Figure 24.—Thames Tunnel. Section through riverbed and tunnel following one of the break-throughs of the river. Inspection of the damage with a diving bell. (Beamish, A Memoir of the Life of Sir Marc Isambard Brunel.)
The Tower Subway at first operated with cylindrical cars that nearly filled the 7-foot bore; the cars were drawn by cables powered by small steam engines in the shafts. This mode of power had previously been used in passenger service only on the Greenwich Street elevated railway in New York. Later the cars were abandoned as unprofitable and the tunnel turned into a footway ([fig. 32]). This small tunnel, the successful driving due entirely to Greathead’s skill, was the forerunner of the modern subaqueous tunnel. In it, two of the three elements essential to such work thereafter were first applied: the one-piece movable shield of circular section, and the segmental cast-iron lining.
The documentation of this work is far thinner than for the Thames Tunnel. The most accurate source of technical information is a brief historical account in Copperthwaite’s classic Tunnel Shields and the Use of Compressed Air in Subaqueous Works, published in 1906. Copperthwaite, a successful tunnel engineer, laments the fact that he was able to turn up no drawing or original data on this first shield of Greathead’s, but he presents a sketch of it prepared in the Greathead office in 1895, which is presumably a fair representation ([fig. 33]). The Tower Subway model was built on the basis of this and several woodcuts of the working area that appeared contemporaneously in the illustrated press. In this and the adjacent model of Beach’s Broadway Subway, the tunnel axis has been placed on an angle to the viewer, projecting the bore into the case so that the complete circle of the working face is included for a more suggestive effect. This was possible because of the short length of the work included.
Henry S. Drinker, also a tunnel engineer and author of the most comprehensive work on tunneling ever published, treats rock tunneling in exhaustive detail up to 1878. His notice of what he terms “submarine tunneling” is extremely brief. He does, however, draw a most interesting comparison between the first Thames Tunnel, built by Brunel, and the second, built by Greathead 26 years later:
| FIRST THAMES TUNNEL | SECOND THAMES TUNNEL (TOWER SUBWAY) |
| Brickwork lining, 38 feet wide by 22½ feet high. | Cast-iron lining of 8 feet outside diameter. |
| 120-ton cast-iron shield, accommodating 36 miners. | 2½-ton, wrought-iron shield, accommodating at most 3 men. |
| Workings filled by irruption of river five times. | “Water encountered at almost any time could have been gathered in a stable pail.” |
| Eighteen years elapsed between start and finish of work. | Work completed in about eleven months. |
| Cost: $3,000,000. | Cost: $100,000. |
Figure 25.—Transverse section through shield, after inundation. Such disasters, as well as the inconsistency of the riverbed's composition, seriously disturbed the alignment of the shield's individual sections. (Law, A Memoir of the Thames Tunnel.)