the water
blown out by compressed air, it was very stable. A special timbering
method was devised, and Tunnel _B_ was driven from Station 66 + 10 to
the shaft with compressed air, but without a shield. In the meantime the
shield was re-erected in Tunnel _A_ and was shoved through the soft
ground from Station 65 + 48 nearly to the river shaft, where it was
dismantled.

There was nothing unusual about the shield work; it was about the same
as that under the river, which is fully described elsewhere. In spite of
great care in excavating in front of the shield, and prompt grouting
behind it, there was a small settlement of the building above, amounting
to about 1-1/2 in. in the walls and about 5 in. in the ground floors
which were of concrete laid like a sidewalk directly upon the ground.
Whether this settlement was due to ground lost in the shield work or to
a compacting of the ground on account of its being dried out by
compressed air, it is impossible to say.

The interesting features of this work from East Avenue to the river
shafts are the mining methods and the building of the iron tube without
a shield.

EXCAVATION IN ALL ROCK.

Where the tunnel was all in good rock two distinct methods were used.
The first was the bottom-heading-and-break-up, and the second, the
top-heading-and-bench method. The first is illustrated by Figs. 1 and 2,
Plate LXIII. The bottom heading, 13 ft. wide and 9 ft. high, having
first been driven, a break-up was started by blasting down the rock,
forming a chamber the full height of the tunnel. The timber platform,
shown in the drawing, was erected in the bottom heading, and extended
through the break-up chamber. The plan was then to drill the entire face
above the bottom heading and blast it down upon the timber staging, thus
maintaining a passage below for the traffic from the heading and
break-ups farther down the line. Starting with the condition indicated
by Plate XIII, the face was drilled, the columns were then taken down
and the muck pile was shoveled through holes in the staging into muck
cars below. The face was then blasted down upon the staging, the drill
columns were set up on the muck pile, and the operation was repeated.
This method has the advantage that the bottom heading can be pushed
through rapidly, and from it the tunnel may be attacked at a number of
points at one time. It was found to be more expensive than the
top-heading-and-bench method, and as soon as the depre