side walls or heads at
each end of the lock were executed by aid of compressed air. The
foundations for these heads were carried down to an impermeable
stratum by means of two bottomless caissons, filled eventually with
concrete, 213-1/2 ft. long across the lock and 105 ft. wide in the
line of the lock at the upper end, and 206-3/4 ft. long and 116-1/2
ft. wide at the lower end, to a depth of 18 ft. below the sill at the
upper end, and 41 ft. at the lower end, owing to the dip down seawards
and southward of the water-tight stratum. These caissons were provided
for their sinkage with temporary dams of masonry closing the opening
of the lock at the extremities of each caisson, enabling the gates to
be subsequently erected under their shelter. The junctions between the
foundations of the heads and the adjacent foundations were effected by
small movable caissons carried down in recesses provided in the buried
caissons. The connexions with the adjacent quay walls were
accomplished by two supplementary side caissons at the end of each
head; and the north side wall of the lock was founded by means of
seven bottomless caissons sunk by aid of compressed air, on account of
the proximity of the tidal harbour on that side. The south side wall
was founded for a length of about 200 ft. at its western end in an
excavated trench kept dry by pumping; but the greater portion was
founded in a dredged trench in which bearing piles were driven under
water, on which the masonry was built in successive layers, about
3-1/4 ft. thick, in a movable caisson 93-1/2 ft. long and 37-3/4 ft.
wide; whilst a bottomless caisson, left in the work, was employed for
founding about 100 ft. of wall at the eastern end. The bed of concrete
also, 10 ft. thick, forming the floor of the chamber, was carried out
for 82 ft. at the western end in the open air, and the remainder in
the same movable caisson as used for the south wall. Two sluiceways on
each side running the whole length of the lock, differing 6-1/2 ft. in
level, communicate with the lock-chamber through openings in the side
walls, 67-1/4 ft. apart, and provide for the filling and emptying of
the chamber.

[Illustration: FIG. 20.--Florida Lock, Havre Docks, Sections and
Plan.]

[Illustration: FIG. 21.--Wooden Dock Gate.]

[Illustration: FIG. 22.--Iron Segmental Dock Gate.]

[Illustration: FIG. 23.--Straight Iron Dock Gate.]