Pile Design and Construction Practice, Fifth edition

154 Resistance of piles to compressive loads
soil swells at the time of drilling the hole, but it increases as concrete is placed in the shaft.
Because of these constantly changing values of K, and the varying pore pressures (and hence
values of ��vo), ‘pure’ soil mechanics methods cannot be applied to practical pile design for
conventional structures without introducing empirical factors and simplified calculations to
allow for these uncertainties.
A method has been developed at Imperial College, London, for determining the ultimate
bearing capacity of piles driven into clays and sands. The method was developed primarily
for piles carrying heavy compression and uplift loads in offshore platforms for petroleum
exploration and production. The procedure for piles in clays is based on the use of rather
complex and time-consuming laboratory tests, with the aim of eliminating many of the
uncertainties inherent in the effective stress approach as noted above. The method for piles
in clays and sands is described in Section 4.3.7.
In the case of piles which penetrate a relatively short distance into the bearing stratum of
firm to stiff clay, that is piles carrying light to moderate loading, a sufficiently reliable
method of calculating the ultimate shaft friction, Qs, on the pile shaft is to use the equation:
Q s � �c u A s
(4.7)
where � is an adhesion factor, is the characteristic or average undisturbed undrained shear
strength of the soil surrounding the pile shaft, and As is the surface area of the pile shaft
contributing to the support of the pile in shaft friction (usually measured from the ground
surface to the toe).
The adhesion factor depends partly on the shear strength of the soil and partly on the
nature of the soil above the bearing stratum of clay into which the piles are driven. Early
studies (4.1) showed a general trend towards a reduction in the adhesion factor from unity or
higher than unity for very soft clays, to values as low as 0.2 for clays having a very stiff consistency.
There was a wide scatter in the values over the full range of soil consistency and
these seemed to be unrelated to the material forming the pile.
Much further light on the behaviour of piles driven into stiff clays was obtained in the
research project undertaken for the Construction Industry Research and Information
Association (CIRIA) in 1969. (4.2) cu Steel tubular piles were driven into stiff to very stiff
London clay and were subjected to loading tests at 1 month, 3 months and 1 year after driving.
Some of the piles were then disinterred for a close examination of the soil surrounding the
interface. This examination showed that the gap, which had formed around the pile as the
soil was displaced by its entry, extended to a depth of 8 diameters and it had not closed up
a year after driving. Between depths of 8 diameters and 14 to 16 diameters the clay was
partly adhering to the pile surface, and below 16 diameters the clay was adhering tightly to
the pile in the form of a dry skin 1 to 5 mm in thickness which had been carried down by
the pile. Thus in the lower part of the pile the failure was not between the pile and the clay,
but between the skin and surrounding clay which had been heavily sheared and distorted.
Strain gauges mounted on the pile to record how the load was transferred from the pile to
the soil showed the distribution of load in Figure 4.5. It may be noted that there was no transfer
of load in the upper part of the pile, due to the presence of the gap. Most of the load was
transferred to the lower part where the adhesion was as much as 20% greater than the
undrained strength of the clay. For structures on land, the gap in the upper part of the
pile shaft is of no great significance for calculating pile capacity because the greater part of
the shaft friction is provided at lower levels. In any case much of the clay in the region of