Pile Design and Construction Practice, Fifth edition

Types of pile 63
completion the results are provided on a printout of the pile log which records the construction
parameters and under- or over-supply of concrete (Figure 2.32). Most specifications for CFA
piles require the rig to be provided with such instrumentation, although, because of anomalies
which inevitably exist in ground conditions, some authorities require all CFA piles to be
tested by non-destructive integrity tests. Regular calibration of the instrumentation is
essential (2.5) . In certain ground conditions doubts may exist as to whether or not the injected
material has flowed-out to a sufficient extent to cover the whole drilled area at the pile toe.
For this reason it may be advisable either to assume a base diameter smaller than that of the
shaft or to adopt a conservative value for the allowable end-bearing pressure. In addition,
‘polishing’ of the shaft can occur in stiff clays due to over-rotation and ‘flighting’ (i.e.
vertical movement of the soil on the auger relative to the soil on the wall of the borehole)
in loose silty sands where over-rotation disturbs the surrounding soil and can reduce shaft
resistance by 30%.
The instrumentation systems which have been successfully used to record the pile
installation are now being applied to control the CFA process, taking some of the decisionmaking
away from the operator in the cab, particularly to ensure that the target volume
concrete is achieved throughout the pile length during withdrawal.
The CFA pile is best suited for ground conditions where the majority of the working load
is carried by shaft friction, and the ground is free from large cobbles and boulders. The standard
CFA system may have difficulty in penetrating stiff clayey soils and glacial till, with
‘refusal’ encountered before reaching the design depth and problems of flighting, shaft
waisting and discontinuities occurring. Bustamante et al. (2.18) have shown that the double
rotary CFA system can overcome such conditions by installing a temporary casing using a
second rotary head on the rig while simultaneously drilling in the auger. The results indicated
that stiff marl could be effectively penetrated, the verticality was better controlled, and
the overall performance was similar to conventional bored and CFA piles. The shaft friction
capacity of CFA piles in chalk has been assessed by Lord et al. (2.19) It is considered that there
should be little difficulty in forming satisfactory CFA piles in better quality structured chalk,
but in chalks with low penetration resistance there may be problems of softening and hole
instability, particularly below water table.
Further information on installation and monitoring of CFA piles is given in a paper by
Fleming (2.20) .
Concrete materials and mix proportions for cast-in-place piles generally are specified in
BS EN 1536 and strength grades should range between C20/25 and C30/37. Cement contents
equal to or greater than 325 kg/m 3 are required for placement in dry conditions and equal to
or greater than 375 kg/m 3 in submerged conditions; water/cement ratios are specified to be
less than 0.6 and have good flow and self-compaction properties. As noted in Section 2.3.2,
these mixes are stronger than BS 8004 requirements which limits the working stress in the
concrete to 25% of the characteristic cube strength at 28 days. Structural design stresses in
EC7 are specified to conform to EC2, EC3, and EC5 for the relevant material; for example,
EC2-1-1 Clause 3 defines the ultimate design compressive stress of concrete in piles as the
characteristic cylinder strength divided by a partial factor of 1.5 � 1.1.
2.4.3 Drilled-in tubular piles
The essential feature of the drilled-in tubular pile is the use of a tube with a medium to thick
wall, which is capable of being rotated into the ground to the desired level and is left