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Analyses of a pile-supported embankment over soft clay:
Full-scale experi-
ment, analytical and numerical approaches
M.A. Nunez, L. Briancon, D. Dias
PII: S0013-7952(12)00319-5
DOI: doi: 10.1016/j.enggeo.2012.11.006
Reference: ENGEO 3487
To appear in: Engineering Geology
Received date: 17 March 2012
Revised date: 6 November 2012
Accepted date: 17 November 2012
Please cite this article as: Nunez, M.A., Briancon, L., Dias,
D., Analyses of a pile-supported embankment over soft clay:
Full-scale experiment, analytical and numericalapproaches,
Engineering Geology (2012), doi: 10.1016/j.enggeo.2012.11.006
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http://dx.doi.org/10.1016/j.enggeo.2012.11.006http://dx.doi.org/10.1016/j.enggeo.2012.11.006http://dx.doi.org/10.1016/j.enggeo.2012.11.006http://dx.doi.org/10.1016/j.enggeo.2012.11.006
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Analyses of a pile-supported embankment over soft clay:
Full-
scale experiment, analytical and numerical approaches.
M.A. Nunez, PhD
LGCIE, INSA de Lyon, Villeurbanne, France
L. Brianon, Assistant Professor
Cnam-3SR, Paris, France
D. Dias, Professor
Joseph Fourier University, LTHE, UMR 5564 BP 53, 38041 Grenoble
cedex 9, France
Corresponding author:
Dr. Daniel DIAS, Geotechnical professor,
Joseph Fourier University, LTHE Laboratory (UMR 5564), Equipe
TransPore
Tel : +33 (0)4 76 63 51 35, Fax: +33 (0)4 76 82 52 86,
e-mail: [email protected]
KEYWORDS: embankment, reinforcement, geosynthetic, pile,
numerical modelling.
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ABSTRACT: The reinforcement of soils using rigid inclusions is a
technique used to reduce
settlements and to ensure the stability of an embankment built
over soft soils. This technique
reduces construction delays and is an economical and reliable
solution, which has led to its
widespread use. Thus, many design methods have been developed to
assess the performance
of these reinforced structures. These methods are mainly based
on results from small scale
models and numerical analyses. The reliability of these methods
must be validated under in-
situ conditions.
This paper presents an analytical and numerical study of
full-size experiments at the Chelles
test site (France). The work presented in this paper is part of
the ASIRI French National
Research Project. The experiment consisted of a 5-m-high
embankment built over soft alluvial
ground improved by rigid vertical piles. The embankment is
divided into four zones that
illustrate the influence of the piles and the geosynthetic
reinforcements on the soils behavior.
The performance of the embankment support system is assessed by
monitoring data (total
stresses, horizontal and vertical displacements). Several
in-situ and laboratory soil
investigations were performed using two axially loaded test
piles. These tests verified the
geotechnical hypothesis used for the numerical model and defined
the soil-pile interaction
parameters.
Several analytical methods and numerical models were tested to
assess the arching effect.
Comparisons between the experimental data and these design
methods are presented in
terms of stress and the settlement efficacy of the improved
system. The results show that
these methods overestimate the stress efficacy but that the
settlement efficacy is a reliable
parameter to assess the overall performance of the rigid
inclusions technique.
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1 INTRODUCTION
Embankments constructed over soft soils induce a significant
load over a large area. The
technique of reinforcing soil with columns has proven to be an
interesting solution that
prevents failure or excessive deformations of embankments
(Kempfert et al., 2004, Alexiew &
Vogel 2002). This technique combines three components: (1)
embankment material, (2) a load
transfer platform (LTP), and (3) vertical elements extending
from the LTP to the stiff
substratum. Optional configurations can be made by adding
geosynthetics inside the LTP or
pile caps. The surface and embankment loads are partially
transferred to the piles by arching
that occurs in the granular material constituting the LTP. This
causes homogenization and the
reduction of surface settlements. Friction along the piles is
also involved in the improvement
mechanism, leading to a complex soil/structure interaction
phenomenon (Smith 2005, Jenck et
al., 2005, Combarieu 2008). Although this technique is widely
used, the mechanisms involved
are still poorly understood.
This paper presents an analytical and numerical study at the
Chelles experimental test site in
France that was carried out in 2007. This experiment was part of
the ASIRI research project,
which has the ultimate goal of developing guidelines for the use
of vertical rigid piles in France
(Simon, 2009). The purpose of this paper is to compare the
predictions from several design
methods to measurements made on a full size experiment in a
pile-reinforced embankment to
assess their pertinence for design.
2 BACKGROUND
Many authors have been interested in the technique of
reinforcing soil using columns. Their
papers have mainly been concentrated on transferring loads to
the pile head by the
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phenomenon of arching. Low et al. (1994), Zaeske (2001), Jenck
et al. (2007) and Chen et al.
