Tunneling: A Practical Treatise.

by Charles Prelini

THE PNEUMATIC CAISSON METHOD.—THE TUNNEL UNDER THE HARLEM RIVER.

In the early seventies Prof. Winkler proposed to construct a tunnel under the River Danube to connect the various portions of the Vienna, Austria, underground railway, and to use caissons in the construction. Prof. Winkler proposed to build caissons from 30 ft. to 45 ft. long, with a width depending upon the lateral dimensions adopted for the tunnel masonry. The caisson was to be made of metal plates and angle iron with riveted connections on all sides except those running vertically transverse to the tunnel axis, whose connections were to be bolted. In the middle of the roof an opening was to be left; this was for the shaft having the air-locks to allow the passage of men, materials, and compressed air.

Across the river two parallel rows of piles were to be driven into the river bed, to fix the place where the caisson was to be sunk. Then the first caisson near the shore was to be lowered in the ordinary way, and a second caisson was to be immediately sunk very close to the first one. When both caissons had reached the plane of the tunnel floor, the sides which were in contact were to be unbolted and removed, and the small space between made water-tight. The chambers of the two caissons were to be opened into a single large one communicating above by means of two shafts. At the same time that the masonry was being built in the first two caissons, from the inverted arch up, a third caisson was to be sunk; and when by excavation it had reached the plane of the projected tunnel floor, the partitions were to be removed so that the three caissons were in communication, forming a large single caisson. Then the outer partition of the first caisson was to be removed, and the masonry of the submarine tunnel connected with the portion of the tunnel built on land. In a similar manner all the caissons were to be sunk; and when the last one was placed, and the masonry lining constructed, and connected with the portion of the tunnel built on the other shore of the river, the partition walls were to be battered down, and the submarine tunnel completely constructed and open to traffic.

The Harlem River Tunnel.

—The pneumatic caissons method was employed in the construction of the tunnel under the Harlem River for the New York Rapid Transit Railway. The tunnel proper consisted of two parallel tubes riveted to each other and surrounded by a cradle of concrete as shown in [Fig. 121], page 216. The tunnel was built in three sections:—The first, from the Manhattan shore well towards the middle of the river; the second, from the shore of the Bronx towards the middle of the river; and the last, the section uniting the other two and completing the tunnel.

Each section was built within a specially constructed working-chamber, consisting of timber side walls forming a wooden caisson, so constructed that compressed air could be used. This working-chamber of Mr. McBean presented some novel features, inasmuch as the caisson was not built on land, but under water.

In building the tunnel, the Harlem River was dredged to a certain depth, so as to leave only 6 ft. or 8 ft. of excavation to be done before reaching the line of sub-grade of the proposed structure. Two service platforms were built on piles 10 ft. apart longitudinally, and cut off at a point above mean high-water mark, braced in the usual manner, and covered with heavy planks, to serve as the floor of the platform. On this platform were placed rails for the trains used in the transportation of materials. These platforms were also used in maintaining the perfect alignment of the caissons.

Within the platforms and along the dredged channel four longitudinal rows of piles were sunk. These piles were accurately brought to line by beams bolted together, and placed across and above the water-level. A few beams were also added for the purpose of bracing the piles transversely, after which they were cut off under water and capped.

Fig. 142.—Showing Working Platforms and Piles Sunk in the Dredged Channel.

[Fig. 142] shows the manner in which the working platforms were constructed, and also the rows of piles sunk in the dredged channel. Between the piles a very strong frame was placed, made up of waling pieces and two transverse beams 14 ins. by 14 ins. each, placed one below the other at a distance of 5 ft. 8 ins., and strongly braced together. Guiding-beams were fixed on each side of the frame for the sheeting piles. The frames were built in sections of different lengths, and placed directly above the cap-pieces of the pile-bents sunk in the dredged channel.

The longitudinal sides of the caisson were constructed by sinking two rows of sheeting piles, each row being close to a service platform. The sheeting piles were made up of yellow-pine timbers 12 ins. by 12 ins.; three piles bolted together formed a section 3 ft. wide. Each section was grooved and tongued, so as to be firmly connected with the adjacent sections to be sunk. The lower ends of the piles were cut wedge-shaped, with a sharp edge to offer a small resistance while penetrating the soil. The sheeting-piles were then cut off under water, which operation was successfully carried out by means of a circular saw operated by a pile-driving machine. The sheeting was also extended between two platforms to make a bulkhead, and in this way the four sides of the caisson were built up. Particular attention was always given to the alignment of the sheeting piles, which was obtained by guiding the piles with the timbers placed longitudinally, one below the water-line in connection with the frames located between the pile-bents, and the second along the inner edge of the service platform, as shown in [Fig. 143].

