and it must be concluded that the action here will be similar to that of
sand and pure water, giving a larger value to the properties of water
than actually exists. If, for instance, it should be found that such a
mixture contained 40% of pure water, the writer would estimate its
pressure on or against a structure as (_a_) that of a moist sand
standing at a steep angle of repose, and (_b_) that of clear water, an
allowance of 60% of the total volume being assumed, and the sum of these
two results giving the total pressure. Until more definite data can be
obtained by experiments on a larger scale, this assumed value of 60% of
the total volume for the aqueous portion may be taken for all conditions
of semi-aqueous materials, except, of course, where the solid and
aqueous particles may be clearly defined, the pressures being computed
as described in the preceding pages.

As to the question of pure quicksand (if such there be) and other
aqueous materials of Class C, such as water, oil, mercury, etc., it has
already been shown that they are to be considered as liquids of their
normal specific gravity; that is, in calculating the air pressure
necessary to displace them, one should consider their specific gravity
only, as a factor, and not the total weight per volume including any
impurities which they might contain undissolved.

In order to have a clearer conception of aqueous and semi-aqueous
materials and their action, they must be viewed under conditions not
ordinarily apparent. For instance, ideas of so-called quicksand are
largely drawn from seeing structures sinking into it, or from observing
it flowing through voids in the sheeting or casing. The action of sand
and water under pressure is viewed during or after a slump, when the
damage is being done, or has been done, whereas the correct view-point
is under static conditions, before the slump takes place.