UNDERPINNING.
219
“The portions of the north and south walls of the old tidal harbour,
extending along the side of the new dock, were retained, but having been
founded on clay they were underbuilt to the rock with rubble conerete, to
a depth varying between 2 feet and 20 feet. The clay below the walls was
excavated back 5 feet from the face of the wall, and the front of the wall
was supported from the rock by raking shores. The rubble conerete under-
building has a uniform thickness of 5 feet, where the depth is less than 10
feet, but for greater depths the underbuilding is 6 feet thick at the top,
increasing downwards with the batter of the wall. The excavation was
taken out in alternate lengths of about 10 feet, and the clay left between
until the blocks on each side were thoroughly set ; then the intermediate
ROCK
.Vart
Fig. 160. —Underpinning at Liverpool.
Cra/te Roeret
Reck
førtest torter leret uiDock
spaces were excavated and built up. The rubble conerete was built in layers
of about 18 inches or 2 feet, until too close to the underside of the old
walls for men to go in below ; it was then built from the front and the con-
crete carefully rammed into the back. When the conerete was within 1
incli or 2 inches of the underside of the wall, an additional board, bevelled
outwards, was put in the front of the frame; liquid grout of cement and
sand was poured in, filling up the small space between the conerete and the
wall. This proved most satisfactory, as shown by an examination of the
side of each block on excavating the intermediate space. No trouble was
experienced in supporting the walls, and no settlement took place during
the underbuilding.”
Equally satisfactory, though attended by more risk, has been the result