the water may change somewhat the properties of the material, by
increasing or changing its cohesion, angle of repose, etc. That is, in
substance, those particles which rest solidly on the bottom and are in
contact to the top of the solid material, do not derive any buoyancy
from the water, while those particles not in contact with the bottom
directly or through other particles, lose just so much weight through
buoyancy. If, then, the vertical depth of the earthy particles or sand
above the bottom is so small that the arching effect against the sides
is negligible, the full weight of the particles in contact, directly or
vicariously, with the bottom acts as pressure on the bottom, while the
full pressure of the water acts through the voids or on them, or is
transmitted through material in contact with the bottom.

Referring now to materials such as clays, peats, and other soft or
plastic materials, it is idle to assume that these do not possess
pressure-resisting and arching properties. For instance, a soft clay
arch of larger dimensions, under the condition described early in this
paper, would undoubtedly stand if the rods supporting the intrados of
the arch were keyed back to washers covering a sufficiently large area.

The fact that compressed air can be used at all in tunnel work is
evidence that semi-aqueous materials have arching properties, and the
fact that "blows" usually occur in light cover is further evidence of
it.

When air pressure is used to hold back the water in faces of large area,
bracing has to be resorted to. This again shows that while full
hydrostatic pressure is required to hold back the water, the pressure of
the earth is in a measure independent of it.