10.--Liverpool Dock Wall.]

Dock walls are constructed of masonry, brickwork or concrete, or of
concrete with a facing of masonry or brickwork. Masonry is adopted
where large stone quarries are readily accessible, in the form of
rubble masonry with dressed stone on the face, as for instance at the
Hull and Barry docks, and forms a very durable wall; but strong
overhead staging carrying powerful gantries is necessary for laying
large blocks. Brickwork has been often used where bricks are the
ordinary building material of the district or can be made on the
works, and requires only ordinary scaffolding; and harder or pressed
bricks are employed for the facework. Concrete is very commonly
resorted to now where sand and stones are readily procured; and where
clean, sharp sand and gravel are found in thick layers in the
excavations for a dock, as in the alluvial strata bordering the
Thames, dock walls can be constructed cheaply and economically with
concrete deposited within timber framing, dispensing with regular
scaffolding and skilled labour. Such walls require to be given a
facing of stronger concrete, or of blue bricks, as at Tilbury, to
guard against abrasion by vessels, chains and ropes; and dock walls
are commonly provided at the top with granite or other hard stone
coping where the wear is greatest. The foundations for dock walls are
excavated in a trench below dock-bottom, only lined with timbering
where the faces of the trench cannot stand for a short time without
support, and with sheet piling through very unstable silt or sand; and
the trench is conveniently filled up solid with concrete, carried out
in short lengths in untrustworthy ground. To reduce the amount of
filling behind the wall, the excavation at the back above dock-bottom,
preparatory for the trench, is given as steep a slope as practicable,
supported sometimes towards the base by timbering and struts; but
occasionally the wall is built within a timbered trench carried down
to the required depth, before the excavation for the dock in front of
it has been executed, as effected at Tilbury. The filling at the back
is thus reduced to a minimum, and the lower portion of the excavation
can be accomplished by dredging, if expedient, after the admission of
the water, the dock wall in this way being exposed to the least
possible pressure behind.

[Illustration: FIG. 11.--Tilbury Basin Wa