. Shield and compressed air tunneling . sTRrcTrny 273 66. Jack Pressures.—On an av(M-iige tlio shields wore shovedby ten or twcKc of the bottom jaeks with a pressure of 4, square inch. As th(> phnigeis weic of 9-in. diameter theaverage total foree on the shield was 2,S()(),0()() lb. Where theshutters were used it would take over 6,000 lb. per square inch onall the 27 jacks so that the total forward thrust required was10.;-5()(),000 lb. 67. Progress.—By the method outlined 4,195 linear ft. oftunnel were driven by four shields in 120 days, an average ft. per day pei- shield. P
. Shield and compressed air tunneling . sTRrcTrny 273 66. Jack Pressures.—On an av(M-iige tlio shields wore shovedby ten or twcKc of the bottom jaeks with a pressure of 4, square inch. As th(> phnigeis weic of 9-in. diameter theaverage total foree on the shield was 2,S()(),0()() lb. Where theshutters were used it would take over 6,000 lb. per square inch onall the 27 jacks so that the total forward thrust required was10.;-5()(),000 lb. 67. Progress.—By the method outlined 4,195 linear ft. oftunnel were driven by four shields in 120 days, an average ft. per day pei- shield. Pennsylvania Railroad, River (A-IS) 68. Pennsylvania Railroad Hudson River Tunnel Shield.—Thechief engineer for the tunnels under the Hudson River of thePeniLsvlvania Railroad was Charles M. Jacobs, and the con-tractor was The ORourke Engineering Construction shields used are shown in Fig. 118. The design was workedup in the engineers office and adopted by the contractor. POSITION OF BREASTBOARDS BEFORE SHOVING. ELEVATION OF FACE SHOWING BREAST BOARDS FOR NEXT SHOVE Fig. 119. Work in waterbearing sand. Pennsylvania Railroad Hudson RiverTunnels (A-IS). (From Trans Am. Soc. C. vol. 68) 69. Comparison with East River Shield.—The shield is asimpler one than the East River type. This is reflected in theirweights. The structural portion of the East River shield weighed185 tons and that of the Hudson River shield 135 tons, a differ-ence of 37 per cent. The hydraulic fittings for the HudsonRivei- shield weighed 58 tons additional. 70. Method of Work in Sand, Hudson River.—The system oftimbering is shown in Fig. 119. The ground was excavated 18 274 SHIELD TUNNELING 2 ft. 6 in. (the length of one ring of the Hning) ahead of the cuttingedge. The roof was held on longitudinal pohng boards resting -h-^^^^^i
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