About 2,500 ft. of tunnel, the roof of which was in soft ground, was
excavated in normal air by the mining-and-timbering method. In the
greater part of this the rock surface was well above the middle of the
tunnel. The method of timbering and mining, while well enough known, has
not been generally used in the United States.
[Illustration: PLATE LXIII]
Starting from the break-up in all rock, as described above, and
illustrated on Plate XIII, when soft ground was approached, a top
heading was driven from the rock into and through the earth. This
heading was about 7 ft. high and about 6 ft. wide. This was done by the
usual post, cap, and poling-board method. The ground was a running sand
with little or no clay, and, at first, considerable water, in places.
All headings required side polings. The roof poling boards were about
2-1/2 or 3 ft. above the outside limit of the tunnel lining, as
illustrated by Figs. 3, 4, and 5, Plate LXIII. The next step was to
place two crown-bars, _AA_, usually about 20 ft. long, under the caps.
Posts were then placed under the bars, and poling boards at right angles
to the axis of the tunnel were then driven out over the bars. As these
polings were being driven, the side polings of the original heading were
removed, and the earth was mined out to the end of these new transverse
polings. Breast boards were set on end under the ends of the transverse
polings when they had been driven out to their limit. Side bars, _BB_,
were then placed as far out as possible and supported on raking posts.
These posts were carried down to rock, if it was near, if not, a sill
was placed.
A new set of transverse polings was driven over these side bars and the
process was repeated until the sides had been carried down to rock or
down to the elevation of the sills supporting the posts, which were
usually about 4 ft. above the axis of the tunnel.
The plan then was to excavate the remainder of the section and build the
iron lining in short lengths, gradually transferring the weight of the
roof bars of the iron lining as the posts were taken out. This meant
that not more than four rings, and often only one ring, could be built
before excavation and a short length of cradle became necessary. Before
the posts under the roof bars could be built and the weight transferred
to the iron lining, a grout dam was placed at the leading end of the
iron lining, and grout was brought up to at least 45 deg. from the top. Such
workings were in progress at as many as eight places in one tunnel at
the same time. Where there was only the ordinary ground-water to contend
with, the driving of the top heading drained the ground very thoroughly,
and the enlarging was done easily and without a serious loss of ground.
Under these conditions the surface settlement was from 6 in. to 2 ft.
Under Borden Avenue, there was more water, which probably came from a
leaky sewer; it was not enough to form a stream, but just kept the
ground thoroughly saturated. There was a continued though hardly
perceptible flow of earth through every crevice in the timbering during
the six or eight weeks between the driving of the top heading and the
placing of the iron lining; and here there was a settlement of from 4 to
8 ft. at the surface.