2 l8
DOCK ENGINEERING.
figure. The Strange thing was that the wall showed no signs of collapsing
altogether. The cause of the mischief is still obscure. An examination
of the stability of the section by theoretical principles revealed no weak-
ness. Apparently had there been excessive compression on the face, the
upper part of the wall, deprived of its support, should have collapsed, but
this is what did not happen. The wall was repaired by a refacing of
concrete, 3 feet thick, and as an improvement of the dock was in contempla-
tion at the same time, advantage was taken of the opportunity to deepen
the foundations of the wall by an operation about to be described.
Fig. 158. —Dock Wall at Liverpool.
Fig. 159. —Underpinning at Ardrossan.
Underpinning.—Occasionally an engineer has to face the problem, not of
constructing a new wall, but of adapting an oid one to conditions far
other than those contemplated at the time of its construction. A common
instance is that in which it is requisite to deepen an existing dock in
order to accommodate vessels with greater draught. This necessitates a
corresponding lowering of the quay wall and its foundations, a process
called underbuilding or underpinning.
When the work can be carried out in the open—that is, with the dock
run dry—it is attended by no more than the usual difficulties, though much,
of course, dépends on the nature of the strata to be undercut. More arduous
and less secure is the operation when it has to be performed with the dock
under normal conditions.
Fig. 169 is a section of a quay wali of Eglinton Dock, Ardrossan, to
which the following extract refers :— *
Robertson on “ Ardrossan Harbour Extensions,” Min. Proc. Inst. C.E., vol. cxx.