Fig. 143.—Showing Sheeting-Piles for the Sides of the Caisson and Trussed Beam for the Roof.

The caisson was completed by placing a roof covering the sides. This roof was 40 ins. thick, made up of three layers of 12-in. beams placed transversely to the axis of the caisson, while between the beams planks 2 ins. thick were placed lengthwise and bolted together, so as to make a firm, solid structure. The roof was built ashore, in sections each varying from 39 ft. to 130 ft. long. The edges of the roof fitted the sides of the caisson perfectly; and when each section was towed to the proper spot, it was sunk and made secure. Under the roof were placed six longitudinal beams, 12 ins. by 14 ins., called “rangers,” resting on the cap-pieces of the pile-bents that were laid across the space of the proposed tunnel; while the extreme rangers were used for the purpose of fitting above the sheeting-piles of the caisson, in order to make the latter water-tight. The two extreme rangers were provided with T-irons, the flat side being laid on the sheeting-piles, while the web penetrated the ranger by reason of the weight of the load resting on the roof, for the purpose of sinking it to the required point. Earth was next heaped on the roof, and in this way a large working-chamber was prepared, as shown in [Fig. 144].

Fig. 144.—Showing the Caisson with the Working-Chamber.

The working-chamber built on the Manhattan side of the Harlem River was 216 ft. long, provided with two rectangular shafts 7 ft. by 17 ft., rendered water-tight, and rising above the high-water mark of the river. Within these shafts the air-locks of the tunnel tubes were placed, so that the work could be carried on by means of compressed air. The pressure of the air was used to expel the water, being sufficiently intense to equilibrate a column of water equal to the depth of the lowest point of the roof of the caisson.

When the working-chamber was constructed, the tunnel proper was begun by excavating the soil down to the required level; the concrete was then laid on. It was just at this point, when a large part of the roof was constructed and supported only by the sheeting-piles of the sides of the caisson, that the writer of this article feared that this novel method of tunneling would prove a failure. The tendency of the timber to float, aided as it was by the air pressure within the caisson, was counteracted only by the weight of the earth heaped on the roof, and by the friction of the soil against the feet of the sheeting-piles. This friction was only a small quantity, as the soil was loose, so that it was considered rather risky and dangerous to place reliance on such a feeble quantity. This fear was, unfortunately, justified on two occasions, when on cutting off a portion of the pile-bents some of the sheeting-piles got loose and water flooded the whole chamber, but, happily, without loss of life. As the chamber was one of large dimensions, the workmen had time enough to effect their escape. It may be remarked that during these troubles only a few of the sheeting-piles were displaced, while the caisson itself offered a stout and successful resistance, due to its being strongly braced transversely. The accidents were, therefore, limited to a few piles, instead of affecting the entire caisson. On the occasion of the first, the repairs were effected by sinking the piles to a greater depth, continuing down until rock was encountered. After that, the water was pumped out and the work resumed. In repairing the second accident, the sheeting-piles were driven down to bed-rock, and the surrounding soil strengthened by cement forced through the loose soil around the piles. This remedy proved effective, and no further trouble occurred to delay the work on the Manhattan side of the Harlem River.

Fig. 145.—Showing the Tunnel Constructed within the Caisson.

On the concrete bed of the tunnel the segments of the metal lining were placed and surrounded by concrete, as required by the plans and specifications ([Fig. 145]). The contractors had planned to unbolt the roof from its holdings, to remove by means of dredgers the earth which had been heaped on it, and thus set the roof afloat, after which it was to be towed within the two working platforms already erected on the Bronx shore. But Mr. McBean devised a simpler and more economic, but at the same time more dangerous, way of constructing this second section of the tunnel. He thought that the upper half of the tunnel proper could be used instead of the timber roof, thereby reducing the capacity of the working chamber, and limiting the use of compressed air. In this way he dispensed with the removal of timber, and also of the earth heaped on the roof.

In building this Bronx section, a channel was dredged along the line of the tunnel to a depth of 5 ft. from the foundation-bed of the proposed tunnel. The working platforms were constructed on both sides of this channel, quite similar to those erected on the other half of the tunnel; and between them pile-bents were sunk, capped with 12-in. by 12-in. beams. Over the cap-pieces rangers were placed longitudinally, which also rested on the sides of the wooden working caisson, [Fig. 146]. The sheeting-piles were cut off at level, but much lower down than in the first half of the tunnel.

