Civil Engineering Reference
In-Depth Information
7.2.2.4 Problems in soft ground
A variety of defects may arise when forming cast-in-place piles in very soft alluvium
exhibiting undrained shear strengths less than 15 or 20 kN/m
2
. These defects relate
to the lateral pressure exerted by the fresh concrete, which in turn is dependent on
the workability of the mix. The lateral pressure of the concrete can easily exceed the
passive resistance of soft soils, and bulges on the pile shaft will almost certainly occur.
Such defects may be detected by a close check on the volume of concrete used or by
sonic integrity testing.
Near the head of the pile the lateral pressure of the concrete may be low, and
further reductions in pressure can be caused by friction as the casing is removed. In
such situations it is possible for soft soil to squeeze the pile section, leading to local
waisting of the concrete (Figure 7.17). Again, the likelihood of this type of defect may
be overcome by casting concrete to ground level or by the use of permanent light steel
casings.
These risks can be analyzed if reliable soils information is available and steps can
be taken to avoid them.
7.2.2.5 Effects of groundwater
The main effect of groundwater seepage, particularly in soils containing thin bands of
silt, is to cause local slumping of the bore. The resulting debris may not be completely
removed before concreting, or the material may become included in the body of the
concrete.
Leaching of cement from fresh concrete is most unusual as high water veloci-
ties are required. Occasional cases of damage have been reported in made ground
(Figure 7.18). A more common effect is illustrated in Figure 7.19, where the pile
Figure 7.17
Waisting of a cast-in-place pile. (Photographs courtesy of CIRIA, London.)
