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 
2—(6010)