ted at from 600 to 1,000 lb. As the tunnel wall-plates
described in this paper were subject to occasional saturation, and
always to a moist atmosphere, they could never have been considered as
equal to dry material. Had the full loading shown by the foregoing
come on these wall-plates, they would have been subjected to a stress of
about 25 tons each, or nearly one-half of their ultimate strength. In
only one or two instances, covering stretches of 100 ft. in one case and
200 ft. in another, where there were large areas of quicksand sufficient
to cause semi-aqueous pressure, or pockets of the same material causing
eccentric loading, did these wall-plates show any signs of heavy
pressure, and in many instances they were in such good condition that
they could be taken out and used a second and a third time. Two
especially interesting instances came under the writer's observation: In
one case, due to a collapse of the internal bracing, the load of an
entire section, 25 ft. long and 19 ft. wide, was carried for several
hours on ribs spaced 5 ft. apart. The minimum cross-section of these
ribs was 73 sq. in., and they were under a stress, as noted above, of
50,000 lb., or nearly up to the actual limit of strength of the
wall-plate where the rib bore on it. When these wall-plates were
examined, after replacing the internal bracing, they did not appear to
have been under any unusual stress.

[Illustration: PLATE XXV, FIG. 1.--NORMAL SLOPES AND STRATA
OF NEWLY EXCAVATED BANKS.]

[Illustration: PLATE XXV, FIG. 2.--NORMAL SLOPES AND STRATA
OF NEWLY EXCAVATED BANKS.]

In another instance, for a distance of more than 700 ft., the sub-grade
of the sewer was 4 ft. below the level of the water in sharp sand. In
excavating for "bottoms" the water had to be pumped at the rate of more
than 300 gal. per min., and it was necessary to close-sheet a trench
between the wall-plates in which to place a section of "bottom." In
spite of the utmost care, some ground was necessarily lost, and this was
shown by the slight subsidence of the wall-plates and a loosening up of
the wedges in the supports bearing on the arch timbers. During this
operation of "bottoming," two men on each side were constantly employed
in tightening up wedges and shims above the arch timbers. It is
impossible to explain the fact that these timbers slackened (without
proportionate roof settlement) by any other theory than that the arching
was so nearly perfect that it reliev