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BREAKWATER CONSTRUCTION.
185
riprap of harder material, which would more effectively resist crushing and
grindiug under the combined action of the weight of the blocks and the shock
of tlio waves.”
The cost of tlie earlier type of breakwater (rubble mound, with the same
superstructure as that in fig. 157 above the caisson block) was .£142 per
linear yard, while the cost of the later caisson type was £182 per yard.
Breakwaters and Piers at Dover.1—The breakwaters and piers are
formed of concrete blocks of from 26 to 40 tons each, according to the
exigcncies of the bond to which the work conforms, and the average weight
throughout may be taken at 32^ tons. The blocks were built, in the yard,
of 6 to 1 concrete, and those for face-work have granite fronts. All the
blocks from foundation level upwards are bonded and dowelled with 4 to 1
Flos. 158, 159. —Sections of Breakwater at Dover.
concrete dowels of circular section, while above low water the courses are
bedded and grouted in cement.
The blocks were set by means of Goliath overhead travellers running on
temporary staging. In the case of the Admiralty works, the roads on the
stagings were 27 feet 6 inches above high water of spring tides and 46 feet
3 inches above low water. The piles were arranged in trestles or clusters of
six to form spans of 50 feet 3 inches, and the Goliaths, both on the stagings
and in the workyards, were all of 100 feet gauge.
The system of operations, carried on simultaneously, was as follows. At
the outer end of the work was a stage-erecting machine, followed by a Goliath
crane working a grab; then a second Goliath, from which the diving-bell was
worked. Following this came a third Goliath, setting blocks under water;
and behind that, a fourth, setting blocks above low-water level.
The diving-bells used on the works were 17 feet 6 inches by 10 feet, with
6 feet 6 inches headroom. They weighed about 35 tons out of water, and
1 Vide Matthews on Harbours, Trans. Am. Soc. C.E., vol. liv.