Book-bot.com - read famous books online for free

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%)
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
DigitalOcean Referral Badge