(2008b) developed physical test models to assess the load
distribution between piles and
foundation soils. However, difficulties arise in reproducing the
behavior of the reinforced soil
at a small scale, leading most of the studies to ignore the
effect of soil-pile interactions
(Brianon and Simon, 2010).
Other authors have proposed analytical methods to improve the
design of this technique.
Combarieu (1988, 2008), Chen et al. (2008a), Russell and
Pierpoint (1997) and Russell et al.
(2003) modified Terzaghis method based on the trapdoor
experiment (Terzaghi, 1943) to
assess arching on the pile improvement problem (Figure 1.a).
Thus, the plane strain
formulation proposed by Terzaghi was updated to take into
account the three dimensional
aspect of the pile problem. An axisymmetric formulation was
proposed by Combarieu (1988,
2008) and Chen et al. (2008a), and a three-dimensional
formulation was proposed by Russell
and Pierpoint (1997). British standards (BS8006, 1995) are also
based on arching caused by
shear load transfer and have adopted the methods by John (1987)
and Jones et al. (1990) for a
2D plane strain design. Their works are based on Marstons
formula for soil arching on top of a
buried pipe (Marston and Anderson, 1913).
Figure 1. Groups of analytical methods.
Other analytical methods propose an idealization of the arching
effect between piles (Figure
1.b). In these cases, arching phenomenon that develops on the
embankment is assumed to
have a predefined shape, such as semi-cylindrical domes (Hewlett
and Randolph, 1988),
spherical shells (Kempfert et al., 2004), or log spiral shells
(Naughton, 2007). The new version
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of the BS8006 (2010) standard includes the 3D shape analysis
developed by Hewlett and
Randolph (1987).
Other analytical approaches are based on an analogy with a
plate-loading test, such as those
by Guido et al. (1987) on geosynthetic reinforced layers. Jenner
et al. (1998) and Bell et al.
(1994) reversed the Guido tests so that the piles punch the mat
and form a soil prism over the
pile caps. Svano (2000), based on the work of Carlsson (1987),
proposed that the slope of the
prism varies and depends on the soil characteristics (2.5 <
< 3.5 - Figure 1.c). Collin (2007)
proposed constructing a rigid geosynthetic reinforced mat. In
this configuration, the slope of
the sides of the prism will correspond to =1. The geosynthetic
layers are individually
dimensioned to support the corresponding wedge of soil under the
critical height. This will
ensure that the entire embankment passes the loads to the piles
and induces small structure
settlements if the piles are firmly fixed to the rigid
substratum.
A few analytical methods have proposed a global approach to
calculate the pile embankment
technique. Combarieu (1988), Filz and Smith (2007) and Chen et
al. (2008a) presented
methodologies to assess the stress distribution at the
embankment base (e.g., an adapted
Terzaghi method) combined with commonly used techniques to
assess the settlement of the
soil and the pile.
Combarieu (1988) used the principles of negative skin friction
(Combarieu, 1974) to calculate
the stress distribution in the foundation soil and the one
dimensional consolidation formula to
assess settlement of the foundation soil. Filz and Smith (2007)
used the elastic solution for a
solid cylinder (pile) surrounded by a thick walled cylinder
(soil) proposed by Poulos and Davis
(1974) to calculate the stress distribution and the settlements.
In this approach, a Mohr-
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Coulomb yield criterion controls slippage and the settlements
can optionally be calculated
using the one dimensional consolidation formula. Chen et al.
(2008a) also combined a modified
Terzaghi method, which includes an equal settlement level, with
frictional and normal stress
distribution conditions between the foundation soil and the
piles. These concepts are all
related to the differential settlements and can be included in a
system of differential equations
to solve the one dimensional compression case. The solution that
assures equilibrium of the
system is presented by these authors. The soil behavior
conditions used in this method were
obtained by Randolph and Wroth (1978) to calculate the shear
friction stiffness and normal
stiffness of the soil under the pile toe, by Teh & Wong
(1995) to calculate the ultimate skin
friction, and by Terzaghis ultimate bearing capacity formula
(1943) for the ultimate toe
resistance.
Comparisons between these design methods show differences in
their load transfer
predictions and in the behavior of the reinforced embankment, as
mentioned by Russell and
Pierpoint (1997), Kempton et al. (1998), Brianon et al. (2004)
and Filz & Smith (2007).
Three dimensional calculation methods to assess the influence of
geosynthetics (if used) are
included in the analytical methods by Kempfert et al. (2004) and
Filz & Smith (2007). Kempfert
et al. (2004) considered geosynthetic behavior as an elastic
cable. Thus, differential equations
are defined for the loading system of the geosynthetic
reinforcement that includes the
foundation-soil effect. Filz & Smiths (2007) methodology to
assess geosynthetic tension and
strain is based on calculations of the deflection of a
geosynthetic material under linear elastic
conditions. The deflection calculation also includes the
influence of soft soil; thus, the
foundation soil contributes to the support of the embankments
residual load.
