permit the river bed to consolidate.

This was the most serious difficulty encountered on any part of the
work, and, coming at the very start, was exceedingly discouraging.
During the shut-down the broken plates were reinforced temporarily with
steel ribs and reinforced concrete (Fig. 1, Plate LXXIII) which, on
completion of the work, were replaced by cast-steel segments, as
described elsewhere. Practically, no further movement of iron took
place, and the loss of grade caused by the settlement of the shield,
which was by far the largest that ever occurred in this work, was not
sufficient to require a change in the designed grade or alignment of the
track. Work was resumed with the shutters in use at the face as an aid
to excavation. The features of extreme seriousness did not recur, but
for two months the escape of air continued to be extremely large, an
average of 15,000 cu. ft. per min. being required on many days during
this period.

[Illustration: PLATE LXXIII, FIG. 1.--TEMPORARY REINFORCEMENT OF BROKEN
PLATES AND REMOVAL OF A PLATE IN SECTIONS.]

[Illustration: PLATE LXXIII, FIG. 2.--HEAVY CAST-STEEL PATCH ATTACHED TO
BENT SEGMENT OF CUTTING EDGE.]

[Illustration: PLATE LXXIII, FIG. 3.--INFLOW OF SOFT CLAY THROUGH
SHIELD.]

[Illustration: PLATE LXXIII, FIG. 4.--REINFORCEMENT OF BROKEN PLATE WITH
LONG POLT AND TWISTED STEEL RODS.]

In Tunnel _B_, after passing out from under the bulkhead line, in April,