Page 192 - Moving the Earth_ The Workbook of Excavation

BASEMENTS

4.32 THE WORK

FIGURE 4.26 Detail of bracing.

Cofferdams consisting of single rows of interlocked steel pilings that enclose areas, with interior
bracing, have been used for depths up to 60 feet, although ordinary practice limits them to 40.
They may be installed by hammering the piling in undisturbed ground until it reaches bedrock, or
to sufficient depth below the excavation floor to be considered safe. All the sections should be
placed and driven to moderate depth before any of them are driven all the way, to make sure that
all joints interlock properly.
Excavation is likely to be done by clamshells. Bracing is placed against the inside of the wall
as it becomes exposed.
If the soil is very porous, great difficulty may be experienced getting the water down the first
few feet, as the joints between sections leak quite freely until forced together by water pressure.
More or larger pumps may be used at this stage of the job than at any later time. It may be neces-
sary to trench outside the wall to place a clay seal partway down, or to partially seal the soil with
cement grout.