mound in a similar manner by the aid of compressed
air (fig. 15). Quay walls also on the Clyde have been founded on
caissons, consisting of a bottomless steel structure, surmounted by a
brick superstructure having hollows filled with concrete, in lengths
of 80 ft. and 27 ft., and widths of 18 ft. and 21 ft. respectively,
carried down by means of compressed air from 54 to 70 ft. below
quay-level, on the top of which a continuous wall of concrete, faced
with brickwork, and having a granite coping, was built up from near
low-water level (fig. 16). In many cases where soft strata extend to
considerable depths, river quays and basin walls have been constructed
by building a light quay wall upon a series of bearing and raking
piles driven into, and if possible through, the soft alluvium. Thus
the walls along the Seine, and round the basins at Rouen, were built
upon bearing piles carried down through the alluvial bed of the river
to the chalk. The lower portion of the quay wall was constructed of
concrete faced with brickwork within water-tight timber caissons,
resting upon the piles at a depth of 9-3/4 ft. below low water; and upon
this a rubble wall faced with bricks was erected from low water to
quay-level, backed by rubble stone laid on a timber flooring supported
by piles, together with chalk, to form a quay right back to the top of
the slope of the bank of the deepened river (fig. 17). The quay walls
of the open basins bordering the Hudson river at New York have had, in
certain parts, to be founded on bearing piles combined with raking
piles, driven into a thick bed of soft silt where no firm stratum
could be reached, and where, therefore, the weight could only be borne
by the adherence of the long piles in the silt. Before driving the
piles, however, the silt round the upper part of the piles and under
the quay wall was consolidated by depositing small stones in a trench
dredged to a depth of 30 ft. below low water; the piles were driven
through these stones, and were further kept in place by a long toe of
rubble stone in front and a backing of rubble stone behind carried
nearly up to quay-level, behind which a light filling of ashes and
earth was raised to quay-level. The slight quay wall resting upon the
front rows of bearing piles was carried up under water by 70-ton
concrete blocks deposited by means of a floating derrick; and the
upper part of the