[Larger illustration]

Fig. 138.—General Elevations and Sections of Shield.

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.