ncy of the sand to arch itself
horizontally over small areas. The same operation cannot be performed on
dry sand, as the arching properties, while protecting the pile from
excessive pressure due to excessive length, will not prevent the loose
sand immediately surrounding the pile from exerting a constant pressure
against the pile, and it is of this pressure that 45% may be taken as
the real value of skin friction on piles in dry sand.

In soft clays or peats which are displaced by driving, the tendency of
this material to flow back into the original space causes pressure, of
which the friction will be a measured percentage. In this case, however,
the friction itself between the material and the clays or peat is
usually very much less than 40%, and it is for this reason that piles of
almost indefinite length may be driven in materials of this character
without offering sufficient resistance to be depended on, as long as no
good bearing ground is found at the point.

If this material is under water, and is so soft as to be considered
semi-aqueous, the pressure per square foot will increase in diminishing
proportion to the depth, and the pressure per area will soon approach
and become a constant, due to the resistance offered by the lateral
arching of the solid material; whereas, in large circular caissons, or
caisson shafts, where the horizontal arching effect is virtually
destroyed, or at least rendered non-effective until a great depth is
reached, the pressure must necessarily vary under these conditions
proportionately to the depth and size of the caisson in semi-aqueous
material. On the other hand, in large caisson shafts, especially those
which are square, the pressure at the top due to the solid material will
also increase proportionately to the depth, as already explained in
connection with the pressures of earth against sheeting and retaining
walls.

The writer believes that the pressure on these surfaces may be
determined with reasonable accuracy by the formulas already given in
this paper, and with these pressures, multiplied by the coefficient of
friction determined by the simplest experiment on the ground, results
may be obtained which will closely approximate the actual friction on
caissons at given depths. The friction on caissons, which is usually
given at from 200 to 600 lb. per sq. ft., is frequently assumed to be
the same on piles 12 in. or less in diameter, whereas the pressures on
these surfaces, as