g the abutments, and solidly cleated and braced frames were
placed across their ends about 2 ft. back of each abutment. A false
bottom, made to slide freely up and down between the abutments, and
projecting slightly beyond the walls on each side, was then blocked up
snugly to the bottom edges of the sides, thus obtaining a box 3 by 4 by
7 ft., the last dimension not being important. Bolts, 44 in. long, with
long threads, were run up through the false bottom and through 6 by 15
by 2-in. pine washers to nuts on the top. The box was filled with
ordinary coarse sand from the trench, the sand being compacted as
thoroughly as possible. The ends were tightened down on the washers,
which in turn bore on the compacted sand. The blocking was then knocked
out from under the false bottom, and the following was noted:

As soon as the blocking was removed the bottom settled nearly 2 in., as
noted in Fig. 1, Plate XXIV, due to the initial compacting of the sand
under the arching stresses. A measurement was taken from the bottom of
the washers to the top of the false bottom, and it was noted as 41 in.
(Fig. 1). After some three or four hours, as the arch had not been
broken, it was decided to test it under greater loading, and four men
were placed on it, four others standing on the haunches, as shown in
Fig. 2, Plate XXIV. Under this additional loading of about 600 lb. the
bottom settled 2 in. more, or nearly 4 in. in all, due to the further
compression of the sand arch. About an hour after the superimposed load
had been removed, the writer jostled the box with his foot sufficiently
to dislodge some of the exposed sand, when the arch at once collapsed
and the bottom fell to the ground.

Referring to Fig. 2, if, instead of being ordinary sand, the block
comprised within the area, _A U J V X_, had been frozen sand, there can
be no reason to suppose that it would not have sustained itself, forming
a perfect arch, with all material removed below the line, _V E J_, in
fact, the freezing process of tunneling in soft ground is based on this
well-known principle.

[Illustration: FIG. 2.]

[Illustration: FIG. 3.]

If, then, instead of removing the mass, _J E V_, it is allowed to remain
and is supported from the mass above, one must concede to this mass in
its normal state the same arching properties it would have had if
frozen, excepting, of course, that a greater thickness of key should be
allowed, to offset a greater tendency to compress