THE KANSAS CITY BRIDGE. ' 53

shifted fifty feet to the south, reversing the distances between it and the two
adjoining piers, and placing the long span, 250 feet, between Piers Nos. 4 and 5.

At the location now selected, the rock was assumed to be at an elevation
of 55 feet and to be overlaid, during the best working season, with about 40 feet
. of sand, which would probably make it necessary to do some portions of the
work in 50 feet of water. Borings taken indicated rock at 58 or 59, but were
not wholly satisfactory. The methods by which the other deep sand foundations
had been put in, though successful, had been very slow, and were likely to prove
impracticable when the depth of sand became doubled, while, even if a bottom-
less caisson could be sunk to the depth now required, the season between two
floods would be found too short to complete the work by putting in a subaqueous
foundation of beton, 30 or 40 feet deep. For these reasons a plan was pre-
pared resembling in many respects the process which was first introduced in
founding the piers of the bridge over the Rhine at Kehl, and which has since
been very generally employed by European engineers ; in all previous works,
however, the excavation has been made by laborers working in a pneumatic
chamber—machinery, if used at all, serving only to remove the material which
had first been handled by the men; but in these plans the machinery was so
arranged as to be self-feeding, and the excavation was carried on without the
use of compressed air. A pier of masonry was to be built in position above
water, and sunk to the rock, by excavating the underlying sand with dredges
working through wells left in the masonry, guiding the mass in its descent by
suspension screws, and keeping the top of the masonry above the surface of the
water by building on the successive courses as the sinking continued.

A caisson was designed which should serve as a support for the pier in its
descent, and which, while of such form as should furnish the best facilities for
excavation below, should bear, without yielding, the weight of 40 feet of masonry
above, and the pressure of the sand and water against its sides. The construc-
tion of this caisson was begun on the 25th of June, 1868, on the north bank of
the river, 400 yards below the bridge line. It measured 70 feet from nose
to nose, 20 feet 6 inches in width, and 11 feet in height.* The sides were

 

* The plans of this caisson are given on Plate Y.