Department of International Improvement [pp. 117-130]

INTERNAL IMPROYEMENT.'
out of it, and hence it is not braced across from side to side, but is simply
designed to exclude the sand. It will be suspended from the wooden framework by a number of rods which will be worked by screws above, and by
which it will be lowered into the sand. It will be held against the current
by chains until it sinks by its own weight in the sand. When it has sunk
to a sufficient depth, the work of pumping out the sand is commenced,
which will be done with powerful sand-pumps, and as the caissons will be
slightly larger at the bottom than at the top, its inclined sides will facilitate
its sinking as the sand within is removed. In this way it will finally be
sunk to the rock, and, reaching above the surface, will completely inclose
the water within it. The sand being removed from the rock, a bed of concrete mortar will be spread over the rock and carefully levelled off. This
mortar-bed will be one or two feet in thickness above the highest parts of
the rock. Things being thus arranged to receive the pier, a portion of the
down stream end of the caisson will be tenvorarily removed to admit
within it a huge flat-bottomed boat. This boat will have its bottom constructed of squared timbers, placed solid, and of about eighteen inches in
thickness, and thoroughly caulked. It will form the base of the pierwood, as our readers are doubtless aware, being practically indestructible
under water. The sides of the boat will be nearly vertical and made of
strong timbers and plank, and caulked tight. In this vast vessel, the interior of which will be of the size and shape of the intended pier, the
masonry-work of the pier will be laid as fast as ten or twelve steam-hoisting
machines on the frameworks above can supply stone and other materials to
the masons working within it. As the masonry progresses, the boat slowly
settles down with its gigantic load, and as it sinks the sides are built up to
exclude the water, and will ultimately reach a height of 100 feet. It is
estimated, when about forty feet of the pier has been constructed, the boat
will have reached the bed prepared for it within the caisson, and will then
rest upon the rock. The masonry will be built up rapidly, the massive
stones being thoroughly grouted, course after course with hydraulic cement,
until the structure of the pier finally emerges above the surface of the water.
The sides of the boat will be secured to its bottom by screws, which can be
loosened from the top, and when the stone-work has reached above the surface, the boat will be permitted to fill with water, when the screws will be
disengaged and the sides will be taken out. The next step will be to remove the iron caisson. The screws supporting it will be put in motion and
it will be drawn up from the sand, unbolted and removed, to be used in the
construction of the other pier. The upper framework will then be taken
down and the screw-piles withdrawn. The pier will be then completed a
few feet above the s-urface of the water, and will be then left alone to withstand the rush of ice and flood, and will be completed in proper season.
The construction of these piers is certainly the most hazardous and difficult
part of the undertaking, but, previous to the commencement of the work,
every possible preparation which scientific foresight can suggest to insure
against delay will be taken.
VWe have thus gone over at some length the main features in the building
of the piers. We have, of course, been compelled to omit many interesting
details, but have said enough to enable our readers to form a general idea
of the magnitude of the undertaking, the obstacles to be overcome, and the
plan to be pursued in the construction.
Each of the great spans will be formed of four-ribbed arches of cast steel,
having a rise of about one-tenth of the span. Each one of these will be
formed of two ribs placed seven feet apart, one above the other, and strongly
braced between with diagonal steel braces. The carriage-way and railways
of the bridge will be supported over the wharf on either side of the river by
five stone arches of thirty feet span. These terminating arches will form
an architectural feature of an important character, and will add much to the
impressive appearance of the great structure.
The cost of the bridge and tunnel, exclusive of real estate, is estimated
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