open caissons sunk to a depth of a few feet and cleaned out and filled
with concrete, which offer much better resistance than spreading the
foundation over four or five times the equivalent area.

He has found that small steel piles and coffer-dams, from 1-ft.
cylinders to coffer-dams 4 or 5 ft. square, sunk to a depth of only 1 or
2 ft. below adjacent excavations in ordinary sand, have safely resisted
loads four or five times as great as those usually allowed.

He believes that short cylinders, cleaned out and filled with concrete,
or coffer-dams of short steel piling with the surface cleaned out to a
reasonable depth and filled with concrete horizontally reinforced, will,
in many instances, give as good results as, and, in most cases, very
much better than, placing the foundation on an equivalent number of
small long piles or a proportionately greater spread of foundation area,
the idea being that the transmission of pressure to the sides of the
coffer-dam will not only confine the side thrust, but will also transfer
the loading in mass to a greater depth where the resistance to lateral
pressure in the ground will be more stable; that is, the greater depth
of foundation is gained without the increased excessive loading, or
necessity for deep excavation.

As to the question of the bearing value and friction on piles, the
writer believes that while the literature on engineering is full of
experimental data relating to friction on caissons, there is little to
show the real value of friction on piles. The assumption generally made
of an assumed bearing value, and the deduction therefrom of a value for
the skin friction is fallacious. Distinction, also, is not made, but
should be clearly drawn between skin friction, pure and simple, on
smooth surfaces, and the friction due to pressure. Too often the bearing