Compressed Air Work And Diving 1909
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52
COMPRESSED AIR WORK.
diameter bclow ground, and founded 011 rock at a depth
of 84 ft. below high water. During the sinking one of
the cylinders cracked, and had to be struttecl with timber
and stiffened with a wrought-iron hoop.
For the centre pier of the Saltash Bridge over the
Tamar, Mr Brunei again usecl compressed air, and
designed a caisson on rather more elaborate lines than
had previously been usecl. The pier itself consisted of
a masonry column, 35 ft. in diameter, carriecl up above
high water. O11 it were placed four smaller octagonal
Fig. 4.—Saltash Bridge Circular Caisson, 37 ft. diameter.
cast-iron columns 10 ft. in diameter, reaching to the
underside of the girders. For the construction of this
pier a cylinder 96 ft. high and 37 ft. in diameter was
designed (Fig. 4). The cutting edge was made 6 ft.
lower on one side than the other, in order to conform to
the. slope of the rock below the mud, as ascertained by
careful borings. About 14 ft. from the highest point of
the cutting edge was a domed roof, the top of which com-
municated with a 10-ft. diameter shaft open to the air at
atmospheric pressure. 1 he portion of caisson below the