 lbs., while the average working pressure was 3500 lbs. per sq. in. The forward movement of the shield was obtained by means of 24 single-acting rams 8¹/₂ in. in diameter and with 38 in. stroke. Each ram exerted a pressure of nearly 100 tons, so that the combined action of the 24 rams was equal to 2400 tons. Each sliding platform was operated by two single-acting rams 3¹/₂ ins. in diameter and with 2 ft. 9 in. stroke. The rams were attached to the rear face of the shield and the front ends of the cylinders to the front ends of the sliding platforms, and since the cylinders were movable and free-sliding so also were the platforms.

Erector.

—The erector, a box-shaped frame mounted on a central shaft, revolved in bearings attached to the shield. Inside this frame there was a differential hydraulic plunger of 4 in. and 3 in. diameters and 48 in. stroke. To the plunger head were attached two channels which slide inside the box frame and to the projecting ends of which the grip was attached. At the opposite end of the box frame was attached a counter-weight which balances about 700 lbs. of the tunnel segment at 11 ft. radius. The erector was revolved by two single-acting rams fixed horizontally to the back of the shield, above the erector pivot, through double chains and chain wheels which were keyed to the erector shaft.

Air Locks.

—Two bulkhead walls, forming the rear closure of the pneumatic sections, were built in each end of each tunnel, one just ahead of the shield chamber, the other about 1200 ft. ahead of the first. The walls were built of Portland concrete 10 ft. thick, and they were grouted with Portland cement, under a pressure of nearly 100 lbs. per sq. in., to make them thoroughly air-tight. Each wall had in it three locks; for man, material and emergency. Each was equipped with hand valves arranged to be operated from either outer end or from within. The floors of the man and material locks were on a level with the working platform of the tunnel, about 3 ft. 6 ins. above the invert; the floor of the emergency lock was about 5 ft. above the horizontal axis of the tunnel. The locks were made of steel plates and shapes, with iron fittings riveted and bolted together. The man lock was 11 ft. long of elliptical cross-section, 6 ft. vertical diameter and 5 ft. horizontal; the material lock was 25 ft. long, with circular cross-section, 7 ft. diameter, and the emergency lock was 20 ft. long, of elliptical cross-section, 4 ft. vertical and 3 ft. horizontal diameters. Fig. 139 shows the elevation of the air lock used in the Pennsylvania tunnel.

Excavation.

—In driving these tunnels almost any kind of material was encountered, viz., rock, partly rock, and partly loose soil, sand and gravel, and finally silt.

Rock.

—Much of the rock excavation was made before the shields were erected in order to avoid the handling of rock through the narrow openings of the shield doors. Throughout the cross-section the shield traveled on a cradle of concrete in which 2 or 3 steel rails were imbedded. At the points where the excavation had been made for the full section of the tunnel, it was only necessary to trim off the projecting corners of rock. Where only the bottom heading had been driven the excavation was completed just in front of the shield; the drilling below the axis level being done from the heading itself, and above that from the front sliding platforms of the shield. The holes were placed near together and were drilled short; very light charges of powder were used in order to lessen the chance of knocking the shield about too much.

Mixed Face.

—When the rock dipped to such an extent that the front of the tunnel was excavated partly in rock and partly in loose soil, the compressed air was turned on, starting with a pressure varying from 12 to 18 lbs. When the surface of the rock was penetrated, the soft face was held up at first by horizontal boards braced from the shield until the shield was shoved. The braces were then taken out and, after the shield had been shoved, were replaced by others. As the amount of soft ground in the surface increased, the system of timbering was gradually changed to one of 2-in. poling-boards. These rested on top of the shield and were supported by vertical breast-boards which in turn were held by 6-in. by 6-in. walings, braced through the upper doors to the iron lining and from the sliding platforms of the shield.

Sand and Gravel.

—Sand and gravel were only met at Weehawken, where two different methods were used. The first method was employed when the roof of the excavation was through sand. It consisted of excavating the ground 2 ft. 6 ins. ahead of the cutting-edge, the roof being held in place by longitudinal poling-boards. These boards rested on the outside of the skin at their back end, and at the forward end on vertical breast-boards, braced from the sliding platforms and through the shield doors to cross timbers in the tunnel.

The second method of timbering was used in the presence of gravel at the upper part of the excavation. In such a case, the excavation was only carried 1 ft. 3 ins. (half a shove) ahead of the cutting-edge, the roof being supported by transverse boards held by pipes which rested in holes left in the shield. After a small section of the ground had been excavated a board supported by a pipe that was inserted underneath and wedged to it was placed against the ground. These polings were kept below the level of the hood, so that when the shield was shoved, they would come inside of it; in addition they were braced with vertical posts from the sliding platform. The upper part of the face was held by longitudinal breast-boards braced from the sliding platform by vertical pieces. The lower part of the face was supported by vertical sheeted poling, braced to the tunnel through the lower doors. Straw and clay were used in front of the boards to prevent the escape of air which was very large, when the tunnel was excavated through sand and gravel. The average rate of progress in these materials was 5.1 ft. per day.

