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A Numerical Study on Settlement of Large Pile Groups
B.B. Sheil and B.A. McCabe
College of Engineering and Informatics, NUI, Galway
b.sheil1@nuigalway.ie
Abstract
The interaction factor method is one of the most widely
used approaches in the analysis of pile group settlement
for its applicability to almost any pile group
configuration. The finite element method is used to
investigate the applicability of this method to large pile
groups by comparing results of a direct analysis to
those obtained by superposition of interaction factors.
An investigation is also carried out on the influence of
intermediate piles on results obtained by this method.
1. Introduction
It has now been long established that a loaded pile
will carry additional loads when located within the
displacement field of other loaded piles. One of the
most common and tractable approaches in the analysis
of pile group interaction settlement has been the use of
the concept of interaction factors and the principle of
superposition, originally established by Poulos [1]. The
successful application of the interaction factor method
to small pile groups has been well documented in the
literature [2]. The finite element method is used to
investigate the applicability of this method to the
analysis of large pile groups.
2. Finite Element Study
The soil profile modeled for the finite element study
was the Kinnegar test site in Belfast. The soil stratum
consists of a layer of made ground which extends to a
depth of ~ 1.0–1.5 m and is underlain by 8.5 m of
slightly overconsolidated soft estuarine silt (referred to
as „sleech‟). The adopted soil parameters which have
been well documented in the literature [3] were
validated by simulating the pile load test carried out by
McCabe and Lehane [3] at the same test site. The
advanced elastic-plastic hardening soil model was used
in the finite element software package PLAXIS 3-D
Foundation. Results of the settlement of a group of 3, 5,
13, 25, 41 and 61 piles were calculated by a direct
analysis and compared to those calculated by the
interaction factor method.
3. Results
From Figure 1, it is clear that settlement predictions
obtained using the interaction factor method are in
almost perfect agreement with predictions by the direct
analysis. In order to reinforce the findings of Figure 1,
an investigation of the effect of intermediate piles on
the value of interaction was also considered (Figure 2).
It can be seen that the presence of an intermediate pile
has an insignificant impact on the interaction factor for
a floating pile group.
138
Centre pile settlement by interaction
factor method (mm)
5
4
3
2
1
0
5 piles
3 piles
13 piles
25 piles
Perfect agreement
line
0 1 2 3 4 5
Centre pile settlement by direct method (mm)
Figure 1. Comparison of results by interaction factor
method and direct predictions by PLAXIS
Interaction factor α
0.2
0.1
0
41 piles
61 piles
4 5 6 7 8 9
s/d
Figure 2. Effects of an intermediate pile on the value of
α.
4. Conclusions
The results from the finite element study confirm the
applicability of the interaction factor method to large
pile groups. The blocking effect of intermediate piles on
the value of interaction factor was also shown to be
insignificant for the case of a floating pile group.
5. Acknowledgements
The authors wish to acknowledge the financial
support provided by the CoEI Postgraduate Scholarship.
8. References
[1] Poulos, H.G., “Analysis of the settlement of pile groups”,
Geotechnique, 1968, pp. 449-471.
[2] Lee, C.Y., “Settlement of pile groups - practical
approach”, ASCE J. of Geotech. Eng., 1993, pp. 1449-1461.
[3] McCabe, B.A., Lehane, B.M., “Behavior of axially loaded
pile groups driven in clayey silt”, J. Geotech. and Geoenv.
Eng., 2006, pp. 401-410