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138 HARBOUR ENGINEERING.
(3) By the uplifting and dislocation of a horizontal course, or layer ; and
(4) By fracture and shattering.
Assuming, for the sake of convenience and simplicity, that we are dealing
with a single block or monolith of assigned dimensions, we may express the
value of the adhesiveness of the bed-joint or base in its resistance to shear as
from 3 to 5 tons per square foot if the cementing material be hydraulic mortar
of good quality, and from 6 to 9 tons per square foot if Portland cement
mortar, further assuming that, in each case, the proportion of sand to the
matrix does not rise above 3 to 1.
Accordingly, the condition for equilibrium is that the horizontal com-
ponent of wave pressure measured in tons shall not exceed the area of the
bed-joint in square feet multiplied by some coefficient ranging from 5 to 9
in accordance with the nature of the cementing material of the joint.
This is in regard to shearing action. If the joint be already fractured,
or if the adhesiveness be neglected, then resistance to movement can only be
forthcoming through the agency of friction. The coefficient of friction has
already been stated at '7 (vide p. 126) for smooth concrete blocks, and a value
of ’65 to '7 will hold for all surfaces of masonry and brickwork in contact.
For stone on rock the same value will suffice, but for brick or stone on moist,
unctuous clay, the coefficient must be reduced as low as '3. If the weight of
a given block be W, then something like 7 W is the force required to move it
over a masonry or rocky surface, and from -3W to -5W over an earthen one.
A distinction must, however, be made as regards the weight of the block,
whether it be submerged entirely, partially, or not at all. Substances
immersed in water lose a part of their weight équivalent to the weight of the
volume of water which they displace. Consequently, the effective weight of
a completely immersed block is less than its weight in air by the weight of
an equal volume of water, which, in the case of sea-water, it is customary to
estimate at the rate of 64 Ibs. per cubic foot.
The fact may be expressed in another form by stating that the weight of
the block is equal to (d - 1) times the weight of an equal volume of water, d
being the density of the block compared with that of water as unity. This
relationship has an important bearing on our next considération.
Parenthetically, it may be pointed out that sliding action is very materially
assisted by small smooth stones and pebbles more or less spherical in shape,
which not infrequently intrude themselves between the detached blocks
protecting the outer slopes of certain breakwaters.
The resistance of breakwaters or their component parts to overturning
arises from their (effective) weight and from the tensional streugth of the
joints. This latter source should, however, not be counted upon. Beyond
affording some slight additional margin of security, its assistance is so slight
as to be negligible, especially when compared with the inertia of the mass.
The overturning1 effort is due to the horizontal pressure of the wave,
which exerts a moment about any point of the base measurable as Fæ, where
a: is the height above the base at which the effect of impact is assumed to be