Silt.

—When silt was encountered, the shield was shoved into the ground without any excavation being done by hand ahead of the diaphragm. As the shield advanced the silt was forced through the doors into the tunnel. Forcing the shield through the silt resulted in raising the bed of the river, the amount that the bed was raised depending on the quantity of material brought into the shield. When the whole volume of the excavation was brought in, the surface of the bed was not affected; when about 50% was taken in, the surface was raised about 3 ft.; if the shield was driven blind, the bed was raised about 7 ft. When the shield was driven blind, the tunnel began to rise for about 2 ins., and the iron lining was distorted, the vertical diameter increasing and the horizontal one decreasing by about 1¹/₄ ins. It was found, however, that the tunnel was not affected when part of the excavation was taken, but if all of it was taken in or the shield was shoved with open doors, the tunnel was lowered. A powerful aid was thus found for the guidance of the shield; for, if high, the shield could be brought down by increasing the quantity of muck taken in, if low, by decreasing it.

The junction of the shields under the river was made as follows: When the two shields of one tunnel, which had been driven from opposite sides of the river, approached within 10 ft. of each other, they were stopped; a 10-in. pipe was driven between them, and a final check of lines and levels was made through the pipe. One shield was then started up with all doors closed, while the doors of the stationary shield were opened for the muck driven ahead by the moving shield. This was continued until the cutting-edges came together. All doors in both shields were then opened and the shield mucked out. The cutting-edges were taken off and the shields moved together again, edge of skin to edge of skin. As the sections of the cutting-edges were taken off, the space between the skin edges was poled with 3-in. stuff. When everything except the skin had been removed, iron lining was built up inside the skins; the gap at the junction was filled with concrete and long bolts were used from ring to ring on the circumferential joint.

Lining.

—The tunnels were lined with cast-iron circular rings of the segmental bolted type. In some special cases, cast steel was used instead of cast iron. The rings were made 30 ins. long, with an internal diameter of 21 ft. 2 ins. and an external one of 23 ft. The rings were composed of nine equal segments of 77¹/₂ ins. external circumferential length each, except the two segments adjoining the key which were equal to the other segments with the difference, that one end joint was not radial but formed so as to make an opening 12.25 ins. wide at the outside and 12.60 ins. at the inside, which was closed by the key segment. Each segment had six bolts in the circumferential joint, the key had one, so that there were 67 bolts in one circumferential joint. Each of the twelve longitudinal or radial joints had five bolts, in all 127 bolts per ring. The circumferential flanges of each plate were strengthened by two transverse webs or feathers on each flange. Each segment was provided with a 1¹/₂ in. grout hole closed with a screw plug. In order to pass around curves, whether horizontal or vertical, or to correct deviation from the line or grade, tapering was used; by this is meant the placing of rings in the tunnels which were wider than the standard rings, either at one side (horizontal tapers or liners), or at the top (depressors), or at the bottom (elevators). Tapers ¹/₂, ³/₄ or even 1 in. were used. The taper rings were made by casting a ring with one circumferential flange much thicker than usual and then machining it off to the taper.

Grouting.

—From the exterior of the tunnel already lined with cast-iron rings, grout was forced through the holes closed by screw-plugs, at a pressure of 90 lbs. per sq. in. The grout was composed of 1 Portland cement and 1 sand by volume and was forced in by a specially constructed machine, so it formed a shell of cement nearly 3 ins. thick around the exterior of the iron lining. The grouting began at the lower segment; the cement was forced in until it reached the hole above, then the hole was plugged, and the grouting was carried on from the consecutive hole and so on until all the tunnel was finally encased in grout, as it filled every crevice between the outside of the lining and the ground as excavated. The cast-iron rings of the tunnel were covered with a concrete lining which was placed in the following order: First, on the invert; second, on the duct benches; third, on the arch; fourth, on the ducts; fifth, on the face of the bench. Before any concrete was placed, the surface of the iron was cleaned by scrapers and wire brushes and by washing it with water. The invert was built in sections 30 ft. long and the duct benches were constructed soon after. These duct benches were built with several steps for the ducts to be laid later. They were built by means of a traveling stage on wheels which ran on tracks on the working platform of the tunnel. The arch was constructed soon after. First the portion from the duct benches to the haunches, then the arch proper, was built on traveling centers on tracks laid on the steps of the duct benches. The concrete was received in ³/₄-cu.-yd. dumping buckets, from the flat cars on which they were run; the buckets were hoisted to the level of the lower platform of the arch by a small Lidgerwood compressed air hoister. At this level the concrete was dumped on a traveling car or stage and moved in that to the point on the form where it was to be placed. For the lower part of the arch the concrete was thrown directly into the form from this traveling part of the stage. Fig. 140 shows the cross-section of the tunnel with the iron lining and concrete.

