The center pier is octagonal, and is built in the same general manner
as to foundations as the shore piers, but the piles are cut off 22
feet below water, and there are eighteen courses of timber in the
grillage. The diameter of the platform between parallel sides is 53
feet, while that of the lower course of battered masonry is but 37
feet. The areas are as 2,332 to 1,147, or as 2 to 1 nearly. The
pressure per square inch of timber on the heads of the piles is about
the same as stated above for the shore piers. The number of piles
under the center pier is 483.

The risks and delays by this method of constructing the foundations
were much less, and the cost also, than if an ordinary coffer dam had
been used. Also the total weight of the piers is much less, as that
portion below a point about two feet below the water adds nothing to
their weight.

The piles were driven with a Cram steam hammer weighing two tons, in a
frame weighing also two tons. The iron frame rests directly upon the
head of the pile and goes down with it. The fall of the hammer is
about 40 inches before striking the pile. The total penetration of the
piles into the clay averaged 27 feet. The settlement of the pile
during the final strokes of the hammer varied from one quarter to
three quarters of an inch per blow.

There are 122 masonry pedestals, of which eight are large and heavy,
carrying spans of considerable length. They will all be built upon
concrete beds, except a few near the river on the north side, where
piles are required.

The four abutments with their retaining walls are of first-class