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BREAKWATER CONSTRUCTION.
173
“The diving-bell now in use is 12 feet long by 9 feet wide by 6 feet high,
and four men work in it at a time. The pressure, which, of course, varies
with the depth of water, is about 20 Ibs. on the square inch above that of
the atmosphère, and up to now (1902) there has been no case of sickness due
to working under air-pressure. When the bed is prepared, the blocks are set
by helmet divers, and as great care is taken to get them level and true as if
they formed part of an architectural structure above water. Ihe reason for
commencing the work at some distance seaward of the junction, was to admit
of the work being carried on at two faces, and thus extended seaward and
shoreward simultaneously.”
Breakwater Construction at Alderney.—The following detailed
description of the operations in connection with the construction of the wall
or superstructure of Alderney breakwater will be found extremely instructive.
It is quoted from an account1 by Mr (now Sir) John Jackson, who acted as
contractor’s agent on the works for a period of nine years. The breakwater
unfortunately subsequently acquired an unenviable reputation on account of
the large annual expenditure incurred in its maintenance.
“ In building the walls, as no machinery of any kind could remain out
during the winter, the works had to be recommenced every year. The first
operation was taking down a machine called the ‘Samson, invented in
Alderney. This was like a railway turn-table on wheels, with balks of timber
76 feet long, placed across and over-trussed ; one end, which projected past
the side of the table farther than the other, was called the jib, and at the
other end was the balance-weight. A double-purchase crab was fixed in the
centre, which worked a chain over a travelling sheave near the end of the jib ;
and the whole revolved on the under frame. The gauge was 15 feet: just the
space left between the travellers or gantries spanning the sea and harbour
walls. This machine was capable of lifting a weight of 4 tons in the water
at a distance of 30 feet from the outside edge of the turn-table. The first
operation was to stretch the jib outside the end of the previous season’s work,
the foreman labourer standing on the outer end. This man held a copper wire
attached to a large cast-iron plumb-bob, which, at slack tide, be let down to
the place where the first pile or upright of the stage was to stand, and at this
spot a helmeted diver excavated a hole in the bank to receive the pile, to the
end of which was fastened a stone weighing 15 cwts. When the hole was
excavated to the required depth, the divers retired, and the pile was lowered
by the Samson into its place. Four piles were set in a row 30 feet apart, and
longitudinal-trussed beams, 2 feet 4 inches by 1 foot 2 inches, were placed from
pile to pile and formed a bay of staging, which consisted of 1050 cubic feet
of timber and 3 tons 8 cwts. of wrought iron in trussed rods, knees, bolts,
straps, etc. The carpenters erected a bay of staging in a week. When the
stage had advanced seawards three lengths or bays, six travellers or gantries,
each capable of lifting 20 tons, were taken down the wall—an operation per-
forraed by the carpenters generally in one day. The gantries spanned the sea
1 Min. Proc. Inst. C.E., vol. xxxvii. p. 87 et seq.