. Shield and compressed air tunneling . Fig. 69.—Grouting stripattached to head of ram.(From Eng. News, Dec. 13,1906.). Fig. 70.—The housing of a shield jack. Pennsylvania Railroad Hudson lliverTunnels (A-18). (.From Eng. News, Dec. 13, 1906.) 47. Jack Housing.—In shields with a cast iron stiffening ringas described in par. 20 and 21, the jack cylinders are cast with aflange as shown in Fig. 60, so that they may be bolted to theshield structure. In shields with an annular box-girder construc-tion the jacks are inserted from the rear of the box-girder, and tofacilitate their proper placing a st
. Shield and compressed air tunneling . Fig. 69.—Grouting stripattached to head of ram.(From Eng. News, Dec. 13,1906.). Fig. 70.—The housing of a shield jack. Pennsylvania Railroad Hudson lliverTunnels (A-18). (.From Eng. News, Dec. 13, 1906.) 47. Jack Housing.—In shields with a cast iron stiffening ringas described in par. 20 and 21, the jack cylinders are cast with aflange as shown in Fig. 60, so that they may be bolted to theshield structure. In shields with an annular box-girder construc-tion the jacks are inserted from the rear of the box-girder, and tofacilitate their proper placing a steel casting is provided in eachcompartment furnished with a flaring socket, as shown in Fig. 70which guides the jack to its proper seat. At the rear the cylinder 158 SHIELD TUNNELING is supported on the annular web plate of the box-girder and isheld in place by plates cut to shape and fastened to the web-plateby means of stud ])olts. 48. Piping for Shield Jacks.—The piping for the shield jacksinvolves usually two connections to each jack, one for the supplyto the main cylinder and one for the pull-back. If a
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