Civil Engineering Reference
In-Depth Information
Ground level
Pile
Clay overburden
Blocky mudstone
with clay infill
between blocks
Effective pile extension
along shear surface
during driving
Settlement under load is high
because of squeezing out of the clay
Figure 7.5
Schematic of squeezing-out or consolidation of clay.
7.1.3 Associated ground movements
When large groups of displacement piles are to be driven, heave of the ground may be
a significant factor. Careful levelling of the pile heads and the ground surface should be
instigated if significant heave is expected. Cases have been reported of heave reducing
the bearing capacity of piles, or in extreme cases inducing tensile failure in the shaft.
Provided the pile shaft is not damaged, bearing capacity can usually be recovered
by re-driving the piles to an acceptable set. If tensile failure of the pile shaft occurs,
repairs can be expensive and may involve drilling down the axis of the pile to grout
the fractures. The problem of heave can be alleviated by driving the centre piles of
a group first and working out to the perimeter piles, selecting a low displacement
type of pile, or pre-boring holes to remove a volume of soil equal to 50 to 100%
of the volume of the piles. It should be noted that pre-boring can lead to a loss of
skin friction, although this may not be significant in large groups of closely spaced
piles.
The effects of heave may extend several metres beyond the perimeter of a pile group
and damage to adjacent structures has been recorded.
When piling close to sensitive structures it is usual to adopt low-displacement piles
to minimize heave.
