The rock near
the shaft contained many wet rusty seams, and settlement was detected in
the segmental tunnel timbering soon after the widening of the heading
was completed. Short props were placed under the timbers, and the street
surface was opened with a view of stripping the earth down to the rock
and thus lightening the load on the timbering. Street traffic was
maintained on a timber structure with posts eventually carried down to
the rock surface, and the walls of the buildings on the north side of
the street were underpinned to rock. The settlement of the tunnel
timbering was checked for a time, and the bench was excavated as shown
on Plate LXI. In this work the cut in the center was first made, and the
short props were replaced by struts, as shown; after this the berms were
removed and the side posts were placed. While building the brick arches,
holes were left in the masonry around the struts. After the masonry had
hardened, piers were built on the arches to support the segmental
timbers. The struts were then removed and the openings filled with
masonry. The voids above the arch were packed with rock and afterward
thoroughly grouted.
The timbers near the shaft continued to settle, and, although they had
been placed from 9 to 12 in. above the level of the top of the masonry,
by October 1st, they encroached 9 in.
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