wall was built of concrete faced with ashlar masonry
(fig. 18). The basin and quay walls at Bremen, Bremerhaven and Hamburg
were built on a series of bearing and raking piles driven down to a
firm stratum, the wall being begun a few feet below low water. At
Southampton, ferro-concrete piles were employed in constructing the
deep quays; and a wharfing of timber pilework has been frequently used
for river quays.

[Illustration: FIG. 16.--Glasgow River Quay Wall.]

[Illustration: FIG. 17.--Rouen Quay Wall.]

Where the increase of trade is moderate and the conditions of the
traffic permit, and also at coal-shipping ports, economy in
construction is obtained by giving sloping sides to a portion of a
dock in place of dock walls, the slope being pitched where necessary
with stone; and the length of the slope projecting into a dock is
sometimes reduced by substituting sheet piling for the slope at the
toe up to a certain height. By this arrangement jetties can be carried
out across the slope as required, enabling vessels to lie against
their ends; and coal-tips are very conveniently extended out across
the slope at suitable intervals (fig. 8).

Failures of dock walls.

As dock walls, especially before the admission of water into the dock,
constitute high retaining walls, not infrequently founded upon soft or
slippery strata, and backed up with the excavated materials from
alluvial beds, into which water is liable to percolate, they are
naturally exposed under unfavourable conditions to the danger of
failure. A dock wall erected on unsatisfactory foundations is liable,
where the bottom is soft, to settle down at its toe, owing to the
pressure at the back, and to fall forwards into the dock, as occurred
at Belfast; or where the silty bottom slips forward under the weight
of the backing, the wall may follow the slip at the bottom and settle
down at the back, falling to some extent backwards, as exemplified by
the failure of the Empress basin wall at Southampton. The most common
form, however, of failure is the sliding forwards of a dock wall, with
little or no subsidence, on a silty or slippery stratum under the
pressure imposed by the backing. Thus the Kidderpur dock walls furnish
an instance of sliding forwards on muddy silt, and part of the South
West India dock walls on two underlying, detached, slippery seams of
London clay.

To avoid these