with a cutting
edge underneath, and gradually sunk by excavating inside; and
eventually the central hollow is filled up solid with concrete or
masonry.

Dock walls.

The walls round a dock serve as retaining walls to keep up the quays;
and though they have the support of the water in front of them when
the docks are in use, they have to sustain the full pressure of the
filling at the back on the completion of the dock before the water is
admitted. They have, accordingly, to be increased in thickness
downwards to support the pressure increasing with the depth. This
pressure, with perfectly dry material, would be represented by the
weight of half the prism of filling between the natural slope of the
material behind and the back of the wall; but the pressure is often
increased by the accumulation of water at the back, which, with fine
silty backing, is liable to exert a sort of fluid pressure against the
wall proportionate to the density of the mixture of silt and water.
The increase of thickness towards the base used formerly to be
effected by a batter on the face, as well as by steps out at the back;
but the vertical form now given to the sides of large vessels
necessitates a corresponding fairly vertical face for the wall, to
prevent the upper part of the vessel being kept unduly away from the
quay. Examples of the most modern types of dock walls are given in
figs. 9 to 12.

[Illustration: FIG. 9.--Havre Bellot Dock Wall.]

The height of a dock wall depends upon the depth of water always
available for vessels, at tideless sea-ports and at ports removed from
tidal influences, such as Manchester, Bruges and the ports on the
Rhine; this depth should not be less than 28 to 30 ft. for large
sea-going vessels, together with a margin of 5 to 8 ft. above the
normal water-level for the quays, and the foundations below. At tidal
ports, however, an addition has to be made equal to the difference in
height between the high-water levels of spring and neap tides; so that
at ports with a large tidal range, such as the South Wales ports on
the Severn estuary and Liverpool, specially high dock walls are
necessary. Under normal conditions, a dock wall should be given a
width at a height half-way between dock-bottom and quay-level, equal
to one-third of its height above dock-bottom, and a width of half this
height at dock-bottom.

[Illustration: FIG.