The ICP method for piles driven into clays is based on effective stresses and takes into
account the effects on the interface shaft resistance of the radial displacement of the clay and
the gross displacement of the clay beneath the base. To determine shaft resistance the ICP
method calculates the local shear stress at failure on the interface after equalization of pore
pressure changes brought about by the pile driving. The calculations are made for a succes-
sion of layers over the embedded length of the shaft. They are then integrated to give the
total shaft resistance from the equation:
(4.20)The peak local interface shear stress is obtained from the equation:
= (Kf/Ko) tan (^) f (4.21)
where
Kf= coefficient of radial effective stress for shaft at failure = /
Ko= coefficient of earth pressure at rest = /
= equalized radial effective stress = Kc
(^) f= operational interface angle of frictional failure
Kcis obtained from the equation:
Kc= [2.2 0.016YSR – 0.870 Ivy]YSR0.42(h/R)0.20 (4.22)
where
Ivy= relative void index at yield = log 10 St
Ivo= relative void index
YSR = yield stress ratio or apparent over-consolidation ratio
St= clay sensitivity
h= height of soil layer above pile toe
R= pile radius
andKf/Kc= 0.8
An alternative to equation 4.22 which is marginally less conservative is
K= [2 – 0.625 Ivo]YSR0.42(h/R)0.20 (4.23)
YSR, vyand voare obtained either from oedometer tests in the laboratory on good
quality undisturbed samples or from a relationship with consolidated anisotropic undrained
triaxial compression tests or by estimation from CPT or field vane tests. The clay sensitivity
is determined by dividing the peak intact unconsolidated undrained shear strength by its
remoulded undrained shear strength.
The operational interface angle of friction at failure (^) flies between the peak effective shear
stress angle and its ultimate or long strain value. The actual value used in equation 4.21 depends
on the soil type, prior shearing history and the clay to steel interface properties. It is influenced
by local slip at the interface when the blow of the hammer drives the pile downwards and at
rebound when the hammer is raised at the end of the stroke. A further influence is progressive
failure when the interface shear stress near the ground surface is at the ultimate state, but near
the toe the relative pile–soil movement may be insufficient to reach the peak stress value.
rc vorf vorf (^) vo
f rc
f
Qs Df dZ
182 Resistance of piles to compressive loads