ubble mound.

Construction of the superstructure.

Formerly, in constructing a large superstructure upon a rubble mound,
it was a common practice to build a sea wall and a harbour wall
several feet apart, and to fill up the intermediate,. space between
them with rubble, so as economically to form a wide structure on the
top of the mound, and provide an adequate width for a quay along the
top. A sheltering wall was also generally erected on the sea side.
This, for instance, was the system of construction adopted for the
superstructures, founded at low water, of Holyhead breakwater,
Portland inner breakwater, and St Catherine's, Jersey, breakwater.
Alderney breakwater, the Tyne breakwaters and Colombo south-west
breakwater were also commenced with a precisely similar method of
construction. The system, however, possesses a Very serious defect for
exposed situations, namely, that if once the sea can force a small
opening through the sea wall, the scooping out of the rubble filling,
and the overthrow of the thinner harbour wall are rapidly accomplished
if the storm continues or recurs before repairs can be effected.
Experience soon proved at Alderney and Tynemouth the unsuitability of
the system for very exposed situations; and the intermediate rubble
filling was replaced by solid hearting down to a certain depth. At
Colombo, after the first 1326 ft. of the south-west breakwater had
been built with two walls and intermediate rubble for the
superstructure, as the exposure proved greater than had been
anticipated, and a slight displacement of part of the sea wall, 24 ft.
wide, had occurred, the rubble filling was discontinued, and the two
walls were united into a solid superstructure 34 ft. in width.

sloping block system.

A difficulty experienced in constructing a solid superstructure on the
top of a rubble mound consists in the settlement of the mound which
takes place when the weight of the superstructure comes on it, in
spite of the consolidation of the rubble under the action of the sea
for one or two years before the erection of the superstructure on it
is undertaken. When the superstructure is carried out in long
stepped-forward courses, irregular settlement is particularly liable
to occur, as the weight is progressively imposed in an uneven manner
on the yielding rubble, in proportion to the height of the rubble base
and its deficie