kin friction on caissons and piles, it may
be of interest to mention an experiment which the writer made during the
sinking of the large caissons for the Williamsburg Bridge. These
caissons were about 70 ft. long and 50 ft. wide. The river bottom was
about 50 ft. below mean high water, and the caissons penetrated sand of
good quality to a depth of from 90 to 100 ft. below that level. On two
occasions calculations were made to determine the skin friction while
the caissons were being settled. With the cutting edge from 20 to 30 ft.
below the river bottom, the calculations showed that the skin friction
was between 500 and 600 lb. per sq. ft. The writer agrees with Mr. Meem
that, in the sinking of caissons, the arch action of sand is, in a great
measure, destroyed by the compressed air which escapes under the cutting
edge and percolates up through the material close to the sides of the
caissons.

With reference to the skin friction on piles, the writer agrees with Mr.
Meem that in certain classes of material this is almost a negligible
quantity. The writer has jacked down 9-in. pipes in various parts of New
York City, and by placing a recording gauge on the hydraulic jack, the
skin friction on the pile could be obtained very accurately. In several
instances the gauge readings did not vary materially from the surface
down to a penetration of 50 ft. In these instances the material inside
the pipe was cleaned out to within 1 ft. of the bottom of the pile, so
that the gauge reading indicated only the friction on the outside of the
pipe plus the bearing value developed by its lower edge. For a 9-in.
pipe, the skin friction on the pile plus the bearing area of the bottom
of the pipe seems to be about 20 tons, irrespective of the depth. After
the pipe had reached sufficient depth, it was concreted, and, after the
concrete had set, the jack was again placed on it and gauge readings
were taken. It was found that in ordinary sands the concreted steel pile
would go down from 3 to 6 in., after which it would bring up to the full
capacity of a 60-ton jack, showing, by gauge reading, a reaction of from
70 to 80 tons.

It is the writer's opinion that, in reasonably compact sands situated at
a depth below the surface which will not allow of much lateral movement,
a reaction of 100 tons per sq. ft. of area can be obtained without any
difficulty whatever.

FRANK H. CARTER, ASSOC. M. AM. SOC. C. E. (by letter).--Mr.
Meem has contributed