The roof was built on floats made of 12-in. by 12-in. timber laid transversely 4 ft. apart and supporting a floor of 3-in. by 12-in. planks rendered water-tight. The sides of the floats were made by verticals, 4 ins. by 6 ins., and planks, 3 ins. by 12 ins., carefully caulked. A temporary floor was built on the base of the float, consisting of transverse beams, 16 ins. by 16 ins., placed 8 ft. apart. A center piece, 10 ins. by 16 ins., was laid so as to correspond with the axis of the tunnel; and on each side of it, other parallel beams, 16 ins. by 16 ins., corresponding to each center of the circular metal lining of the tunnel; the beams, longitudinal and transversal, were strongly bolted together. The temporary floor was completed by boarding the spaces left between the various beams.

Fig. 146.—Showing Sides of the Caisson and Supports for the Roof.

On this float, the upper half of the tunnel was constructed by erecting the segments of the metal lining, which were strongly supported, so as to prevent any settling or distortion; the concrete was then built up in a large flange with vertical suspension rods, four to each bar. The rings of the tunnel were 6 ft. each, the weight of each lining being 41,000 lbs., the concrete covering 618 cubic feet. The second part of the tunnel was 300 ft. long, with roof constructed in three sections—two of 90 ft. in length each and the third of 84 ft. Each of these sections alternated with a smaller section, 12 ft. long, provided with air-locks. The shortest of the three sections was the first one set up, and was constructed close to the Bronx side of the Harlem River. For this purpose the two extreme ends of the section were closed by means of steel plates forming diaphragms, built 6 ft. inward, thus leaving one ring projecting out at each end. Openings were left on the top of these projecting rings for access by divers. The exterior of the upper half section of the permanent tunnel was filled with water until it was lowered into position. It was directed by means of tackles attached to vertical eye-bars, which were strongly fixed to the flanges of the springing line of the arch, and bolted to the beams of the temporary floor. In this way the roof was towed into place, and lowered by means of stone ballast, until it rested on the cap-pieces and frames of the pile abutments, the sides of the roof remaining just on top of the sheeting-piles that formed the sides of the caisson, as shown in [Fig. 147]. Perfect alignment was obtained by wires strung at each end and along the side of the roof, corresponding to points fixed on the working platforms and sighted with transits. Such accuracy was obtained that the circumferential flanges of the outer 6-ft. ring were brought into contact with those of the 12-ft. section already constructed. A diver then entered by the opening left in the projecting ring, and bolted this section of the roof to the preceding one. By removing the iron diaphragm, the consecutive sections were united into one. When the diver completed his work, the opening was closed up, and compressed air used to keep the water out of the box included between the roof and the temporary flooring.

Fig. 147.—Showing the Roof of the Caisson Formed by the Upper Half of the Tunnel.

The remaining sections of the tunnel roof were built in the same way, until the last abutted against the part of the work constructed within the caisson under the high wooden roof on the Manhattan side of the river. The following method was adopted for the purpose of connecting the few parts of the tunnel which had been differently constructed. The diaphragm at the end of the last section of the tunnel roof was constructed so as to abut against the last circumferential flanges of the iron lining without leaving a projecting ring. It was continued above the metal and concrete lining of the roof in a rectangular form, and of the same height and width as the wooden bulkhead of the working-chamber on the Manhattan side of the river. The diaphragm was made of riveted plates and angles, with an opening 20 ins. by 30 ins., bolted so as to be removable at will. The diaphragm was of the same height as the roof and was connected with a roof-plate to the rangers supporting the thick wooden roof. Other steel plates, placed vertically, were riveted to the diaphragm and bolted to the caisson. All this work was carried on by divers. The wooden bulkhead was cut to the springing-line of the arch; and between the two parts of the tunnel, built by different methods, a bulkhead was placed, made of steel plates 14 ins. long, which prevented the entrance of water into the working-chamber.

Fig. 148.—Showing the Tunnel Completed by Building the Lower Half within the Caisson.

When the different sections were joined together, and all the openings closed and made water-tight, cement-grout was poured on the roof, and earth was heaped up to a height of 5 ft. The 300 ft. of the roof, resting on sheeting-piles and provided with diaphragms at the extreme ends, formed a water-tight working-chamber, or caisson, communicating with the exterior by means of the shafts and air-locks. The lower portion of the tunnel was built under air-pressure. The pile-bents were first cut off at the plane of the tunnel sub-grade, after which the foundation-bed of concrete was laid. The lower segments of the iron lining were then placed in position, and the structure made continuous by building up the lateral walls, consisting of concrete ([Fig. 148]). No accidents occurred while building the second part of the tunnel.

The Harlem River tunnel was completed in contract time, although the opening of the subway was delayed by difficulties encountered in tunneling through rock in the borough of the Bronx. The writer endeavored to obtain information regarding the expense per linear foot, but all his efforts were rewarded with a general assurance that it proved to be the cheapest method.