Engineering Wonders of the World
Volume I

THE RIVER TUNNELS Fig. 5.—COMPRESSED-AIR HOSPITAL. IN WHICH MEN ATTACKED BY THE “ BENDS ” ARE PUT UNDER AIR PRESSURE TO CURE THEM BY VERY SLOW DECOMPRESSION. (PENNSYLVANIA RAILROAD HUDSON RIVER TUNNELS.) OF NEW YORK CITY. 107 f mud and water, and ultimately it was necessary to employ the powerful agency of shields. A shield, 10 feet 10 inches outside diam- eter and 7 feet 2 inches long, was built in a recess of the brick lining, under the pro- tection of a heavy timber wall closing off the face. With this the work went through successfully. But it was by no means easy even now. The following interesting passage from a paper by the engineer in charge, Mr. W. I. Aims, mentions some of the troubles encountered in the New York heading :— “ At the point where the shield entered the soft black mud on top there still remained about 12 feet of hard rock in the bottom. Blasting had therefore to be continued in the bottom pockets of the shield after the top had entered the much-softened material. It was with great difficulty that the bottom pockets could be kept clear of the black slush from overhead. The material had become so soft- ened along the rock face that it was almost impossible to confine it, and several rushes of inflowing material occurred, until finally an open connection with the river was estab- rock were separated by rock which had been ground up and decomposed to a soft, earthy, water-bearing residue. The engineers first tried driving forward into the soft material with careful “ timber- ing ”—that is, sheeting the roof, sides, and face with boards supported A Check. inside, and digging away cau- tiously with only a small area of soil exposed at any one time. But the tremendous inward pressure of the water softened the soil so much that it soon became impossible to advance further, even with the use of arched steel roof plates (Fig. 6). Compressed air was resorted to. A bulk- head wall of brick, 6 feet through, was built across the finished part of the tunnel, and compressed air forced into the “ heading ” to hold back the inflow. Even this proved in- sufficient, however, to control the semi-fluid Fig. 6.—WORKING IN SOFT GROUND WITHOUT SHIELDS. (east river gas tunnel.) Steel lining plates, supported by struts on a longitudinal timber, were put in as the face excavation progressed, and a strong brick lining was added inside the steel-plate work. The method soon proved inadequate, and shields had to be used.