failures with untrustworthy foundations, great care has
to be exercised in selecting the best hard material available,
unaffected by water, for the backing, which should be brought up in
thin, horizontal layers carefully consolidated; and where there is a
possibility of water accumulating at the back, pipes should be
introduced at intervals near the bottom right through the wall in
building it, and rubble stone deposited close to the back of the wall,
so as to carry off any water from behind, these pipes being stopped up
just before the water is let into the dock. These precautions,
moreover, are assisted by reducing the amount of backing to a minimum
in the construction of the wall, best effected by building the wall
inside a timbered trench. The liability to slide forwards can be
obviated by carrying down the foundations of the wall sufficiently
below dock-bottom to provide an efficient buttress of earth in front
of the wall, and also by making the base of the wall slope down
towards the back, thereby forcing the wall in sliding forwards to
mount the slope, or to push forward a larger mass of earth; whilst a
row of sheet piling in front of the foundations offers a very
effectual impediment to a forward movement, and, in combination with
bearing piles, prevents settlement at the toe in soft ground. In very
treacherous foundations it may be advisable to defer the completion of
the backing till after the admission of the water; but the additional
stability given to a retaining wall or reservoir dam by an ample
batter in front, is precluded in dock walls by the modern requirements
of vessels.

[Illustration: FIG. 18.--New York Quay Wall, Hudson river.]

Maintenance of depth.

Silt accumulates in docks where the lowering of the water-level by
locking, the drawing down of half-tide basins, and the raising of the
water at spring tides, involve the admission of considerable volumes
of tidal water heavily charged with silt, which is deposited in still
water and has to be periodically removed by dredging. To avoid this,
the water is sometimes replenished from some clear inland source, an
arrangement adopted at some of the South Wales ports opening into the
muddy Severn estuary, and at the Alexandra dock, Hull, to exclude the
silty waters of the Humber. At the Kidderpur docks on the Hugli, the
water from the river for replenishing the docks is conduct