With a
rise of tide at springs of 18-3/4 ft., the average depth is thus
approximately 66 ft. at high tide, necessitating a pressure of 29 lb.
on the square inch, which is the limit at which men can work without
inconvenience in the diving-bells. The breakwaters are raised about 11
ft. above high water of springs. The detached southern breakwater was
finished off at this level; but the extended western breakwater, or
Admiralty pier, is provided with a promenade parapet on its exposed
side, rising 13 ft. above the quay; and the eastern breakwater also
has a parapet on its exposed eastern side, raised, however, only 9 ft.
above its quay. The breakwaters are protected from scour along their
outer toe by an apron of concrete blocks, extending 25 ft. out from
their sea face.

[Illustration: Dover Breakwater.

FIG. 13. South Breakwater.

FIG. 14. Admiralty Pier Extension.]

Concrete bag foundations.

The levelling of the foundations for laying the courses of an
upright-wall breakwater is costly and tedious, even in chalk; and the
expense and delay are considerably enhanced where the bottom is hard
rock. Accordingly, in constructing two breakwaters at the entrance to
Aberdeen harbour on a bottom of granite in 1870-1877, concrete bags
were laid on the sea-bed; and these bags, by adapting themselves to
the rocky irregularities, obviated levelling the bottom. They formed
the foundation for the concrete blocks in the south breakwater; and by
the deposit of successive layers of 50-ton concrete bags till they
rose above low water, they constituted the whole of the submerged
portion of the north breakwater. The 50-ton bags were deposited from
hopper barges towed out to the site; and the portions of both
breakwaters above low water were carried up with mass concrete.
Subsequently, the breakwater at Newhaven was constructed on a
foundation of chalk, with lop-ton concrete bags up to low water, and
mass concrete above. Still later, the two breakwaters sheltering the
approach to the river Wear (see HARBOUR) and the Sunderland docks were
built with a foundation mound of concrete in bags, 56 to 116 tons in
weight, on the uneven sea-bottom, raised slightly above low water of
spring tides, on which a solid upright wall was erected, formed of
concrete blocks on each side faced with granite, filled in the centre
and capped on the top with mass concrete. The mos