ugge and Scheveningen
harbours, large open metal caissons, built inland, ballasted with concrete,
floated out into position, and then sunk and filled with concrete, have
been employed for forming very large foundation blocks for the breakwaters
(see BREAKWATER). Open iron caissons are frequently employed for enclosing
the site of river piers for bridges, where a water-tight stratum can be
reached at a moderate depth, into which the caisson can be taken down, so
that the water can be pumped out of the enclosure and the foundations laid
and the pier carried up in the open air. Thus the two large river piers
carrying the high towers, bascules, and machinery of the Tower Bridge,
London, were each founded and built within a group of twelve plate-iron
caissons open at the top; whilst four of the piers on which the cantilevers
of the Forth Bridge rest, were each erected within an open plate-iron
caisson fitted at the bottom to the sloping rock, where ordinary cofferdams
could not have been adopted.

Where foundations have to be carried down to a considerable depth in
water-bearing strata, or through the alluvial bed of a river, to reach a
hard stratum, bottomless caissons sunk by excavating under compressed air
are employed. The caisson at the bottom, forming the working chamber, is
usually provided with a strong roof, round the top of which, when the
caisson is floated into a river, plate-iron sides are erected forming an
upper open caisson, inside which the pier or quay-wall is built up out of
water, on the top of the roof, as the sinking proceeds. Shafts through the
roof up to the open air provide access for men and materials to the working
chamber, through an air-lock consisting of a small chamber with an
air-tight door at each end, enabling locking into and out of the
compressed-air portion to be readily effected, on the same principle as a
water-lock on a canal. When a sufficiently reliable stratum has been
reached, the men leave the working chamber; and it is filled with concrete
through the shafts, the bottomless caisson remaining embedded in the work.
The foundations for the two river piers of the Brooklyn Suspension Bridge,
carried down to the solid rock, 78 and 45 ft. respectively below
high-water, by means of bottomless timber caissons with compressed air,
were an early instance of this method of carrying out subaqueous
foundations; whilst the Antwerp quay-walls, commenced many years ago in the
river Scheldt at some