Geology Reference
In-Depth Information
Driving resistance and set are, however, part of the process of quality
control during construction. Pile driving analysers (PDAs) using accel-
erometers and other instruments attached to the pile can be used to
estimate driving resistance in a more sophisticated way than the tradi-
tional method of measuring the quake with a pencil, although the same
limitations apply regarding whether or not dynamic behaviour is a
reliable indicator of future performance. PDAs are sometimes used
after the pile has been installed (both driven and bored piles) to test
its capacity, but this can be somewhat of a black art with many
assumptions being made and the method is certainly not foolproof or
as reliable as full static load tests, as discussed below.
6.4.3.1.2 BORED PILES
Bored piles are excavated as described earlier. Temporary or perma-
nent steel tubes (casing) may be used to prevent collapse of the hole
and, if the hole is formed below the water table, often bentonite or
some other mud or polymer is used to support the sides of the hole.
Once the hole has been completed and cleaned out, then a steel
reinforcing cage is introduced and,
finally, concreting carried out.
Concrete needs to be tremied by a pipe from the surface to the
bottom of the hole. This avoids the concrete disaggregating, and the
concrete will hopefully displace soft sediment that might have accu-
mulated at the bottom of the bored hole after the
final clean out. It will
also displace the bentonite slurry or water from the bored pile excava-
tion, so this can be a very messy operation. Despite best efforts, soft
toes of sediment will still sometimes occur (perhaps associated with the
removal of temporary casing) and sometimes ground movements
occur causing necking of piles. Clearly, there is a need for high-quality
work and for close supervision. Currently, in Hong Kong, all bored
piles are installed with steel tubes attached to the reinforcing cage
(Figure 6.7)
. After concreting, rotary drilling is carried out down one
of the tubes, through the concrete and into the underlying natural
ground, to prove that the pile is founded as designed and that there
are no soft sediments. If there are, then remedial measures such as
pressure grouting might be needed. Other tubes installed through the
concrete are used to carry out geophysical cross-hole tests (seismic) to
check for necking and other construction defects. In severe cases, piles
may prove inadequate to carry the loads and remedial works are
required. This might not be discovered until the superstructure is
constructed. In one extreme case in Hong Kong, two 44-storey tower
blocks had to be demolished. Such problems may be put down to
workmanship, the inherent dif
culties of the operation, poor investigation
and design and sometimes fraud (Hencher
et al
., 2005).
Once the piling is completed, a pile cap is constructed as a reinforced
box of concrete that bridges between several piles to support major
columns in the superstructure.
