[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 relieved the bracing of a large part of
the load, the ordinary loose material being held in place by the arching
or wedging together of the 2-in. by 3-ft. sheeting boards in the roof,
arranged in the form of a segmental arch. The material above this roof
was coarse, sharp sand, through which it had been difficult to tunnel
without losing ground, and it had admitted water freely after each rain
until the drainage of a neighboring pond had been completed, the men
never being willing to resume work until the influx of water had
stopped.

The foregoing applies only to material ordinarily found under ground not