4.3.5 Bored and cast-in-place piles in coarse-grained soils
If drilling for the piles is undertaken by baler (see Section 3.3.7) or by grabbing under water
there is considerable loosening of the soil beneath the pile toe as the soil is drawn or slumps
towards these tools. This causes a marked reduction in end-bearing resistance and shaft
friction, since both these components must then be calculated on the basis of a low relative
density .Only if the piles are drilled by power auger or reverse-circulation
methods in conjunction with a bentonite slurry or by drilling under water using a base
grouting technique as described in Section 3.3.9 can the end-bearing resistance be calcu-
lated on the angle of shearing resistance of the undisturbed soil. However, the effects of
entrapping slurry beneath the pile toe(3.13)must be considered. Loading tests should be made
to prove that the bentonite technique will give a satisfactory end-bearing resistance. If there
are indications that the entrapment of slurry beneath the toe cannot be avoided, the appro-
priate reduction in resistance should be made. Fleming and Sliwinski(4.12)suggest that the
shaft friction on bored piles, as calculated from a coefficient of friction and the effective
lateral pressure, should be reduced by 10% to 30% if a bentonite slurry is used for drilling
in a sand.
The effects of loosening of the soil by conventional drilling techniques on the interface
shaft friction and base resistances of a bored pile in a dense sand is well illustrated by
the comparative loading tests shown in Figure 4.17. Bored piles having a nominal shaft
diameter of 483 mm and a driven precast concrete shell pile with a shaft diameter of
508 mm were installed through peat and loose fine sand into dense sand. The bored piles
with toe levels at 4.6 and 9.1 m failed at 220 and 350 kN respectively, while the single
precast concrete pile which was only 4 m long carried a 750 kN test load with negligible
settlement.
When determining the ULS resistance of bored piles in coarse soils by EC7 rules, the
direct use of in-situ tests is unpracticable because these tests measure the soil properties in
a relatively undisturbed state compared with the gross disturbance which can occur when
drilling the pile boreholes. Only when drilling is performed under a slurry can the undis-
turbed soil properties be used in the calculation, subject to applying a factor to take account
of the process.
The most reliable method to obtain compliance with EC7 is to obtain the ULS resistance
from static load tests. Dynamic tests are impractible because of the likely variations in the
cross-sectional area of the shaft and different elastic properties between the concrete at the
pile head and in the body of the shaft.
Design by calculation using the DA1 approach is similar to that described in Section 4.3.2
with Nqand tan (^) rin equation 4.17 being obtained from values based on SPT or CPT
relationships and judgement used to estimate the reduction in caused by the pile drilling.
Values of Ksin equation 4.17 are obtained from Table 4.10 with the assumption that Ko
represents the loosening of the sand.
4.3.6 The use of in-situ tests to predict the ultimate resistance
of piles in coarse-grained soils
It has been noted that the major component of the ultimate resistance of piles in dense coarse
soils is the base resistance. However, Figures 4.13 and 4.14 show that the values of Nqare
( 28 to 30 )Resistance of piles to compressive loads 175