on
hard pan, which varies in thickness from o to 18 to 20 feet. In such
cases where the hard pan was very thick we put the concrete bed on this
hard pan after removing- about 4 feet of it. Of course, where we
encountered rock without an overlying layer of hard pan we founded it
on this rock." (Hard pan is here understood as a very consolidated
mixture of clay and boulder rock.)
The
foundations of the Hudson Terminal Building rest on the Manhattan
schists or gneiss 85 to 110 feet below the street curb (J. V. Davies,
C. E.), passing through, above the gneiss, 10 to 15 feet of " hard
pan," above which were sand beds, mud, and filling. Manhattan schist
was taken 390 feet below curb at Cortlandt and Church Streets (A. S.
Coffin), and a flake of granite from granite vein at 240 feet below
curb at Cortlandt and Broadway (A. S. Coffin).
General
conditions respecting the foundations of buildings at the lower end of
Manhattan Island have been described in the following language, by
Engineer T. Kennard Thomson:
"
Under the New York quick-sand, which is from thirty to sixty feet
thick, we find from two to thirty feet of hard pan. Sometimes this is
directly on the rock, and sometimes we find masses of sand and boulders
under the hard pan. Good hard pan, if on bedrock, will hold up any
building that ever will be raised in New York, but nothing except
personal examination of each individual caisson foundation on the job
is sufficient to decide whether the hard pan is really good, and even
then it is necessary to determine if the good hard pan extends to the
rock or not. This opinion is the result of having been down in the air
chamber more than two thousand times.
The
deepest caisson foundation in New York is under the Mutual Life
building, where the depth is one hundred feet below the curb and
eighty-five feet below the surface water, through thirty feet of
quick-sand, twenty-three feet of hard pan, and then thirty-two feet of
sand, boulders and decomposed rock, which we took out with a shovel."
Fossil evidences as to age and past conditions of the Man-
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