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Numerical modeling has also been used to understand the physical
phenomenon at the origin
of the load transfer. Numerical models have been developed to
reproduce physical laboratory
tests and the behavior of real projects. However, few studies
have focused on the bearing
capacity of the piles, the tip resistance and the shear
resistance of the shaft. Indeed, the data
needed to carry out these analyses are rarely available, and
studies are often focused on one
aspect of the system, e.g., load transfer on the embankment, and
not on the system as a
whole.
In general, analytical methods are interesting and easy-to-use
tools to design pile-reinforced
embankments; however, few of these methods have been validated
with on site
measurements. Available data on full-scale experimental models
is rare because of the high
costs involved. Among the few reported full-scale experiments,
such as those presented by Liu
et al. (2007), Almeida et al. (2007), and Wachman et al. (2010),
some commonalities can be
seen:
In-situ soil characterization using cone penetration, vane shear
and pressuremeter
tests are predominant in these projects, though only odometric
and shear tests allow
reliable data on the consolidation behavior of soils to be
obtained.
All of the experimental projects have investigated only
geosynthetic reinforced
platforms. Parallel reference tests without any reinforcement
(piles or geosynthetics)
are not presented. Consequently, it is not possible to assess
the natural arching range
and the settlement reduction ratio.
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3 GENERAL PROJECT INFORMATION
The Chelles test site was described by Brianon et al. (2009) and
Brianon & Simon (2012).
Details on its construction, ground conditions, and experimental
data may be obtained by
consulting these references. In the present paper, only the
information required to describe
the numerical models and the analytical design is presented.
The experiment was built prior to a bridge construction project
on compressive type alluvial
soils. Soil data has been collected by geotechnical programs in
the experimental area. Cone
Penetration Tests (CPT), pressuremeter tests and vane shear
tests were performed at the site
and were supplemented by odometric and triaxial laboratory
tests.
The experiment consists of a 5-m-high embankment. Figure 2 shows
the general configuration.
The embankment was divided into four zones. Three zones were
reinforced with vertical piles
(2R, 3R and 4R), while the fourth (1R) was not improved and was
used as a reference.
Horizontal geosynthetic reinforcements were used in two of these
zones (3R and 4R). The piles
were driven down through 8 m of compressible soil and were
embedded in the stiff gravel-
sandy layer. The total length of the inclusions averaged 8.4 m.
The elastic modulus of the
inclusions was 18 GPa, and they had a Poissons ratio of 0.2;
these parameters were
determined by extensometer recordings. Their specific weight was
set to 23kN/m3
. The LTPs of
zones 3R and 4R were reinforced by a geotextile layer (GTX) and
by two geogrids (GGR),
respectively, with individual stiffnesses (J) of 750 kN/m and
520 kN/m, respectively.
Most of the monitoring concentrated on the stresses and
displacements at the pile head level.
Settlement transducer devices (T), magnetic settlement plumbs
(TM) and earth pressure cells
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(EPC) were employed on the central grid located under the axis
of the embankment of all four
test zones. Lateral displacements were recorded at the
embankment toe by inclinometers. The
water table was recorded by piezometers. The positions of the
monitoring devices are
presented on Figure 2. Other devices, such as optical strain
sensors in the geosynthetics, were
also installed, but they will not be presented in the present
paper.
The Chelles test site has some advantages compared to other
experiments on piled-
embankments. For instance, a reference zone exists where the
compressible soil is not
improved by piles, the experiment simultaneously tests two
different kinds of LTP
reinforcement, and axially loaded test piles are tested. The
reference zones (1R and 2R) permit
the direct assessment of the performance of the two zones
reinforced by piles and
geosynthetic layers (3R, 4R, Figure 2). To accomplish this, the
reference zones were submitted
to the same loading conditions as the reinforced ones. Piles
were implemented in zone 2R, but
no additional reinforcement was used.
Figure 2. Plan view of the experimental site and configuration
of the tested zones.
3.1 Ground conditionsSeveral in-situ and laboratory soil
investigations were performed. In-situ testing, borings, and
CPT and pressuremeter tests allowed the geological profile to be
defined. Odometric, triaxial
and pressuremeter tests were employed to define the soils
geotechnical parameters, which
are presented in Table 1.
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Table 1. Soil parameters.
The final state was obtained less than two weeks after the end
of construction. This behavior
confirms the rapid consolidation of the alluvial soils, which
could be verified from the
piezometric recordings. Most of the deformation was recorded
during the embankment
construction.
3.2 Load test piles
Analyses of the mechanical behavior of single piles submitted to
axial loads have shown that
the soilpile interface exerts a significant influence on
defining the structural stability
conditions (Said et al., 2009). The fundamental aspects of pile
analysis rely on empirical
correlations based on experimental observations from laboratory
and full scale in-situ testing
(Randolph, 2003). Instrumented piles permit a direct
quantification of the load distribution
along the pile, the axial load and shaft friction so the
soil-pile interaction can be determined.
