The reader is referred to the ICE Specificationfor the various qualifications to the above
tolerances. It will be noted that the casing length rather than the diameter is a factor which
influences casting tolerances. This reflects the problems which occur when extracting the
temporary casing.
The use of a permanent casing in the form of a light-gauge metal sleeve surrounding a
pile shaft in soft clays or peats was described in Section 2.4.2. This sleeving cannot be used
within a temporary lining tube where the latter has to be withdrawn in a long length by
means of a vibrator or by jacking. This involves the risk of distortion or jamming of
the sleeve, which is then lifted while raising the temporary tube with disastrous effects on the
concrete in the pile shaft. The sleeve can be used within an outer temporary liner where the
depth of soft clay is shallow, and it can be used in conjunction with a casing oscillator which
keeps the outer tube free of any jamming by the sleeve. There are no problems of using the
light-gauge sleeve where power auger drilling can be performed to produce a stable hole
without employing a temporary outer lining tube.
Unfortunately, defects in a pile shaft of the type shown in Figure 3.41 are by no means
uncommon, even when placing a workable concrete in the dry open hole of a large-diameter
bored pile. Defects can take the form of large unfilled voids, or pockets of clay and silt in
the concrete. Some causes of these defects are listed below:
(1) Encrustations of hardened concrete or soil on the inside of the lining tubes can cause
the concrete to be lifted as the tubes are withdrawn, thus forming gaps in the concrete.
Remedy:The tubes must be clean before they are lowered down the bore-hole.
(2) The falling concrete may arch and jam across the lining tube or between the tubes and
the reinforcement. Remedy:Use a concrete of sufficient workability to slump easily
down the hole and fill all voids.
Piling equipment and methods 131Figure 3.41Defective shaft of bored pile caused by cement being washed out of unset concrete.