2723
The Performance of Helical Pile Groups Under Compressive Loads: A Numerical
Investigation
Performance d’un groupe de piles héliocoïdales sous chargement axial : une étude numérique
Elsherbiny Z.
Natural Resources, AMEC Americas Ltd., Calgary, Canada
El Naggar M.H.
Department of Civil and Environmental Engineering, University of Western Ontario, London, ON, Canada
ABSTRACT: An extensive finite element analysis (FEA) study on helical piles is conducted to evaluate the performance of helical
pile groups subjected to axial compressive loads. Three-dimensional nonlinear analysis is conducted using the FE program ABAQUS.
The Mohr-Coulomb plasticity model is used to represent the mechanical behaviour of soil. The numerical models are calibrated and
verified using: full-scale load testing data of single piles; representative soil properties obtained from the borehole logs; and realistic
modeling assumptions. A parametric study is then conducted on a wide range of varying parameters including: soil types (dry sand
and saturated clay); and pile spatial parameters (inter-helix spacing and pile spacing). The numerical results are compared to available
methods in the literature for conventional piles and design recommendations are provided.
RÉSUMÉ : Une étude numérique par éléments finis (MEF) sur des piles hélicoïdales est entreprise pour évaluer la performance d’un
groupe de piles soumis à des charges de compression. Une analyse tridimensionnelle, non linéaire, est conduite en utilisant le code
ABAQUS et le modèle de plasticité de Mohr-Coulomb est utilisé pour représenter le comportement mécanique du sol. Les
modélisations numériques sont calibrées sur des essais complets sur simple pile avec les caractéristiques du sol obtenu sur des carottes
en laboratoire et avec des hypothèses réalistes. Une étude paramétrique est alors entreprise sur un large éventail de paramètres
comprenant: les types de sol (sable sec et argile saturé) et des paramètres géométriques (espacement inter-hélice et espacement des
piles). Les résultats numériques obtenus sont comparés aux résultats issus de la littérature pour des piles conventionnelles et des
recommandations de conception sont fournies.
KEYWORDS: helical pile, numerical modeling, group effect, interaction factor, settlement ratio, displacement ratio, efficiency factor
1 INTRODUCTION
Helical piles represent an efficient deep foundation system used
in a wide range applications varying from anchors for
transmission towers to foundations for bridges and large
industrial installations. Helical piles are made of a steel shaft;
either a solid square shaft or circular pipe, with one or multiple
helices attached to it. They are installed by employing rotational
force applied through a drive head. The piles could be installed
to any depth and at any angle provided that the soil conditions
are tolerable and the pile is designed to withstand the applied
torque from a suitable drive head.
The current design methods of single helical piles are based on
the same framework and theories of conventional piles, where
the compressive capacity of the pile is provided by a
combination of shaft resistance and bearing resistance on the
helices (Mitsch and Clemence, 1985; Narasimha Rao, et. al,
1991; Zhang, 1999; and Livneh and El Naggar, 2008).
Pile foundations typically involve a group of piles connected by
a common pile cap. A concrete cap is normally used to connect
the pile heads in the group. Structural loads are applied to the
cap, which in turn transfers them to the piles. The pile group
behaviour is strongly affected by the soil type and the spacing
between piles. However, currently there is no published
research work on the compressive capacity and performance of
helical pile groups which lead the designers to use methods
available for conventional piles (i.e. bored piles and driven
piles) to design helical pile groups.
The load transfer mechanism of helical piles is more complex
than for conventional piles. The lack of particular guidance for
helical piles motivated the present research work herein, with
special emphasis on the group performance of helical piles and
to provide design methods that are tailored for helical pile
groups. This paper examines the effects of: inter-helix spacing;
soil type; and pile spacing on the performance of helical pile
groups.
1.1
Review of Pile group Behaviour
Piles in a group are expected to interact as the stress zones
around the piles overlap. This interaction is strong for small pile
spacing and diminishes as the pile spacing increases. The
overlapped stress zones underneath the cap could affect the
average capacity or average settlement of piles in the group
compared to single piles subjected to average group load.
It is convenient to characterize the group effect on the
performance of pile groups through the settlement ratio,
R
s
, as
follows:
(1)
A practical approximation of the settlement ratio was
derived by Randolph (Rowe, 2001):
(2)
where
n
is the number of piles in the group; and
w
is a factor
depending on pile spacing, pile geometry, relative pile/soil
stiffness, and the variation of soil modulus with depth.
Typically,
w
= 0.5 for friction piles in clay
and 0.33 for friction
piles in sand spaced at 3 x pile diameters center to center.
Poulos and Davis (1980) proposed using the interaction factors,
α
v
, to represent the effect of a pile on a neighboring pile. In
general, the interaction factor is a function of the relative
pile/soil stiffness, pile length, pile diameter, center to center pile
spacing, and the soil elastic modulus along the pile length and at
its base (Poulos, 1988).