Two axially tested piles were built for the Chelles experiment
for this purpose. The results of
these tests can be found on Figure 6. One of the tested piles
did not reach the stiff bedrock
(floating pile). The second pile was embedded and penetrates 0.4
m of the stiff gravel-sandy
layer. Two sections of the pile with different lateral friction
limits qswere identified in the
floating test pile. This lateral friction limit corresponds to
the maximum load that the soil
surrounding the shaft is able to transfer by friction. The
maximum end-bearing load could be
measured from the embedded test pile. In this test, the bearing
capacity of the pile is mainly
assured by tip resistance; the measured lateral friction remains
under the limits determined on
the floating pile.
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4 ANALYTICAL METHODS
Several current design methods permit the calculation of the
stress efficacy or the stress
reduction ratio by assessing the soil arching between the piles.
Most of the methods are
formulated for a three dimensional problem (Combarieu, 1988,
Russell et al., 2003, Chen et al.,
2008a), as is necessary in real applications (Kempton et al.,
1998). Other authors only
considered a two-dimensional problem (Low et al., 1994). The
stress efficacy E of the pile
support is defined as the proportion of the embankment weight
carried by the piles at their
head level (1). The stress reduction ratio is defined as the
ratio of the average stress applied to
the foundation soil between the columns to the overall average
stress applied by the
embankment at the pile head level (2).
R
pP
HA
Ac
W
FE
' (1)
0
'
qHSRR
R
s
(2)
These equations are related by
)1(1 saSRRE (3)
Where FPand p are the load and stress applied to the pile,
respectively; W is the weight of
the embankment on the tributary surface; A is the surface area
of a single inclusion; HRand
are the embankment height and density, respectively; s is the
stress applied to the foundation
soil; and Ac is the area of the pile section.
The results of several analytical methods applied to the Chelles
site were compared to the
experimental data. The methods used in this paper include the
adapted Terzaghi solution given
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by Combarieu (1988, 2007), the modified Marstons formula (2D)
and the Hewlett and
Randolph (3D) formula, both of which were adopted in BS8006
(2010), the Kempfert et al.
(2004) method adopted by EBGEO (2003) and the Filz and Smith
(2007 GeogridBridge method.
4.1 Calculation hypothesis.
The parameters used for the analytical calculations are
presented in Table 1. Most of the
analytical methods only require the characteristics of the
embankment fill. The foundation soil
stiffness and the subgrade reaction needed in the GeogridBridge
and the EBGEO methods are
determined from the non-reinforced zone 1R; thus, Eoed = 2940kPa
and ks = 365kN/m2/m. The
geometrical configuration and dimensions are presented in Figure
2.
4.2 Analytical results
Comparisons between the analytical results and the experimental
results are presented in
Figure 3. The analytical results show that increasing the
embankment height increases the
stress efficacy in all cases. For zone 2R, without LTP, all of
the analytical methods overestimate
the arching effect and therefore the stress efficacy. The
smallest difference between the
analytical and experimental results at the final state is
obtained with Combarieus (2008)
approach, which represents an overestimation of 72%. It is
important to recall that the
majority of these results were obtained without considering the
cohesive strength of the
embankment, which is only included in Combarieus approach. Okyay
and Dias (2010) observed
numerically that the stress efficacy of the system increased
with an increase in cohesion of the
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embankment fill. Thus, the fact that cohesion is not taken into
account in the analytical models
results in a lower value of the stress efficacy than in
reality.
Of the design methods used in reference zone 2R, only the EBGEO
and GeogridBridge methods
consider the influence of geosynthetic reinforcement on the
stress efficacy calculation.
Consequently, the results of test zones 3R and 4R are only
compared to these two analytical
methods. Although British standard BS8006 considers that a
geosynthetic material must be
placed over the piles, this method will not be tested on
geosynthetic reinforced zones because
it cannot calculate the in-situ tension and strain of the
geosynthetic material. For design
purposes, this standard recommends that the geosynthetic
material must be calculated to
resist the entire residual embankment load without any support
given by the foundation soil,
which is clearly not the case in the experiment.
Figure 3. Analytical and experimental results
Table 2. Summary of the analytical and experimental results
The analytical results for zones 3R and 4R show that the stress
efficacy increases due to the
addition of geosynthetics. The calculated increase is greater
with the EBGEO approach than
with GeogridBridge. At the final state, the stress efficacy
increased with EBGEO by 69% and
78% for zones 3R and 4R, respectively, and increased by 37% and
45%, respectively, with
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GeogridBridge. The stress efficacy is higher in zone 4R than in
zone 3R because the individual
geosynthetic stiffnesses are added to allow for a multi-layer
configuration. The EBGEO
response also shows that an important gain in stress efficacy is
obtained at the beginning of
the loading. After this point, a threshold seems to be reached.
