as they are related to the effective overburden pressure (the total overburden pressure minus
the pore water pressure), are only slowly developed. The soft clay displaced by the pile shaft
slumps back into full contact with the pile. The water expelled from the soil is driven back
into the surrounding clay, resulting in a drier and somewhat stiffer material in contact with
the shaft. As the pore-water pressures dissipate and the re-consolidation takes place the
heaved ground surface subsides to near its original level.
The effects in a stiff clay are somewhat different. Lateral and upward displacement again
occurs, but extensive cracking of the soil takes place in a radial direction around the pile.
The clay surrounding the upper part of the pile breaks away from the shaft and may never
regain contact with it. If the clay has a fissured structure the radial cracks around the pile
propagate along the fissures to a considerable depth. Beneath the pile toe, the clay is exten-
sively remoulded and the fissured structure destroyed. The high pore pressures developed in
the zone close to the pile surface are rapidly dissipated into the surrounding crack system
and negative pore pressures are set up due to the expansion of the soil. The latter may result
in an initially high ultimate resistance which may be reduced to some extent as the negative
pore pressures are dissipated and relaxation occurs in the soil which has been compressed
beneath and surrounding the lower part of the pile.
In permissible stress terminology the end-bearing resistance of the displacement pile (the
term Qbin equation 4.1) is calculated from the equation:
(4.4)where Ncis the bearing capacity factor, cubis the characteristic undisturbed undrained
shear strength representative of the fissured strength at the pile toe, and Abis the cross-
sectional area of pile toe. The bearing capacity factor Ncis approximately equal to 9
provided that the pile has been driven at least to a depth of 5 diameters into the bearing
stratum. It is not strictly correct to take the undisturbed strength for cubsince remoulding
has taken place beneath the toe. However, the greater part of the failure surface in end
bearing shown in Figure 4.3 is in soil which has been only partly disturbed by the pene-
tration of the pile. In a stiff fissured clay the gain in strength caused by remoulding is
offset by the loss due to large displacement strains along a fissure plane. In the case of a
soft and sensitive clay the full undisturbed cohesion should be taken only when the working
load is applied to the pile after the clay has had time to regain its original shearing strength
(i.e. after full dissipation of pore pressures); the rate of gain in the carrying capacity of piles
in soft clays is shown in Figure 4.4. It may be noted that a period of a year is required for the
full development of carrying capacity in the Scandinavian ‘quick’clays. In any case the
end-bearing resistance of a small-diameter pile in clay is only a small proportion of the total
resistance and errors due to the incorrect assumption of cubon the failure surface are not of
great significance.
In terms of ‘pure’soil mechanics theory the ultimate shaft frictionis related to the
horizontal effective stress acting on the shaft and the effective remoulded angle of friction
between the pile and the clay. Thus
(4.5)wheresis the unit shaft friction at any point, is the horizontal effective stress, and (^) ris
the effective remoulded angle of friction.
h
s h tan (^) r
Qb NccubAb
152 Resistance of piles to compressive loads