. Shield and compressed air tunneling . ng diaphragmof steel plate with its lower edge about 3 ft. above the floor and4 in. below the upper edge of another diaphragm which rose fromthe floor about 3 ft. behind the hanging plate. There were 40shield jacks for propelling the shield, each with a diameter of 9 a stroke of 3 ft. 6 in. The working pressure was 6,720 lb. persquare inch, giving a total forward thrust of 8,500 tons. Theshield is shown in Fig. 115. The total weight of the shield was425 tons. 55. Ground Penetrated.—This tunnel passed through a seriesof roughly horizontal layers or
. Shield and compressed air tunneling . ng diaphragmof steel plate with its lower edge about 3 ft. above the floor and4 in. below the upper edge of another diaphragm which rose fromthe floor about 3 ft. behind the hanging plate. There were 40shield jacks for propelling the shield, each with a diameter of 9 a stroke of 3 ft. 6 in. The working pressure was 6,720 lb. persquare inch, giving a total forward thrust of 8,500 tons. Theshield is shown in Fig. 115. The total weight of the shield was425 tons. 55. Ground Penetrated.—This tunnel passed through a seriesof roughly horizontal layers or bands of blue clay, mottled clay,sand and shelly clay, rock, pebbles and sand. The serious diffi-culties afforded at Blackwall were not present here. 268 Sin ELI) TUNNELINCr 56. Comparison with Blackwall Shield.—The shield was ofvastly simpler construction than the Blackwall type. A tunnelshield is an instrument of brute force rather than of delicatepersuasion. Simplicity combined with strength should be thekey-note of its 57. The Pilot Tunnel.—The Rotherhithe tunnel is noticeablefor the fact that the contractor decided to drive a pilot tunnelahead of the main bore in order to explore the ground. Thepilot was 12 ft. 6 in. in diameter, driven with a rotary cuttershield of the Price type, and lined with cast iron. The pilot wasstarted from the shore shaft on the north bank of the river andwas stopped when 150 ft. from the south shaft which then hadnot yet been sunk. It was driven, therefore, 1,420 ft. Its cossTurcnos 2().) top was about 2 It. below the top of the main tunnel. The pilotwas started on Oct. 12, 1905 and stopped on Jan. 22, was in action, therefore, 103 days, counting Sundays andholidays and the average progress was ft. per day. The airpressure varied from 12 to 21 lb. per squan^ inch. The timerequired to make a shove for one ring, 20 in. long, was about 20min. in clay and 45 min. in hard ground. A bed of rock from3 to 5 ft. thick overlai
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