Page 325 - Numerical Analysis and Modelling in Geomechanics

306 ANALYSIS AND DESIGN OF PILE GROUPS
and pile-soil separation on the upper soil layer can have a significant influence
on the values of soil stiffness, and hence the values of E adopted for the axial
s
response may be reduced up to 50% or more.
As regards the soil Poisson’s ratio, its effect is quite minor when the analysis
is based on the use of Young’s modulus rather than shear modulus. For saturated
clays under undrained conditions, a value of 0.5 is relevant while, for most clays
and sands, the drained value is usually in the range 0.3–0.4 (Poulos, 1994).
Values of Poisson’s ratio may also be approximated using the empirical formula
(Duncan and Mokwa, 2001):

(10.16)

where the value of ф (friction angle) should be the total stress shear strength
parameter ф for short-term undrained conditions and the effective stress shear
u
strength parameter ф' for long-term drained conditions.
For a non-linear analysis, it is also necessary to assess the axial and lateral pile
shaft resistance, and the end-bearing resistance, as discussed in the earlier section
on limiting pile-soil stresses. Further information on this subject is provided in
the work by Poulos (1989), Fleming et al. (1992) and Tomlinson (1994).

Applications and design analysis
Attention is turned to the application of available numerical analyses to practical
problems involving real soils. Three published case histories are considered,
involving single piles and pile groups subjected to either axial or lateral loading.
In each case, the rationale for the selection of the soil parameters is described
briefly, and then the predictions from selected methods of analysis are compared
with the field measurements.

North London railway viaduct
Before proceeding to the analysis of the case histories, it is found instructive to
discuss the results obtained from different numerical codes in the analysis of a
3× 3 pile group subjected to a combination of vertical loads, horizontal loads and
moments and embedded in London Clay. This project was part of the foundation
design of a high-speed railway viaduct in North London. The bored cast-in-situ
reinforced concrete piles are 17 m long, 0.9 m in diameter, with a centre-to-
centre spacing of three pile diameters, and with the underside of the pile cap
assumed at the top of the London Clay. The assumed Young’s modulus for the
piles is 25 GPa. A profile of undrained shear strength (C ) of 50+9.4z kPa has
u
been adopted, where z is the depth in m below the top of the London Clay. An
adhesion factor of 0.6 is employed, while the hyperbolic curve fitting constants