¾ in. resting on gravel at a depth of from 20 to 30 ft., cleaned out
practically to the bottom, sustain a measured load of 60 tons without
settlement.

As to skin friction in sand, a case came under his observation wherein a
14-in. hollow cylindrical pile which had stood for 28 days at a depth of
about 30 ft. in the sand, was cleaned out to its bottom and subjected to
hydraulic pressure, measured by a gauge, and sunk 2 ft. into the sand
without any pressure being registered on the gauge. It should be
explained, however, that the gauge could be subjected to a pressure of
250 lb., equal to a total pressure of 7,000 lb. on the piston of the
jack without registering, which corresponded, assuming it all as skin
friction, to a maximum of not more than 78 lb. per sq. ft., but it
should be noted that this included bearing value as well, and that the
pressure was very far from 7,000 lb., in all probability, at the
beginning of the test.

In the case of the California stove-pipe wells driven by the Board of
Water Supply on Long Island, the writer is informed that one of these
tubes, 12 in. in diameter, was sunk to a depth of 850 ft. In doing this
work the pile was excavated below the footing with a sand pump and was
then sunk by hydraulic pressure. Assuming the maximum capacity of the
jacks at 100 tons, which is not probable, the skin friction could not
have amounted to more than 75 lb. per sq. ft. It cannot be assumed in
this case that the excavation of the material below the pile relieved
the skin itself of some of its friction, as the operation consumed more
than 6 weeks, and, even if excess material was removed, it is certain
that a large percentage of it would have had time to adjust itself
before the operation was completed.