t exposed northern
Roker breakwater, raised about 11 ft. above high water of springs
where the rise is 14 ft. 5 in., is devoid of a parapet; but a subway
formed near the top in each breakwater gives access to the light on
the pierhead in stormy weather (fig. 15). These concrete bags are made
by lining the hopper of the barge with jute canvas, which receives the
concrete and is sewn up to form a bag whilst the barge is being towed
to the site. The concrete is thus deposited unset, and readily
accommodates itself to the irregularities of the bottom or of the
mound of bags; and sufficient liquid grout oozes out of the canvas
when the bag is compressed, to unite the bags into a solid mass, so
that with the mass concrete on the top, the breakwater forms a
monolith. This system has been extended to the portion of the
superstructure of the eastern, little-exposed breakwater of Bilbao
harbour below low water, where the rubble mound is of moderate height;
but this application of the system appears less satisfactory, as
settlement of the superstructure on the mound would produce cracks in
the set concrete in the bags.

[Illustration: FIG. 15.--Sunderland Southern Breakwater.]

Foundations with large blocks.

Foundation blocks of 2500 to 3000 tons have been deposited for raising
the walls on each side of the wide portion of the Zeebrugge breakwater
(fig. 16) from the sea-bottom to above low water, and also 4400-ton
blocks along the narrow outer portion (see HARBOUR), by building iron
caissons, open at the top, in the dry bed of the Bruges ship-canal,
lining them with concrete, and after the canal was filled with water,
floating them out one by one in calm weather, sinking them in position
by admitting water, and then filling them with concrete under water
from closed skips which open at the bottom directly they begin to be
raised. The firm sea-bed is levelled by small rubble for receiving the
large blocks, whose outer toe is protected from undermining by a layer
of big blocks of stone extending out for a width of 50 ft.; and then
the breakwater walls are raised above high water by 55-ton concrete
blocks, set in cement at low tide; and the upper portions are
completed by concrete-in-mass within framing.

Concrete monoliths.

Sometimes funds are not available for a large plant; and in such cases
small upright-wall breakwaters may be constructed in