Transactions of the American Society of Civil Engineers, vol. LXVIII, Sept. 1910 - The New York Tunnel Extension of the Pennsylvania Railroad. - The East River Tunnels. Paper No. 1159 by S. H. Woodard;Francis Mason;James H. Brace
page 62 of 93 (66%)
page 62 of 93 (66%)
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lowered pressure. Very few blows showed losses approaching this in
quantity, but the inherent inaccuracy of the observations make the foregoing figures only roughly approximate. [Footnote C: _Minutes of Proceedings_, Inst. C. E., Vol. CXXX, p. 50.] SPECIAL DIFFICULTIES. The most serious difficulties of the work came near the start. In Tunnel _D_ blows and falls of sand from the face were frequent after soft ground was met in the top. About six weeks after entering the full sand face, and before the shutters had been installed, the shield showed a decided tendency to settle, carrying the tunnel lining down with it and resulting in a number of badly broken plates in the bottom of the rings. Notwithstanding the use of extremely high vertical leads,[D] the sand was so soft that the settlement of the shield continued for about fifteen rings, the maximum being nearly 9 in. below grade. The hydrostatic head at mid-height of the tunnel was 32-1/2 lb., and the raising of the air pressure to 37 lb., as was done at this time, was attended with grave danger of serious blows, on account of the recent disturbance of the natural cover by the pulling and re-driving of piles in the reconstruction of the Long Island ferry slips directly above. It dried the face materially, however, and the shield began to rise again, and had practically regained the grade when the anticipated blow-outs occurred, culminating with the entrance of rip-rap from the river bed into the shield and the flooding of the tunnel with 4 ft. of sand and water at the forward end. The escape of air was very great, and, as a pressure of more than 28 lb. could not be maintained, the face was bulkheaded and the tunnel was shut down for three weeks in order to |
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