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i66
HARBOUR ENGINEERING.
One caution to observe in
periods of deposit that a mass of
SA 00Jg.
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Cross Section
tidal situations is that of so arranging the
freshly placed ooncrete may not be subjected,
while setting, to the disturbing action
of a choppy sea surface. Less than
18 inches or 2 feet of water is in-
sufficient to prevent even small waves
from exercising a deleterious influence,
chafing the concrete, and robbing it
of its cement. Therefore, wherever
possible, advantage should be taken
of the variation in the tidal level,
surface of an uneven bed destined
during springs and neaps, to suspend
concreting from time to time, either
at some little depth below the sur-
face or altogether out of range. A
quick-setting cement will prove of con-
sidérable value in tidal situations.
It is needless to remark that when
operations are carried on within the
shelter of a diving-bell, the same re-
strictions do not apply, although it
must be admitted that sudden out-
bursts of air may do more damage
than the fretting action of waves.
Yet, with care, these may be avoided.
The work is then only limited by the
convenience of arrangements in regard
to shifts.
Bagwork.—The dispersive action
of waves, and, indeed, the solvent action
of comparatively still water, has led
to the introduction of a system of con-
crete laying in sacks or bags. These
bags have, in certain cases, attained
a very considerable size and weight,
the latter reaching 100 tons and over.
But small bags of 5 or 10 tons, or
thereabouts, are most common. They
are often employed for regularising the
to receive blocks. The bags are of
jute or canvas, strongly made. After being filled with concrete, they
must be deposited immediately, while the material is plastic, so that each
bag may adapt itself to the inequalities of its environment. This adapta-
tion of bulk to various positions is one of the chief advantages claimed
for bagwork. The system is not without certain drawbacks ; the bags are