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1 20
HARBOUR ENGINEERING.
acting individually and collectively upon all obstacles and structures in their
path.
(1) A direct horizontal force, exerting compression.
(2) A deflected vertical force, acting upwards and tending to shcar off any
projections beyond the face line of the obstacle, whether cliff or wall.
(3) A vertical downward force due to the collapse of the wave and
exercising a particularly disturbing effcct ou mounds in shallow water and
beaches.
(4) The suction due to back-draught or after-tow. This also produces its
most noticeable results on foundation beds, whether natural or artificial.
Applying these unmistakable and fundamental phenomena to the question
of breakwater design, it will be reoognised that the forces to which they give
rise are as follows :—
(1) A powerful momentary impact, combined with
0 !0 20 30 40 50 50 70 30 90 100
I II I I I 1 I I I I
Scale of Feet.
Fig. 100. — Section of Leixoes Breakwatei
200
(') Hydrostatic pressure continuons for some short period, however minute,
after the first shock.
Attending these principal forces there will be several subsidiary results,
such as: —-
(1) A vibration of the whole structure, tending to weaken the connection
of the varions parts.
(2) A series of impulses imparted to the water contained in the pores,
joints, and interstices of the structure, producing internai pressures in varions
directions.
(3) The alternate condensation and expansion of the volumes of air which
are confined in cavities and which may be unable to escape freely or not
at all.
The exact determination of these stresses is practically impossible. Some-
thing, however, may be done towards estimating their scope and extent.
How far this lies within the range of definite and effective calculation is our
next concern.