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120 HARBOUR ENGINEERING.
3J tons; and at Buckie (Banffshire) 3 tons. Stevenson noted that the force
of impact on a rising slope is six times as great as the pressure on a steep wall.
Some inferential records may be adduced in confirmation of the above.
From experiments made with concrete blocks sliding upon a well-wetted
concrete floor, Mr Shield determined a frictional coefficient of '7. In 1891,
a section of the breakwater at Peterhead, weighing 3300 tons in a single
mass, was slewed bodily to the extent of 2 inches, without any dislocation of
the substructure. The waves, therefore, must have exerted a pressure of
•----36 0 -----*— 30‘0 ------f— 25 0
|.5f- ---26 0-
27 ' 0
---30 o“—
- 197'0
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40
Fio. 104. —Section of Madras Breakwater.
over 2310 tons, which, upon the surface exposed, works out to an average
pressure of rather more than 2 tons per square foot.1
At Penzance, Mr Frank Latham has noted pressures of from 18 to
20 cwts. per square foot. At Cherbourg, wave pressure is stated to vary
from 5 to 7 cwts. per square foot.
Examples Of Wave Action.—Noteworthy instances of wave action
are numerous, and many of them remarkable to a degree verging on the
incredible. At Genoa, in 1898, blocks of artificial stone weighing 40 tons
each are said to have been driven a distance of over 160 feet.2 At Wick, in
1872, a huge monolith weighing 1350 tons is recorded as having been removed
bodily from its seating, and deposited intact some considerable distance away.
Another enormous mass of 2600 tons at the same place was broken into two
pieces and similarly displaced.8 Yet another instance is afforded by the
movement of the 3300-ton mass at Peterhead, already alluded to.
Storm at Genoa.—It will not be without interest to consider, in some
detail, the description of a great storm which damaged the breakwaters at
Genoa in 1898. The following account is condensed from the report of M.
Bernardini to the International Maritime Congress at Milan in 1905.
The maximum fetch at Genoa is about 600 nautical miles, and the sector
of exposure has an angle of 30° open to the south west.
1 Min. Proc. Inst. C.E., vol. cxxxviii. p. 400.
2 Bernardini on the Galliera Mole, Proc. Inst. Nav. Cong. Milan, 1905.
3 Min. Proc. Inst. C.E. vol. xliii.