The GeogridBridge response is
quite different; in this case, the stress efficacy evolution is
constant and linear.
These results were obtained without taking into account the
pile-foundation soil interaction.
Only GeogridBridge offers the option of including this effect.
If this option is activated, i.e., if
interface strength characteristics are imposed, the geosynthetic
effect on stress efficacy will be
lower due to soil foundation unloading by negative skin
friction. Consequently, the foundation
soil deformation decreases.
A comparison of the experimental results from zones 3R and 4R
(Table 2) shows that
GeogridBridge underestimates the stress efficacy in both zones.
On the other hand, EBGEO
gives satisfactory results for zone 4R but considerably
underestimates the results for zone 3R.
The horizontal multi-layer reinforcement in the analytical
methods is introduced by assuming a
global rigidity equal to the multiplication of the individual
geosynthetic stiffness by the number
of layers. These methods do not take into account the type
(geogrid or geotextile) and the
setup configuration of the horizontal reinforcements. In fact,
the experimental data show that
using one GTX layer, with less rigidity, induces a better stress
efficacy than two GGR layers. This
result is contrary to the results obtained using the analytical
formulas and can probably be
improved by incorporating a factor that considers the
differences between these types of
geosynthetics.
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5 NUMERICAL MODELING
Finite difference numerical models were used to simulate the
Chelles experiments. The
objective was to precisely model the mechanical behavior of the
piled-embankment. The
results presented herein were obtained usingFLAC3D
(Itasca, 2009) from an elementary cell
(CE) and a global model (MG) of the site. These models simulate
the mechanical behavior of
the reinforced subsoil by explicitly considering (i) the
geotechnical characteristics of the soils
and the interface, (ii) the behavior of the piles, and (iii) the
configurations of each test zone.
5.1 Numerical modelsThe finite difference elementary cells are
presented in Figure 4.a & 4.b. The elementary cells
mesh consists of about 3200 zones including the interface
elements along the piles. The
elementary cell presented in Figure 4.a was used for each of the
four zones of the
embankment. This cell represents a quarter of the tributary area
of a pile. This simplification is
justified by the symmetry conditions of a pile located in the
center of a zone (Mestat, 1997).
Another numerical model, presented in Figure 4.b, was developed
for the test piles. This model
presents lateral boundaries placed at a distance of 15 diameters
away from the pile axis and
base boundaries at 10 diameters under the piles tip.
Figure 4. The adopted three-dimensional numerical models.
The global model was constructed by using multiple coarse
elementary cells (Figure 4.c, 4.d
and 4.e). This modification induces less than a 5% difference
from the original elementary cells.
This mesh reduction limits the global model size to 445000
volume elements. The horizontal
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and vertical displacements were fixed on the lateral boundaries
and at the base of the models,
respectively. The embankment fill and the substratum were
simulated as a linear elastic,
perfectly plastic material with a Mohr-Coulomb failure
criterion. Soft soils were modeled with
the Modified Cam Clay constitutive model. The piles were
considered to be linearly elastic. Due
to rapid dissipation of the water pore pressure, drained
conditions are assumed for all of the
calculations. The simulations were performed using large strain
mode to activate the
geosynthetic membrane effects.
5.2 Ground compressibilityFigure 5 presents the numerical
response in zone 1R compared to the experimental data.
Settlements were measured at four different depths corresponding
to the positions of the
magnetic settlement plumbs. As shown in this figure, settlement
of the non-reinforced zone is
accurately assessed by the numerical models. Nevertheless, a
slight overestimation can be
noted in the elementary cell, which is mainly caused by the fact
that this configuration cannot
take into account the lateral load dissipation.
Figure 5. Non-reinforced area (1R) settlement.
5.3 Pile load testsNumerical modeling is often adopted to obtain
a deeper understanding of the pile behavior
and especially the mechanical behavior of the soilpile system
(Bransby and Springman, 1996;
Comodromos et al., 2009; Said et al., 2009). For the studied
case, the lateral friction limit of the
regular piles was determined using the instrumented floating
test pile. In the numerical model,
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interfaces with a Mohr-Coulomb failure criterion were placed
around the pile shaft. A null
friction angle was used for these elements with the cohesion
values presented in Table 3 to
obtain the constant friction limit in two different sections of
the pile. This produced a constant
friction limit over the two sections of the shaft, as was
observed in the experimental response.
The simulation results showed that the embedded pile
load-displacement curve is reproduced
well (Figure 6), validating the hypothesis made about the soft
soils shear properties and the
bedrocks stiffness and resistance.
These preliminary simulations are complex but are important as
they confirm the
compressibility and the shear resistance of the soils. The
quality of the predictions confirms
that the developed models can predict the behavior of the
unreinforced zone and of the
embedded piles.
Table 3. Interface properties
Figure 6. Load-displacement curves and final load distributions
for the test pile (embedded pile).
