Page 334 - Numerical Analysis and Modelling in Geomechanics

F.BASILE 315
cap was located), increasing linearly at the rate of 9.5 MPa/m, as deduced from
the seismic cone penetration test (SCPT) shear wave velocity measurements
using a Poisson’s ratio of 0.35 (from Equation (10.16)). Based on the soil
stratification derived from CPT, and for the purpose of evaluating the response to
lateral loading (for which the soil properties in the top eight pile diameters are
most relevant), it is reasonable to idealise the soil profile as a single cohesionless
layer with a friction angle of 30°. This has been derived from the widely adopted
correlation with standard penetration test (SPT) data reported in Tomlinson
(1995), using an N value of 10 for the soil in the top eight diameters. Other input
parameters for the PGROUPN analysis include a pile-soil interface angle of 25°
(i.e. 5 degrees less than the friction angle), a buoyant unit weight of 10 kN/m 3
(assumed), and a coefficient of horizontal soil stress (K ) of 0.7 (Fleming et al.,
s
1992). The hyperbolic curve fitting constants have been taken as 0.5 and 0.99 for
the axial response of the shaft and the base, respectively, and 0.9 for the lateral
response (it should be noted that the value of the hyperbolic constants for the
axial response has in effect no influence on the lateral response of the group).
Figures 10.15 and 10.16 report the computed and measured pile head load-
deflection response of the single pile and the 6-pile group, respectively. The
agreement for the single pile results is favourable, whereas, for the 6-pile group,
the deflections computed by PGROUPN are slightly overestimated. These
differences may partially be explained with the disregard of any shear resistance
that might have developed along the base of the massive cap. In addition, other
factors such as the cracking of the pile section and the rigidity of the connection
of pile to pile cap can influence the lateral group response, particularly under
large loads. These factors are not readily modelled in the PGROUPN analysis.
Figures 10.15 and 10.16 also report the results obtained by Huang and
colleagues using the computer program GROUP, based on the use of p-y curves.
They found that none of the p-y curves derived from the soil tests dilatometer
test (DMT) yielded reasonable predictions of pile deflection profiles of the single
pile and the pile group. The p-y curves were then adjusted until a good match
between the measured and computed load-deflection profiles was achieved.
A comparison between the bending moment profiles predicted by PGROUPN
and GROUP for the single pile and the 6-pile group is presented in Figures
10.17 and 10.18, respectively, showing a reasonable agreement between the
analyses.
Overall, it may be concluded that the PGROUPN results are of comparable
accuracy to those obtained from GROUP. However, it should be emphasised that
the PGROUPN analysis is based on the assessment of intrinsic soil properties
determined from the soil investigation, whereas the GROUP analysis made use
of backfigured data from loading tests on the single pile and the pile group.