FAILÜRES.
213
longer used for any part of the backing, its place being taken by dry
quarry rubbish. The blocks are made to larger dimensions, but, in order
to facilitate setting operations, they are rendered temporarily lighter than
they would otherwise be by the arrangement of voids or pockets in their
interiors, as shown by the plan in fig. 152. The lowermost blocks weigh
some 50 tons prior to the filling of the pockets with concrète, an operation
which is performed when they are in position. The former face batter of
I in 10, found to be unsuitable for vessels with vertical sides, is now reduced
to 1 in 20.
V" The profile thus adopted may be compared with that of a quay wall at
the neighbouring port of Sfax* in Tunis, similarly constructed, but with the
face receding in a series of offsets as shown in fig. 153.
Fig. 152. Fig. 153. — Quay Wall at Sfax.
The difficulty caused by excessive settlement in walls of this class is
well illustrated by the case of a wall at Smyrna, where no less than six or
seven tiers of blocks had to be superimposed, instead of four, as originally
intended, while the front of the wall had to be supported by a rubble mound
carried up to within 7 feet of mean sea-level.
Failures.
Failures of dock walls are by no means scarce, and they often present
interesting and instructive features, but, in nearly every case, the cause can
be traced to a bad foundation. Movement to a greater or less degree is to
be expected, and has been experienced in all walls founded upon any other
stratum than hard rock. It is stated as the experience of Voisin Bey, the
Engineer-in-Chief of the Suez Canal, that he had never found a long line of
quay wall which, on close inspection, proved to be perfectly straight in line
and free from indications of movement.
* Baron de Rochemont on “ Quelques Ports de la Mediterranée,” Int. Nav. Cong.,
Paris, 1900.