Page 317 - Numerical Analysis and Modelling in Geomechanics

298 ANALYSIS AND DESIGN OF PILE GROUPS

Figure 10.3 Comparison of load-settlement response for single pile (E p =30,000 GPa).
spacings (where k is the ratio of the total vertical load acting on the group to the
p
average settlement of the group, n is the number of piles in the group, s is the
pile spacing and G is the soil shear modulus). In the analyses of PIGLET and
GRUPPALO, it is assumed that axial interaction effects between piles become
insignificant for a pile spacing greater than a limiting value s max equal to
(Randolph and Wroth, 1979):

(10.10)

where v s is the soil Poisson’s ratio and r , for rectangular pile group
g
configurations, may be taken as the radius of the circle of equivalent area to that
covered by the pile group. In the analyses of DEFPIG, PGROUP and PGROUPN,
no limiting value for axial interaction effects has been adopted.
It may be observed that DEFPIG and PIGLET approaches give divergent
results, while a reasonable agreement is obtained between PGROUPN and the
computer program GRUPPALO (Mandolini and Viggiani, 1997). It is worth
noting that results from PGROUPN are in excellent agreement with the rigorous
BEM solution of PGROUP, but the latter is limited to groups of 8×8 piles, due to
the magnitude of computer resources required to analyse larger groups. In
contrast, PGROUPN took about 30 CPUs on an ordinary desktop computer for
the 20×20 pile group, considering the symmetry of the pile arrangement. This
observation is of great significance because it demonstrates the applicability of
the complete BEM approach to large pile groups, whereas previous work (i.e.
PGROUP) was restricted to small pile groups.