Hauling.

—A working platform, made up of 5-ft. sections, was built inside the tunnel and kept close to the shield. On this platform two lines of industrial railway tracks with switches and sidings at the locks, and a heading, were laid for hauling materials and spoils. These lines converged into a single track in passing through the air locks. At the shaft elevators, they terminated in a steel plate floor to avoid switches. Between the locks of the bulkheads was installed an electrically driven cable system, to haul the loaded muck up grade and to empty the flat cars. From the first bulkhead to the shaft, the cars were hauled up grade by a steam hauling engine. At the Manhattan end there was one 10-H.P. engine for each tunnel, while at Weehawken one 25-H.P. engine served for both tunnels. Each shaft contained two elevators driven by a double-cable, reversible single-drum steam-hoisting engine. A grouty frame was built over the shafts, and on the platforms over this frame were narrow-gauge tracks, extending from the elevators to the muck-chutes and to points where the lining segments were loaded on the cars. The elevators were arranged to stop at both the ground and the grouty platform levels. The rolling-stock at each of the tunnels consisted of 75 flat cars for moving the tunnel segments, and of about 50 muck cars, each of 1¹/₄ cu. yd. capacity.

Plant.

—The plants located at each end of the tunnel near the shafts were almost identical. Each consisted of three 500-H.P. Stirtling boilers, which supplied steam at 150 lbs. pressure. Feed water was supplied by three 13¹/₂ metropolitan injectors, and two Blake duplex pumps. Two Worthington surface condensers, each of 2250 sq. ft. condensing surface, took care of the exhaust from the engines and compressors. Condensing water was pumped from the river through a 16-in. pipe. The high-pressure air was supplied by a duplex Ingersoll-Sergeant compressor, with cross-compound steam end 14× 26× 30 ins. and simple water-jacket air cylinders 13¹/₄× 36 ins. Its capacity at 100 r.p.m. was 1085 cu. ft. free air per minute. The maximum pressure was 130 lbs. per sq. in. The air for the pneumatic working was supplied by three 14× 26× 30 in. duplex Ingersoll-Sergeant compressors. The maximum capacity of the three was 12,000 cu. ft. free air per minute at 125 r.p.m. and a discharge pressure of 50 lbs. per sq. in. The suction air was taken from the outside about 10 ft. above the roof of the engine house. Three aftercoolers, 32¹/₂ ins.× 11 ft. 4 ins., each having 809 sq. ft. cooling surface of tinned brass tubes, cooled the low-pressure discharge to within 10° F. of the temperature of the cooling-water. From the aftercoolers, the air passed into three steel receivers each 54× 12 ft., placed outside the engine room and fitted with weighing safety valves. The receivers were connected to two 10-in. mains; one serving the north, the other the south tunnel. A fourth receiver of the same size was built to receive the discharge of the high-pressure compressor, through a 4-in. pipe. The high-pressure water required for the shield was furnished by three Blake direct-acting, duplex pumps with outside packed plungers. The steam end was 16× 18 ins., the water end 2¹/₁₆× 18 ins. At 55 r.p.m. pumping against 5000 lbs. per sq. in., the capacity of each pump was 57 gals. per minute. Two of them, one on each tunnel, were sufficient to run the shields and the third was held in reserve. The high-pressure water was conveyed to the front by means of a 2-in. double, extra strong pipe which was buried between the engine room and the shaft, in a trench, to prevent freezing in cold weather. The electric current for light and power was supplied by two 100-K.W. 250-volt G.E. direct-current generators directly connected to Ball & Wood high-speed engines running at 250 r.p.m. The switchboard had two machine panels, two distributing panels and one panel carrying a circuit breaker for the traction circuit.

Illumination.

—The tunnel was lighted by electricity, there being two rows of lamps, one in the crown and one in the south axial fine. The lamps were 16-c.p., 240-volt, two-wire system, and were spaced 35 ft. apart in the crown and 12¹/₂ ft. apart on the axial line. In addition, five nests of 5 lamps each were used at the front. Candles were supplied for miscellaneous and emergency uses. The sockets for electric globes were fitted to a wooden reflector, coated with white enamel paint on the inside.

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