value on irregular surfaces as well as the bearing due to taper in
piles, and lastly the resistance offered by binding, enter into the
determination of so-called skin friction formulas. The essential
condition of sinking a caisson is keeping it plumb; and binding, which
is another way of writing increased bearing value, will oftentimes be
fatal to success.

The writer believes that a series of observations on caissons sunk plumb
under homogeneous conditions of ground and superficial smoothness will
show a proportional increase of skin friction per square foot average
for each increase in the size of caissons, as well as for increase of
depth in the sinking up to certain points, where it may finally become
constant, as will be shown later. The determination of the actual
friction or coefficient of friction between the surfaces of the pile and
the material it encounters, is not difficult to determine. In sand it is
approximately 40% of the pressure for reasonably smooth iron or steel,
and 45% of the pressure for ordinary wood surfaces. If, for instance, a
long shaft be withdrawn vertically from moulding sand, the hole may
remain indefinitely as long as water does not get into it or it does not
dry out. This is due to the tendency 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,