Pile Design and Construction Practice, Fifth edition

Piling equipment and methods 127
small-diameter holes in the clay. Equipment has yet to be devised which will quickly and
effectively remove the clay adhering to the wall of a pile to a sufficient standard of cleanliness
to achieve a good bond with a concrete plug. The procedure for placing the concrete plug in
the cleaned-out pile or for completely filling a steel tubular or box pile is similar to that
described below for shell piles.
3.4.4 Driving and concreting steel shell piles
Steel shell piles are driven by drop hammers or single-acting hammers acting on the head of
an internal mandrel or core which is collapsed to allow it to be withdrawn before placing the
concrete. Problems arise with heave when driving shell piles in groups, and distortion or
collapse of the shells when driving past obstructions. Shell piles have the advantage that the
interior of the shell can be inspected before concrete is placed. This can be done with the aid
of light reflected down the pile by a mirror, or by a narrow beam lamp. Distortion of the
shells can be detected by lowering a lamp down to the toe. If it disappears wholly or partially
then distortion has occurred. This can be corrected by pulling up the shells and redriving
them or, in the case of tapered shells, by inserting and redriving a new tapered shell assembly.
The problem of heave is discussed in Sections 5.7 to 5.9.
Sometimes some leakage of groundwater occurs through shells in quantities which do not
justify replacing the damaged units. The water can be removed from the shells before
placing the concrete by pumping (if the depth to the pile toe is within the suction lift of the
available pump), by an air lift or by baling. If, after removing the water, the depth of inflow
is seen to be less than a few centimetres in 5 minutes the collected water can again be
removed and concrete placed quickly to seal off the inflow. For higher rates of seepage
the water should be allowed to fill the pile up to its rest level, and the concrete should then
be placed by tremie-pipe as described in Section 3.4.8.
Concrete placed in ‘dry’ shell piles is merely dumped in by barrow or chute. It should be
reasonably workable with a slump of 100 to 150 mm to avoid arching as it drops down a
tapered shell or onto the reinforcing cage. The cement content should be such as to comply
with the requirements in BS EN 1536 or with any special requirements for durability (see
Section 10.3.1). The American Concrete Institute (6.13) states that vibration due to driving
adjacent piles has no detrimental effect on fresh concrete in shell piles. Therefore concreting
can proceed immediately after driving the shell even though adjacent shells are being driven,
provided there are no detrimental effects due to ground heave or relaxation (see Section 5.7).
3.4.5 The installation of withdrawable-tube types of
driven and cast-in-place piles
There are no standard procedures for installing driven and cast-in-place piles of the types
which involve the driving and subsequent withdrawal of a casing tube. However, BS EN
12699 requires that cast-in-place displacement piles shall be concreted in the dry using high
workability concrete or semi-dry concrete as appropriate to the methods for each type of pile
as described in Section 2.3.2. Where the concrete is compacted by internal drop hammer a
mix is required that is drier than that which is suitable for compaction by vibrating the
piling tube. The workability and mix proportions of the concrete should be left to the piling
contractor, subject to compliance with the requirements of BS EN 1536 and the needs regarding
durability (see Section 10.3.1).