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144
HARBOUR ENGINEERING.
somewhat generally the same lines of construction as the Grande Jetée, in so
far as regards the disposition of the material and the external slopes. The
quay, however, is only about half the width of that in the previous case.
The cost of this breakwater, including the outer protection blocks, amounted
of stones of considerable size (some 500
external swell of the sea
to nearly £154 per yard. The
parapet wall with its ashlar work
came to £61, 10s. per yard, and
the formation of the inner quay
involved another £44, making
the total cost, approximately,
£260 per lineal yard.
Breakwaters at Algiers.
—The principle underlying the
design of the north and east
breakwaters at Algiers, is that of
g approximating the rubble mound
as closely as possible to the form
^ and fonctions of an upright wall.
£ The mound has been laid to the
£ very steep slope of 45 degrees
throughout, and the superstruc-
£ ture occupies the whole of the
^ narrow summit. Such a design
§ is open to very strong and grave
g objections. The impulse of the
“ wave, abruptly checked by the
2 face of the parapet wall, is con-
verted into a powerful downward
£ force directed against the outer
slope, or, alternatively, the
waves, rising to an abnormal
height, clear the parapet wall
and break with considerable im-
pact upon the inner side. In
each case the tendency to dis-
turbance is very pronounced,
and movements frequently take
place. Furthermore, the voids
and interstices in the uppermost
layer of rubble, which consists
cubic feet in volume), causes the
to be transmitted through the breakwater to the
interior area, giving rise
to an agitation of the surface,
which is incom-
patible with efficient harbourage.
Accordingly, iu the construction of the inner port at the Agha, begun in