MASONRY PIERS.
279
4,500 tons. The underside of each caisson had a knife edge to penetrate
the ground. The concrete was composed of 33 parts of small stone and
13 of sand to 5 of cement. The caisson was designed with three compart-
ments, and in each of the walls there was provided an orifice for filling
them with water. The orifices were temporarily plugged while the caisson
was being towed into position. On removing the plugs, the block foundered.
The interior was then filled with concrete by means of skips opening at the
bottom. The top layer of 3 feet was deposited in the dry at low water,
with concrete very rich in cement. Large pieces of rock were then sunk
to the seaward of the block, and along its base, to prevent any danger of
undermining by the water. The ground was a clayey sand.
Upon the foundation course thus laid, the upper blocks, of 55 tons weight
each, were set by a Titan crane. The jetty was constructed with horizontal
offsets, in order to partially destroy the downward effect of a breaking
wave upon the foot of the wall (fig. 211).
SECTION or THE BREAKWATER UETTy
tt 5 !0 Metres
0 10 eb 30 Feet:
Fig. 211. — Jetty at Zeebrugge.
Other examples of bag work are to be found at Sunderland (figs. 244
and 245), of block work at Dover (fig. 213), and of mass work at Liverpool
(figs. 221, 222, and 223). The subject of concrete work has also been
treated in the chapter on Dock Walls, and instances are there given of
quays constructed on the same or kindred lines.
Masonry Piers are not so common as they used to be in the days before
the introduction of cement concrete. They are only executed now in
places where suitable stone is very plentiful and skilled labour cheap.
In other situations, concrete offers every inducement for its adoption.
Masonry piers usually have facings of ashlar with heartings of rubble,