Pile Design and Construction Practice, Fifth edition

132 Piling equipment and methods
(3) The falling concrete may jam between the reinforcing bars and not flow outwards to the
walls of the borehole. Remedy: Ensure a generous space between the reinforcing bars.
The cage should be stiff enough to prevent it twisting or buckling during handling and
subsequent placing of concrete. Widely spaced stiff hoops are preferable to helical
binding. Check that the bars have not moved together before the cage is lowered down
the hole.
(4) Lumps of clay may fall from the walls of the borehole or lining tubes into the concrete
as it is being placed. Remedy: Always use lining tubes if the soil around the borehole is
potentially unstable and do not withdraw them prematurely. Ensure that adhering lumps
of clay are cleaned off the tubes before they are inserted and after completing drilling.
(5) Soft or loose soils may squeeze into the pile shaft from beneath the base of the lining
tubes as they are withdrawn, forming a ‘waisted’ or ‘necked’ shaft. Remedy: Do not
withdraw the casing until the placing of the concrete is complete. Check the volume of
concrete placed against the theoretical volume and take remedial action (removal and
replacement of the concrete) if there is a significant discrepancy.
(6) If bentonite has been used for ‘mudding-in’, the hydrostatic pressure of the bentonite in
the annulus, which is disturbed on lifting the casing, may be higher than that of the fluid
concrete, thus causing the bentonite to flow into the concrete. This is a serious defect
and is difficult to detect. It is particularly liable to happen if the concrete is terminated
at some depth below the top of the ‘mudded-in’ casing. Remedy: Keep a careful watch
on the level and density of the bentonite gel when the casing is lifted. Watch for any
changes in level of the concrete surface and for the appearance of bentonite within the
concrete. If inflow of the bentonite has occurred the defective concrete must be
removed and replaced and the ‘mudding-in’ technique must be abandoned.
(7) Infiltration of groundwater may cause gaps, or honeycombing of the concrete.
Remedy: Adopt the techniques for dealing with groundwater in pile boreholes
described in Section 3.4.8.
Further guidance on the installation procedures is given in CIRIA Report PG2 (3.23) .
3.4.7 Installing continuous flight auger piles
CFA piles can be installed in a variety of soils, dry or waterlogged, loose or cohesive, and
through weak rock. The soil is loosened on insertion of the auger and the borehole walls are
supported by the auger flights filled with drill cuttings; bentonite support slurry is not used.
The pile is concreted through a bottom or side exit at the tip of the hollow stem auger (100 or
127 mm bore) using a concrete pump connected by hose to a swivel on the rotary head as
the auger is slowly rotated and withdrawn. Soil is brought to the surface on the auger blades.
The concrete flow rate and feed pressure are continuously measured at the tip; reinforcement
is pushed or vibrated into the fresh concrete. In order to avoid the problems of flighting and
polishing (see Section 2.4.2) which reduce pile capacity, reliable instrumentation and experienced
operators are essential.
For rotary displacement auger piles the displacement tool, which is mounted at the bottom
of a drill tube, is rotated by the high torque top drive and forced into the ground by the
rig crowd thereby compacting the wall of the hole. The pile is concreted through the auger
tip as the tool is rotated out of the hole maintaining the profile. To form the various types of
screw piles, discussed in Section 2.3.5, the thick-flanged continuous auger is screwed into