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BREAKWATER CONSTRUCTION.
i8i
In 1891 the superstructure was commenced, and in 1893 indubitable
evidence was given of the prejudicial, and even disastrous, influence which it
exerted upon the breakwater as a whole. The waves striking against the
vertical face of the wall fell back with great force upon the top of the mound,
disturbing the blocks and laying bare the rubble core, which was then easily
washed away. The experience was renewed and confirmed in the following
year by a storm which has already been alluded to.
Accordingly, a change of plan was decided upon. The superstructure was
commenced at a level of 16| feet below its former level. Most probably this
in itself would have been insufficient to secure immunity from undermining,
resulting from the collapse of waves and their back-draught, had there not
been additional shelter afforded by the setting back of the line of the wall
Equinoctia! Low
Small
Rubble <
Equinoctia/ Hiqh Water
Large Rubble
Small Rubble
Plan oF Caisson.
Design Ultimately Adopted
200
0 102030 4050 60 70 80 80^0
Sca/e oF Feet:
Fig. 155. —Sections, Bilbao Breakwater.
nearly a couple of hundred feet from the seaward face of the artificial blocks
of the original breakwater, which latter thus constituted a sort of advance
guard or outlying defence. The breakwater, in fact, was practically
duplicated with a block mound in front and a wall at the rear, as shown in
fig. 155. The space betweeu the two (about 100 feet) not only reduced the
force and violence of the waves, but it also afforded some constructional con-
venience by providing room for a tugboat to work and facilitate the building
of the wall.
To render the wall as solid and homogeneous as possible, it was decided to
build it with the aid of framed and plated caissons. The dimensions adopted
for these were 42 feet 6 inches long by 23 feet wide by 23 feet deep. The
weight of the caissons was 30 tons, and they had a light draft of 12J inches ;
but before actually towing them into position (they were constructed on the
river bank), they were ballasted with a layer of Portland cement concrète
5 feet thick, which inoreased the draught to a little over 11 feet. When