5.4 Stress and settlement efficacies of the pile-reinforced
area
It is a common practice to use the stress efficacy E to evaluate
system performance, although
this value only shows the rate of load transfer over the piles.
Settlement efficacy (ET) will be
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used to present the results in this section. This parameter is
defined as the ratio of settlement
with piles to settlement without piles (4).
wp
pT
s
sE 1 (4)
where sp and swp are the soil settlement with and without piles,
respectively. This parameter
gives an overall evaluation of the soil reinforcement. Thus, it
takes into account the effects of
the tip resistance and the friction along piles and of the
compressibility of the subsoil.
Figure 7 presents the efficacies obtained with the numerical
models: the elementary cells and
the global model. The efficacies of the global model were
calculated for the three
instrumented piles located in each of the three zones.
Settlements (Figure 10) were only
presented on cross-section BB at the level of the pile heads,
i.e., at the original site surface (see
Figure 2).
Both the elementary cell and the global models of zone 2R
overestimate the experimental
stress efficacy. Thus, the settlements are underestimated. The
elementary cell simulations of
zones 3R and 4R show higher stress efficacies than those of zone
2R due to the presence of
horizontal reinforcement layers. However, the measurements of
load transfer in these zones
were underestimated.
On the other hand, the settlement efficacies showed that the
models give satisfactory
predictions of the performance of the reinforced soil. The use
of a global model improves the
quality of the predictions.
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The symmetry assumption employed to justify the use of
elementary cells for the analysis are
fulfilled in the center of a regularly reinforced embankment
(Jenck et al., 2009a). In an irregular
configuration, such as in the present experiment, particular
predictions must be made with a
global numerical model. For example, the global models results
show that the instrumented
piles in the interior of the embankment are overloaded in the
vicinity of the non-reinforced
zones (Figure 7 and Table 4). This overloading was also observed
in the border piles (Piles F, D,
E in Figure 2), as shown in Figure 8. In these cases, the stress
efficacies are higher than those
obtained with the elementary cells. This is explained by the way
that stress efficacy was
calculated for these piles; the tributary area of the piles in a
regular mesh was used even
though the influence area of the border piles is larger than in
a regular mesh. Figure 9 presents
the experimental and numerical results of the vertical stress
above the heads of the center and
edge piles. This figure confirms, experimentally and
numerically, that the edge piles are
submitted to higher stresses.
Figure 7. Numerical and experimental results of stress
efficacies.
Table 4. Summary of stress efficacies
Figure 8. Numerical results of stress efficacy on selected edge
piles.
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Figure 9. Experimental and numerical results of vertical stress
applied to the heads of the center and
edge piles.
The most relevant displacements were observed on the border
piles (Figure 10). Several tests
demonstrated that when modeling a group of piles, differential
settlement determines the
stress efficacy. The border piles follow this rule until a pile
collapses during the construction of
the embankment. Shear collapse was observed in the subsoil under
the tips of the border piles.
This caused a decrease in the stress efficacy.
It is important to recall that, as mentioned in the analytical
study, a single high-strength
geotextile at the bottom resulted in better experimental stress
efficacies than a two-geogrid
platform over the piles. This occurred even though the
settlement efficacies for both cases are
almost the same (Table 5), demonstrating that the mechanisms of
these two configurations are
different.
Figure 10. Numerical and experimental results of
settlements.
Table 5. Numerical and experimental results of settlement
efficacies.
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Figure 11 shows the experimental measurements made between four
piles. These
measurements were made at the pile head level and over the LTP
for zones 3R and 4R and
show that the stress applied over the LTP is the same for both
geosynthetic-reinforced zones
but that the stress transmitted to the foundation soil is quite
different. This result indicates
that the ways the piles function are different.
Figure 11. Experimental results.
Figure 12.a presents the pile head displacements related to the
axial head pile loads. A
comparison of the numerical results to the experimental
measurements shows that the
loading-pile displacements are reproduced well. When the
embedded test-pile measurements
were superimposed on the numerical results, it is clear that the
toe response approximately
describes the embankment piles behavior. This result indicates
that the pile load test is an
important test to predict the behavior of the piled-embankment
fill.
Figure 12.b shows ET with respect to SRR. The numerical results
obtained with the global model
are represented along with the elementary cell results and the
experimental data. The results
of the global model were recorded at the center of the
embankment along the three pile-
reinforced zones. This figure shows that the experimentally
observed soil stress-settlement
tendency is represented well by the numerical models. Another
important observation is that a
low stress reduction of the reinforced soil causes considerable
settlement reduction. Stress
efficacy is the traditional parameter to assess the performance
of rigid inclusions and to
measure arching. Nevertheless, the results show that the
settlement efficacy ET is an indicative
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parameter that illustrates the global performance of the
reinforced system. The relationship
between these two quantities shows that our case has a
settlement efficacy of around 0.8,
which is equal to two times the stress reduction ratio of
0.4.
