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BREAKWATER CONSTRUCTION. 183
10 feet by 8 feet each, set in two courses as headers and stretchers alternately.
The hearting is of mass concrete. Given favourable conditions and fine
weather, the superstructure could be finished during the fourth tide with the
exception of the parapet wall, which was not undertaken until the full measure
of settlement had been obtained. This was considered to have been achieved
after the lapse of a couple of winters. From the moment a caisson was first
placed to the completion of the superstructure, the settlement averaged about
8 inches. Under the load and vibration of the Titan crane setting blocks
further seaward, and under the influence of winter storms, a further settlement
of 16 inches took place, making 24 inches in all. When this had been
realised, the joints between the caissons, which were 12 inches wide, were
made good with cement concrete, and the parapet wall was built.
The total amount of the contract was équivalent to £962,756, and, as the
length of the breakwater is 4757 feet, the cost works out to £203 per foot
run. The quantifies of material in a caisson length (42 feet 8 inches) were
as follows:—
Cub. yds. Cub. yds.
( Bottoin ballast 42 feet 8 inches x 23 feet x 4 feet 11 inches= 178'54
g ) Twelve blocks each 39'4 cubic yards . . . 470’88
) Filling-in concrete ..... 180’20
O ( Steel bulkheads and tirnber struts . . . 3'55
------833'17
Super- f Eight blocks, each 39’24 cubic yards . . 313’92
structure. 1 Filling-in concrete .... 166’77
1 --------480’69
Total . . . 1313'86
Say 1314 cubic yards in all.
Breakwater at Bizerta.1—The type of structure primarily adopted at
Bizerta for the converging jetties at the entrance to the port, built between
1889 and 1895, was the rubble mound, consisting of a core of pierre perdue
of all sizes, surmounted and protected on the sea face by a revetment of
natural blocks of large size. The site is not so exposed to violent gales as are
other places on the north coast of Africa, and this system of construction was
found to answer very satisfactorily. Unfortunately, the local stone (a marly
limestone of poor quality, flaking rapidly in salt water) was of such a character
as not to commend itself for further use, and a complété departure in design
was made in dealing with the new breakwater and the extension of the north
jetty, the former of which has a length of 2000 feet, and the latter of 660 feet.
These works, as shown in fig. 157, were carried out by means of metallic
caissons with movable upper works, forming ultimately huge artificial blocks
faced with marble, which were laid upon a mass of miscellaneous riprap at a
level of 26 feet below low water. The blocks measured 102 feet by 26| feet
by 26| feet cubing at 70,000 feet, and having a weight of 5000 tons each.
They were set with great precision, and only two blocks out of twenty-three
varied perceptibly from the exact line. The settlement, averaging 27 inches,
was very regular throughout. The slopes of rubble, however, intended to be
1 De Joly on Breakwaters, Min. Proc. Tenth Int. Nav. Gong. Milan, 1905.
I