decreasing since construction, and appears to have been due almost
entirely to the disturbances of the surrounding materials during
construction. The silt weighs about 100 lb. per cu. ft. * * * and
contains about 38% of water. It was found that whenever this
material was disturbed outside the tunnels a displacement of the
tunnels followed."

This in substance confirms observations made in the Battery tubes that
subsidence of the structure followed disturbance of the outside
material, although theoretically the tubes were buoyant in the aqueous
material.

The writer would urge, however, that, in all cases of submerged
structures only partially buried in solid material, excess weighting be
used to cover the contingencies of vibration, oscillation, etc., to
which such structures may be subjected and which may ultimately allow
leads of water to work their way underneath.

On the other hand, he urges that, in cases of floor areas of deeply
submerged structures, such as tunnels or cellars, the pressure to be
resisted should be assumed to be only slightly in excess of that
corresponding to the pressure due to the water through the voids.

The question of pressure, etc., in Class B, or semi-aqueous materials
will be considered next. Of these materials, as already shown, there are
two types: (_a_) sand in which the so-called quicksand is largely in
excess of any normal voids, and (_b_) plastic and viscous materials. The
writer believes that these materials should be treated as mixtures of
solid and watery particles, in the first of which the quicksand, or
aqueous portion, being virtually in suspension, may be treated as water,