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BREAKWATER CONSTRUCTION. ' l6l
tie-rods or bars; and great care should be exercised in construction. The
better course, wherever practicable, is to excavate to the lower level, at which
the harder stratum is found.
It must be borne in mind that some settlement is inévitable. It will take
place, if not in the foundation, at anyrate in the structure itself, especially
in mounds formed of rubble work. The numerous vacuities in the mass and
their proportionately great volume, combined with inequalities of bedding and
support, lead to a shrinkage of the entire mass, which is very considerable in
the earlier stages of its existence, and is more or less a constant characteristic.
The diminution arising from this cause, however, is readily made good in
ordinary cases by the simple deposition of additional material; but it is mani-
fest that where the mound is acting as a substratum or base for a wall, the
effects of shrinkage cannot be so easily effaced, nor can the wall itself escape
a share in untoward conséquences. Hence the obvious necessity of allowing
such mounds adequate time to take a firm bearing.
Moreover, it must not be overlooked that in addition to that arising from
its own inherent tendencies, some further subsidence must occur when the
weight of the wall is imposed upon a mound. Allowance must be made, in
the first instance, for this and for other contingencies.
Settlement, therefore, in some form or other, must be looked upon as
inevitable, and the essential point is to ensure its uniformity. Well-
constructed breakwaters have sunk to the extent of 10 or 12 per cent, of their
total height without appreciably afiecting the appearance or the stability of
the superstructure ; but this has only been so because the process was gradual
aud regular. Sudden and irregular changes cannot fail to produce fracture,
especially in bonded work, concerning which we must speak later.
Permanence.—The second point of a good foundation is permanence,
or unalterability. Certain mineral substances, when exposed to external
influences, undergo physical and chemical changes which naturally modify
their characteristics. The hardest rocks, such as granite, are known to
disintegrate and decay under atmospheric agencies alone. Marine, and
particularly submarine, agencies are much more drastic in action. The
attacks of sea-worms, the erosive power of currents, the dissolving properties
of water, and the percussive action of waves—all these are sources of change
and détérioration.
As far as possible, therefore, a foundation should be guarded from
destructive influences. Even when the ground is naturally firm and durable,
it is very desirable to protect the surface in the immediate neighbourhood of
the breakwater from scour. To this end, in the case of upright walls, rubble
and riprap are deposited alongside, so as to form an apron covering the
toe, and, in more exposed cases, large blocks and monoliths are similarly
utilised.
Wall Foundations.—Before dismissing the subject of foundations, we
must make a few remarks on the manner in which they are prepared for
breakwater piers of the upright wall type.
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