ll.]

The walls of open basins are often constructed out of water precisely
like dock walls, as in the case of the basins forming the Manchester,
Bruges and Glasgow docks; and basin walls open to the tide, as at
Glasgow and in the tidal basin outside Tilbury docks (fig. 7), differ
only from dock walls in being exposed to variations in the pressure at
the back resulting from the lowering of the water-level in front,
which is, indeed, shared to some extent by the walls round closed
docks where the difference in the high-water levels of springs and
neaps is considerable. The walls, however, round basins in tideless
seas, such as Marseilles, occasionally those inside harbours, and
especially quay walls along rivers and round open basins alongside
rivers, have to be constructed under water.

[Illustration: FIG. 12.--Barry Dock Wall.]

[Illustration: FIG. 13.--Marseilles Quay Wall.]

[Illustration: FIG. 14.--Antwerp Quay Wall, founded by compressed
air.]

[Illustration: FIG. 15.--Caracciolo Jetty Quay Wall, Genoa.]

Open basin and river quay walls founded under water.

At Marseilles, the simple expedient was long ago adopted of
constructing the quay walls lining the basins formed in the sea, by
depositing tiers of large concrete blocks on a rubble foundation, one
on top of the other, till they reached sea-level, and then building a
solid masonry quay wall out of water on the top up to quay-level,
faced with ashlar (fig. 13), the wall being backed by rubble for some
distance behind up to the water-level. The same system was employed
for the quay walls at Trieste, and at Genoa and other Italian ports. A
quay wall inside Marmagao harbour, on the west coast of India, was
erected on a foundation layer of rubble by the sloping-block system,
to provide against unequal settlement on the soft bottom (see
BREAKWATER). The quay walls alongside the river Liffey, and round the
adjacent basins below Dublin, were erected under water by building
rubble-concrete blocks of 360 tons on staging carried out into the
water, from which they were lifted one by one by a powerful floating
derrick, which conveyed the block to the site, and deposited it on a
levelled bottom at low tide in a depth of 28 ft., raising the wall a
little above low water. After a row of these blocks had been laid, and
connected together by filling the grooves formed at the sides and the
in