Figure 12. Experimental and numerical responses on the pile head
level.
5.4.1 Lateral displacements.
Figure 13 presents the measured and computed lateral
displacement profiles for the
inclinometers placed at the toe of the embankment in the middle
of each zone (Figure 2). The
numerical and experimental results were compared to the
analytical approach of Bourges et al.
(1980), which is presented in the French standards for
foundations design Fascicule 62-V
(1993). This approach, generally called the g(z) method,
consists of calculating the lateral
displacement at different depths using formula (5).
)().(),( max tgZGtzg (5)
D
zZ
(6)
73.013.269.483.1)( 23 ZZZZG(7)
)()0()( maxmaxmax tggtg (8)
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where z is the depth, D is the thickness of the soil layer and t
is the time after the end of the
construction of the embankment. G(Z) is a fixed function.
gmax(0) and gmax(t) are the maximal lateral displacements
induced during the embankments
construction and after its completion, respectively. gmax(0) is
calculated from the undrained
shear strength of the soil and the dimensions of the embankment,
whilegmax(t) depends on
the measured settlement at the center of the embankment. For our
purposes,gmax(t) will be
taken from the experimental results and will be used alone to
determine gmax(t) for the
reinforced zones (2R to 4R). For the non-reinforced zone (1R),
gmax(t) includes the construction
and post construction displacements.
A comparison of the numerical and experimental results shows
qualitatively that the shape of
the experimental horizontal displacement curve is represented
well by the numerical model.
For the 1R zone, a peak is observed two meters below ground
level; the behavior is different in
the reinforced zone, where the maximum lateral displacement is
registered at the surface.
Quantitatively, the prediction for zone 1R is very accurate.
Nevertheless, the lateral
displacements recorded in the pile-reinforced zones are less
than the experimental results.
This underestimation is directly connected to the low numerical
settlement recorded at the
center of the embankment.
The analytical approach predicts a peak displacement
approximately two meters deep in all
zones. The experimental and numerical results show that this
prediction is only valid for the
non-reinforced zone. The behavior is different in the other
zones and cannot be reproduced by
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this analytical method. The reason for this is that the methods
formulation does not take into
account the heterogeneity of the geologic profile. Nevertheless,
this method gives a good
quantitative prediction for a design that is particularly useful
for zone 2R.
Figure 13 and Table 6 show the maximum vertical and lateral
displacements and the ratios
between these two parameters. These values are presented for the
experimental, analytical
and numerical results. As can be seen, the relations between the
vertical and lateral
displacements in the numerical model are similar to those of the
experimental site. This result
again confirms the reliability of the developed model. To
simulate the behavior of the
reinforced soil in the center of the embankment, the observed
ratios can easily be applied to
assess the maximal lateral displacement even with an elementary
cell.
Figure 13. Experimental, analytical and numerical results of
lateral displacements..
Table 6. Ratios between settlement and lateral displacement.
6 DISCUSSION
As shown by the results obtained from the different models, due
to the 3D nature of the
experimental site, only the 3D global model accurately
reproduces the experimental behavior
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of the Chelles site. This model fit the experimental
measurements while accounting for the
complex mechanisms and interactions of the full-scale test.
The differences in stress efficacies between the experimental
and numerical results in zone 2R
might be caused by the assumptions of a continuum numerical
model. The nodes at the top
corner of the pile are not allowed to slide around the pile
shaft due to the fact that they are
connected to the LTP. Some authors, including Chevalier (2008)
and Jenck et al. (2009b), have
studied load transfer on pile-reinforced soils using discrete
modeling. The latter authors
highlighted the differences between the response of discrete and
continuum models,
particularly near the pile head. In discrete modeling, the
elements can slide along the pile
shaft, which will modify the behavior and lead to the smallest
values of the stress efficacies.
Thus, using the discrete element approach might be an
interesting method to simulate the
critical behavior around the head of the pile.
This problem does not occur in zones 3R and 4R due to the setup
of a granular platform
reinforced by geosynthetics, and the continuum models response
better agrees with reality.
7 CONCLUSION
A full-scale experiment was developed to study a soil
reinforcement technique using vertical
rigid piles. Compressible alluvial soils intended to support a
high embankment were reinforced
by concrete piles and geosynthetics. Numerous devices were used
to monitor the evolution of
the slope. Experience shows that this technique reduces
settlements and improves the stability
and the performance of structures. Analytical and numerical
methods were used to predict the
behavior of the embankment.
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The overall consistency of the experiment has been verified by
comparing the data recorded by
different sensors placed at similar positions throughout the
site. The measurements highlight a
significant improvement of load transfer towards the piles when
there is a reinforced load
transfer platform over the piles.
The results show that all of the tested analytical methods
overestimate the arching effect in
the zone without an LTP. When including an LTP at the base of
the experimental embankment,
the stress efficacy substantially increases; thus, the
analytical results underestimate the
experimental performance. Consequently, the use of these
analytical methods results in a safe
design.
