Pile Design and Construction Practice, Fifth edition

128 Piling equipment and methods
The procedures to be adopted for avoiding ‘waisting’ or ‘necking’ of the shaft, or the
inclusion of silt pockets and laitance layers, are similar to those adopted for bored and castin-place
piles and are described in the following section of this chapter. Precautions against
the effects of ground heave are described in Section 5.8. Because the casing tube is, in all
cases, driven down for the full length of the pile, it is essential to ensure that the interior of
the tube is free of any encrustations of hardened concrete. Even small encrustations can
cause the concrete to arch and jam as the tube is withdrawn. If the reinforcing steel is lifted
with the tube the pile shaft is probably defective and should be rejected. Further guidance is
given in CIRIA Report PG8 (3.19) .
3.4.6 The installation of bored and cast-in-place piles by
power auger equipment
The employment of a power auger for the drilling work in bored and cast-in-place piles
presupposes that the soil is sufficiently cohesive to stand unsupported, at least for a short
time. Any upper soft or loose soil strata or water-bearing layers are ‘cased-off’ by drilling
down a casing or pushing the tubes down into the pre-drilled hole by vibrator or the crowd
mechanism on the kelly bar. If necessary, ‘mudding-in’ techniques are used at this stage (see
Section 3.3.8). After the auger has reached the deeper and stiffer fine-grained soils, the borehole
is taken down to its final depth without further support, until the stage is reached when
a loosely fitting tube is lowered down the completed hole. This loose liner may be required
for safety purposes when inspecting the pile base before placing the concrete; or if an
enlarged base is required, the lining prevents the clay collapsing around the shaft over the
period of several hours or more required to drill the under-ream. The loose liner may not be
needed for straight-sided piles in weak rocks, or in stable unfissured clays, where there is no
risk of collapse before or during the placing of the concrete. However, if the clays are in any
degree fissured there is a risk of the walls collapsing during concreting, and thus leading to
defects of the type shown in Figure 3.40. Lining tubes must be inserted in potentially
unstable soils if a visual inspection is to be made of the pile base. Where it is required to
lower inspection personnel into a shaft (which should be greater than 750 mm diameter)
the requirements of BS 8008: 1996 (safety precautions and procedures for the construction
and descent of machine-bored shafts for piling and other purposes) must be followed.
Particular care is needed if an enlarged base has to be inspected or if concrete spreading has
to be carried out by hand. The support can be in the form of a ‘spider’, consisting of a number
of hinged arms mounted on a ring. The assembly is lowered down the shaft with the arms in
a near-vertical closed position. They are then lifted upwards and outwards and locked in
position to form a cone which is pulled up against the clay surface.
Favourable conditions for stability of the borehole are given by care in setting up the rig
on a firm level base and attention to maintenance of verticality. Tilting of the rig or violent
operation of the auger leads to misalignment and the need for corrective action by reaming
the sides. ‘Working Platforms for Tracked Plant’ (3.20) provides guidance for the design and
construction of ground-supported platforms for piling plant, although selection of design
parameters to provide realistic mat thicknesses is still a problem. The requirements for the
safety of operatives should be rigorously followed as detailed in the British Drilling
Association Health and Safety Manual (3.21) . Casings protecting open pile boreholes should
extend above ground level and should be provided with a strong cover.
The final cleaning-up operation before placing concrete in a bored pile consists of removing
large crumbs of soil or trampled puddled clay from the pile base. Any lumps of clay adhering