that point; and, allowing 50% for voids, halving this area gives the
line, _C D_, between which and the vertical face any horizontal line
measures the water pressure. Extending these pressure areas where they
overlap gives the line, _B D_, which represents the total pressure
against the face, measured horizontally.

Next, as to the question of buoyancy in Class A materials. If a
submerged structure rests firmly on a bottom of more or less firm sand,
its buoyancy, as indicated by the experiments, will only be a percentage
of its buoyancy in pure water, corresponding to the voids in the sand.
In practice, however, an attempt to show this condition will fail, owing
to the fact that in such a structure the water will almost immediately
work under the edge and bottom, and cause the structure to rise, and the
test can only be made by measuring the difference in uplift in a
heavier-than-water structure, as shown in Experiment No. 5. For, if a
structure lighter than the displaced water be buried in sand
sufficiently deep to insure it against the influx of large volumes of
water below, it will not rise. That this is not due entirely to the
friction of the solid material on the sides has been demonstrated by the
observation of subaqueous structures, which always tend to subside
rather than to lift during or following disturbance of the surrounding
earth.

The following is quoted from the paper by Charles M. Jacobs, M. Am. Soc.
C. E., on the North River Division of the Pennsylvania Railroad
Tunnels:[E]

"There was considerable subsidence in the tunnels during
construction and lining, amounting to an average of 0.34 ft.
between the bulkhead lines. This settlement has been constantly