The settlement ratio can then be evaluated using the interaction
approach as follows:
(3)
where
n
is the number of piles in the group; the interaction
factor between reference pile and itself,
α
11= 1
; and
α
1j
is the
It is convenient to characterize the group effect on the
performanc of pile groups through the settlement ratio,
R
s
, as
follows:
= ℎ
≥ 1.0
(1)
A practical approximation of the settlement ratio was
derived by Randolph (Rowe, 2001):
≅
(2)
where
n
is the number of piles in the group; and
w
is a factor
depending on pile spacing, pile geometry, relative pile/soil
stiffness, and the variation of soil modulus with depth.
Typically,
w
= 0.5 for friction piles in clay
and 0.33 for friction
piles in sand spaced at 3 x pile diameters center to center.
Poulos and Davis (1980) proposed using the interaction factors,
α
v
, to represent the effect of a pile on a neighboring pile. In
general, the interaction factor is a function of the relative
pile/soil stiffness, pile length, pile diameter, center to center
pile spacing, and the soil elastic modulus along the pile length
and at its base (Poulos, 1988).
The settlement ratio can then be evaluated using the
interaction approach as follows:
traces of gravel.
extends to 9m be
fine grained at the
The Standard Pe
indicated loose to
The natural mois
depth. The ground
drilling and the
month of October.
The subsurfac
at site (B) compri
mixed with some
number ranging b
is medium to stiff
between 2.3m to
number varying b
lay r that extend
surface. The silty
and the SPT num
table was encount
The
tested
representative of
involve light to me
in Tables 1 and 2 f
The test resu
verify the numeri
the parametric stu
It is convenient to characterize the group effect on the
performance of pile groups through the settlement ratio,
R
s
, as
foll ws:
= ℎ
≥ 1.0
(1)
A practical approximation of the settlement ratio was
derived by Randolph (Rowe, 2001):
≅
(2)
where
n
is the number of piles in the group; and
w
is a factor
depen ing n pile spacing, pile geometry, relative pile/soil
stiffness, and the variation of soil modulus with depth.
Typically,
w
= 0.5 for friction piles in clay
and 0.33 for friction
piles in sand spaced at 3 x pil diameters center to c nter.
Poulos and Davis (1980) rop sed using the int raction factors,
α
v
, to represent the effect of a pile on a neighboring pile. In
general, the interaction factor is function of the rela ive
pile/soil stiffness, pile length, pile diameter, center to center
pile spacing, and the soil elastic modulus along the pile length
and at its base (Poul s, 1988).
The settlement ratio can then be evaluat d using the
interaction approach as follows:
(3)
traces of gravel. Underlying the
extends to 9m below ground su
fine grained at the top to coarse
The Standard Penetration Test
indicated loose to medium dens
Th natural moisture c ntent
depth. The grou dwater table w
drilling and the piles were inst
month of October.
The subsurface soil profile es
at site (B) comprises a surficial
mixed with so e organics and
number ranging between 5 and 6
is medium to stiff brown silt and
between 2.3m o 4.6m below
number varying between 3 and
layer that extends to depths 6.
surface. The silty clay layer gets
and the SPT number ranged fr
tabl was encounter d 1.0 m bel
The
tested
piles
geo
representative of typical helical p
involve light to medium loading c
i Table 1 and 2 for site (A) and s
The test results were used
verify the numerical models tha
the parametric study.
Table 1. Summary of tested piles confi
It is convenient to characterize the group effect on the
performance of pile groups through the settlem nt rati ,
R
s
, as
follows:
= ℎ
≥ 1.0
(1)
A practical approximation of the settlement ratio was
derived by Randolph (Rowe, 2001):
≅
(2)
where
n
is the number of piles in the group; and
w
is a factor
depending on pile spacing, pile geometry, relative pile/soil
stiffness, and the variation of soil modulus with depth.
Typically,
w
= 0.5 for friction piles in clay
and 0.33 for friction
piles in sand spaced at 3 x pile diameters center to center.
Poulos and Davis (1980) proposed using the interaction factor ,
α
v
, to represent th ffect f a pile on a neighboring pile. In
general, the interaction factor is a function of the relative
pile/soil stiffness, pile length, pile diameter, center to center
pile spacing, and the soil elastic modulus along the pile length
and at its base (Poulos, 1988).
The set lem nt ratio can then be evaluated sing the
interaction approa h as follows:
=
+
≥ 1.0
(3)
where
n
is the number of piles in the group; the interaction
factor between reference pile and itself,
α
11= 1
; and
α
1j
is the
interaction factor between reference pile 1 and pile
j
and
j
=
2,…,
n.
traces of gravel. Underlyi
extends o 9m below gr
fine grained at the top to
The Standard Penetrati
indicated loose to mediu
The natural moisture co
depth. The groundwater t
drilling and the piles w
month of October.
The subsu face soil pr
at site (B) comprises a s
mix d with some organic
number ran ing betw en
is medium to stiff brown
between 2.3m to 4.6m
number varying between
layer that extends t de
surface. The silty clay lay
and the SPT number ran
table was encountered 1.
The tested piles
rep es n ative of typical
involve light o m dium lo
in Tables 1 and 2 for site (
The test results wer
verify the numerical mo
the p rametric study.
Table 1. Summary of tested p
Test Pile
Depth
(m)
PA-1
5.5
PA-3
5.6
