[Footnote A: Presented at the meeting of December 15th, 1909.]

[Footnote B: _Transactions_, Am. Soc. C. E., Vol. LXIX. p. 1.]


TUNNELS FROM EAST AVENUE TO THE RIVER SHAFTS.

The sinking of the temporary shaft at East Avenue was a fairly simple
matter. Rough 6 by 12-in. sheet-piling, forming a rectangle, 127 by 34
ft., braced across by heavy timbering, was driven about 28 ft. to rock
as the excavation progressed. Below this, the shaft was sunk into rock,
about 27 ft., without timbering. As soon as the shaft was down, on
September 30th, 1904, bottom headings were started westward in Tunnels
_A_, _B_, and _D_. When these had been driven about half the distance to
the river shafts, soft ground was encountered. (See Station 59, Plate
XIII.) As the ground carried considerable water, it was decided to use
compressed air. Bulkheads were built in the heading, and, with an air
pressure of about 15 lb. per sq. in., the heading was driven through the
soft ground and into rock by ordinary mining methods. The use of
compressed air was then discontinued. West of this soft ground, a top
heading, followed by a bench, was driven to the soft ground at about
Station 66. Tunnel _C_, being higher, was more in soft ground, and at
first it was the intention to delay its excavation until it had been
well drained by the bottom headings in the tunnels on each side. A
little later it was decided to use a shield without compressed air. This
shield had been used in excavating the stations of the Great Northern
and City Tunnel in London. It was rebuilt, its diameter being changed
from 24 ft. 8-1/2 in. to 23 ft. 5-1/4 in. It proved too weak, and after
it had flattened about 4 in. and had been jacked up three times, the
scheme was abandoned, the shield was removed, and work was continued by