Page 312 - Numerical Analysis and Modelling in Geomechanics

F.BASILE 293
Limiting pile-soil stresses
It is essential to ensure that the stress state at the pile-soil interface does not violate
the yield criteria. This can be achieved by specifying the limiting stresses for the
soil.

Cohesive soil

For cohesive soils, a total stress approach is adopted. The limiting shear stress in
the slip zone (i.e. the pile shaft for the axial response) is taken as:

(10.3)

where C is the undrained shear strength of the soil and a is the adhesion factor.
u
The limiting bearing stress on the pile base is calculated as:
(10.4)

The limiting bearing stress on the pile shaft for the lateral response is calculated
as:

(10.5)

where N is a bearing capacity factor increasing linearly from 2 at the surface to a
c
constant value of 9 at a depth of three pile diameters and below, much as was
originally suggested by Broms (1964) and widely accepted in practice (Fleming
et al., 1992).

Cohesionless soil

For cohesionless soils, an effective stress approach is adopted. The limiting shear
stress in the slip zone (i.e. the pile shaft for the axial response) is taken as:

(10.6)

where K S is the coefficient of horizontal soil stress, is is the effective vertical
stress and 8 is the angle of friction between pile and soil. The limiting bearing
stress on the pile base is calculated as:
(10.7)

where N q is calculated as a function of the soil angle of friction (Φ≥) and the
length-to-diameter ratio (L/d) of the pile, much as was originally established by
Berezantzev et al. (1961). The limiting bearing stress on the pile shaft for the
lateral response is calculated as (Fleming et al., 1992):