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l8o HARBOUR ENGINEERING.
reacted detrimentally upon the stability of the onter slope by increasiug the
recoil of the waves. As it is, the sea overrides the breakwater in rough
weather, and this, combined with an unstable foundation, produces movements
in the outermost blocks. Voids are created, and these have to be filled with
fresh blocks tipped as closely as possible into position. Hence, the seaward
face is losing, to a very large extent, its arrangement in regnlar courses, so that
the desirability of creating, or of attempting to maintain, anything of the kind
with detached blocks in an exposed position, is open to question.
The cost of depositing the blocks was as follows: —
Tipped overboard, 57s. per block.
Set without regulär coursing, 78s. 6d. per block.
Set and coursed regularly, 92s. 4d. per block.
The cost of the breakwater complète ranged from £75 to £90 per foot
run. The cost of replenishing the blocks on the outer slope forms a current
charge of about eight guineas per lineal yard per annum.
Scate oF Feet.
Fig. 154. —Section of West Mole at Genoa.
Breakwater at Bilbao. —The protection works in Bilbao Bay, at the
mouth of the River Nervion, afford an illustration of the method of con-
struction by caisson monoliths. The example is the more interesting in that
the original design for the breakwater was very materially modified under the
severe experience gained in the course of its formation.
The original design is shown in fig. 155. It comprised all the features
associated with breakwaters of the mixed type, viz., an inner core of small,
with an outer layer of large, rubble stone surmounted by large conerete blocks
deposited at random, upon which was to be erected a superstructure of mass
conerete with blockwork facings, and an upper parapet wall. The foundation
consisted of mud and sand except in the parts immediately adjacent to the
shore where the rock was exposed. The artificial blocks contained from 40 to
65 cubic yards each, and brought the level of the work up to low water line
of equinoctial tides. Begun in 1888, the substructure was allowed to settle
for a couple of years before any additional weight was imposed upon the
foundation.