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HARBOUR ENGINEERING.
136
can be attached to any specific type of breakwater for general adoption.
Questions of cost and maintenance, the degree of efficiency desired, the nature
of the sea bottom, and the extent of exposure—these are all matters which
have to be individually weighed before any definite decision can be arrived at.
At the present time, there are breakwaters, either in course of construction
or recently completed, of the pure wall type at Dover and Tynemouth, of
the pure mound type at Brest and Marseilles, and of the composite type at
Zeebrugge, Bilbao, and Peterhead.
We now enter upon a discussion of the conditions affecting the stability of
breakwaters.
The Stability of Mounds.—It has already been pointed out that
mounds are lacking in the quality of permanence. This applies more par-
ticularly to their upper portions which are under the constant influence of
hydrodynamic action. The equilibrium of the lower portion is simply a
question of quiescent hydrostatic pressure. Wave influence does not extend
to an indefinite depth. Below the level at which its effeets are felt, it has
been found that rubble mounds will stand at slopes of 45 or 50 degrees.
The limiting depth of wave influence, however, is a matter of some un-
certainty. It has generally been assumed, until recently, that a depth of
30 feet below the surface level marks the extreme boundary of the zone of
appréciable disturbance; but there are on record instances of serious wave
action at greater depths. Thus at Peterhead Harbour, in October 1898,
blocks weighing upwards of 41 tons each were displaced by waves at a depth
of 36^ feet below low water of ordinary spring tides. Instances of this nature,
however, are very rare, and in the majority of cases the standard limit may
still be counted upon as generally reliable.
The disturbing influence of waves is most keenly felt between the levels
of high and low water, and it is in this region that the most trying ordeals
of a breakwater are experienced. A difficulty underlying the situation is that
in proportion as the slope is flattened to maintain its equilibrium, the dis-
ruptive effort of the wave is fostered and increased. Hence the introduction
of huge blocks and monoliths to withstand impact. These blocks, which
rarely weigh less than 25 or 30 tons a-piece, and often considerably more, may
be deposited either in courses or at random. In the former case, they may
be stepped so as to form a general inclination of 1 to 1 ; but if deposited at
random, a flatter slope will be necessary.
The blocks, when artificial, are generally made in the form of rectangular
solids: parallelopipeds in preference to cubes; and they should be laid as
headers—that is, with their ends facing the line of wave action. In this way
the minimum face area is exposed to the stroke, and there is the maximum
resistance to overturning.
Natural blocks are heavier per unit volume than the majority of artificial
blocks, and, for this reason, have clairns to preference. They are also less
liable to disintegration, but they are difficult to procure economically to large
dimensions, and their irregular shapes render it impossible to bed them