PORTS AND HARBOURS ae
this dispersive action calls for constant répleni hment or 72
material. Moreover, the depth of wave a eatin! Lad
down to more than 30 ft. below sea leve \ “locks have
been disturbed at a depth of 36 ft. Bel “thus levelp.
mounds appear to be fairly stable and will star itl side
slopes of 45 degrees or thereabouts.

The wall reflects waves without breaking them, and is
much less liable to damage. In a properly constructed
work, maintenance costs are insignificant. The wall is
more costly to construct, except, possibly, in relatively
shallow situations, where the depth does not exceed, say,
4o ft. It is more effective than a mound, in that wave
undulations, which are transmitted in a modified degree
through the interstices of a mound, are completely
destroyed against the face of the wall.

For depths greater than 40 or 45 ft., it is economical
and usual to employ breakwaters of the composite form,
viz., a mound from the sea bottom up to a level of about
40 ft. below the surface, with a wall superstructure there-
from to the summit.

The convenience and economy arising out of the use of
concrete has caused its almost universal adoption for
breakwater construction, certainly in the case of the wall
type; and a very suitable form is the monolith, or mass
block. This is a hollow shell, generally rectangular or
box-shape, built on land and launched, floated into
position and sunk, thereafter being filled with concrete,
so as to form a solid block of immense size extending from
the foundation to surface level. The force required to
disturb such a gigantic homogeneous structure would
obviously be enormous.

Other forms of breakwater construction consist of
sackwork, in which jute sacks are filled with concrete and
deposited under water by divers; cribwork, in which
rectangular crates, or cribs of timber, are floated out into
position, weighted with stone, and sunk (this is, of course,

a type only suitable in cases of minor importance and is

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