form a slurry with the clay as the drilling tools are lowered down or raised from the hole.
Water can also soften the clay if it trickles down from imperfectly sealed-off water-bearing
strata above the clay, or if hose pipes are carelessly used at ground level to remove clay
adhering to the drilling tools.
The effect of drilling is always to cause softening of the clay. If bentonite is used to sup-
port the sides of the borehole, softening of the clay due to relief of lateral pressure on the
walls of the hole will still take place, but flow of water from any fissures will not occur.
There is also a risk of entrapment of pockets of bentonite in places where overbreak has been
caused by the rotary drilling operation. This would be particularly liable to occur in a stiff
fissured clay.
After placing concrete in the pile borehole, water migrates from the unset concrete into
the clay, causing further softening of the soil. The rise in moisture content due to the com-
bined effects of drilling and placing concrete was observed by Meyerhof and Murdock(4.9),
who measured an increase of 4% in the water content of London Clay close to the interface
with the concrete. The increase extended for a distance of 76 mm from the interface.
This softening affects only the shaft. The soil within the zone of rupture beneath and
surrounding the pile base (Figure 4.3) remains unaffected for all practical purposes and the
end-bearing resistance Qbcan be calculated from equation 4.4, the value of the bearing
capacity factor Ncagain being 9. However, Whitaker and Cooke(4.10)showed that the
fissured structure of London Clay had some significance on the end-bearing resistance of
large bored piles, and they suggested that if a bearing capacity factor of 9 is adopted the
characteristic shearing strength should be taken along the lower range of the graph of shearing
strength against depth. If bentonite is used the effects of any entrapment of slurry beneath
the pile base as described by Reese et al.(3.13)should be allowed for by an appropriate reduction
in end-bearing resistance.
The effect of the softening on the shaft friction of bored piles in London Clay was
studied by Skempton(4.11), who showed that the adhesion factor in equation 4.7 ranged
from 0.3 to 0.6 for a number of loading test results. He recommended a value of 0.45 for
normal conditions where drilling and placing concrete followed a reasonably rapid
sequence. However, for short piles, where a large proportion of the shaft may be in heav-
ily fissured clay, Skempton recommended the lower value of 0.3. Skempton observed
that the actual unit shaft friction mobilized in London Clay did not exceed 100 kN/m^2 ,
and this value should be taken as an upper limit when the unit resistance is calculated
from 0.3 or 0.45 times the average undisturbed undrained shear strength. Alternatively,
the curve for bored piles in Figure 4.8 can be used to obtain the adhesion factor for very
stiff to hard clays.
The authors recommend that the same value of 0.3 should be used for small-diameter
bored piles where there may be a long delay between drilling and placing the concrete, for
example, where piles are drilled in the morning and the borehole is left unlined awaiting the
arrival of the ready-mixed concrete truck at the end of the day. The factor of 0.3 should also
be used for large bored piles with enlarged bases which may involve a long delay between
first drilling and finally concreting the shaft, giving a long period for the swelling and soft-
ening of the clay on the sides of the shaft. It is believed that differences in the method of
drilling, such as between the scoring or gouging of a plate auger and the smoothing of a
bucket auger, can also cause differences in friction. However, the effects of soil swelling and
water from the concrete are likely to be of much greater significance in controlling the
adhesion factor.
Resistance of piles to compressive loads 161