Although the analytical methods give rapid and safe results in
traditional pile-reinforced
embankments (i.e., with geosynthetics), only a few of these
methods have proposed a global
method that takes into account the compressibility of soft soil,
the negative frictional loading
and the pile displacements. This is an important limit that can
lead to important differences
with real structures. As mentioned in the text, few previous
studies take these parameters into
account.
The numerical models have the advantage of being able to
reproduce the phenomenon that
determines the pile-reinforced soils behavior. The numerical
simulations showed that the
behavior depends not only on soil compressibility but also on
the soils shear resistance.
Preliminary numerical investigations of the characteristics of
soil-pile interaction are required
to define reliable simulations of the reinforced soil. Thus, an
axially loaded test pile must be
part of the preliminary study of a pile-embankment project.
The behavior of the non-reinforced area and the soil-pile
interaction are correctly predicted by
the simulations. On the other hand, when dealing with the pile
reinforcement, high stresses
were reported over the piles of the zone without the LTP; this
resulted in an underestimation
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of the settlements in this zone. They also showed that the
behavior of the geosynthetics
cannot be correctly reproduced with the numerical simulations.
Nevertheless, the settlement
efficacy results showed that the global predictions were
satisfactory. Settlement efficacy
appears to be a convenient parameter to evaluate the global
performance of the reinforced
soil; a modest stress reduction in the soil can led to an
important settlement reduction ratio. In
addition, the numerical calculations show that the piles
behavior in the reinforced soil
conforms to the test piles.
The developed global numerical model took into account the real
configuration of the full-scale
experiment. These kinds of models are the only way to observe
the differences between the
performance of interior and border piles in a group and the
lateral response of the reinforced
soil after being submitted to non-homogeneous overburden
loads.
ACKNOWLEDGEMENTS
This work is part of a French National Research Project, ASIRI,
aimed at formulating guidelines
and recommendations for the design of soils reinforced by stiff
vertical piles. The authors
would like to thank the French National Project (ASIRI) for
funding this research within the
partnership between Fondasol, IREX, Keller, LCPC, EGIS, Socotec,
and Tencate Geosynthetics.
This work was made possible thanks to the financial support of
Drast and RGCU and the
Conseil Gnral de Seine et Marne, who kindly allowed us to use
the experimental site.
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Figures
q
s-a
hb
b(
s-a)/2
aa
a) b) c)
s-a aa
t
v
dh
v+dvt hh
Figure 1. Groups of analytical methods.
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ZoneSoil
reinforcement
Load transfer
platformGeosynthetics
1R No No No
2R
Driven back
auger pile
No No
3R Yes Yes
4R Yes Yes
GTXGTX
GGRGGR
GGRGGR
3R ZONE 4R ZONE
10cm
20cm
20cm
20cm
20cm
Load transfer platforms
Figure 2. Plan view of the experimental site and configuration
of the tested zones.
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0,0
0,2
0,4
0,6
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5
Efficacy
Experimental
BS8006 (2D)
BS8006-1 (3D)
Combarieu (1988)Combarieu (2007)
EBGEO
Geogridbridge
)( Ds
h
(a) Zone 2R
0,00
0,20
0,40
0,60
0,80
1,00
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5
Efficacy
Experimental
EBGEO
Geogridbridge
)( Ds
h
(b) Zone 3R
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0,00
0,20
0,40
0,60
0,80
1,00
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5
Efficac
y
Experimental
EBGEO
Geogridbridge
)( Ds
h
(c) Zone 4R
Figure 3. Analytical and experimental results
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Figure 4. The adopted three-dimensional numerical models.
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-10
-8
-6
-4
-2
0
-0,4-0,3-0,2-0,10,0
Depth(m)
Settlement (m)
Experimental
Elementary cell
Global model
Figure 5. Non-reinforced area (1R) settlement.
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-6,0
-5,0
-4,0
-3,0
-2,0
-1,0
0,0
0 100 200 300 400 500 600 700
Pileheaddisplaceme
nt(cm)
Axial load, Q0 (kN)
Experimental
Numerical calculation
(a)
-8,0
-7,0
-6,0
-5,0
-4,0
-3,0
-2,0
-1,0
0,0
0 100 200 300 400 500 600 700
z(m)
Load along the pile (kN)
Experimental
Numerical calculation Qp(z)Axial load
Qs(z)Cumulatedshear load
(b)
Figure 6. Load-displacement curves and final load distributions
for the test pile (embedded pile).
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0,0
0,2
0,4
0,6
0,8
1,0
0 1 2 3 4 5 6
Stressefficacy
Embankment height (m)
MG_2R MG_3R MG_4R
CE_2R CE_3R CE_4R
Experimental final
results
Numerical results
EPC1
EPC10
EPC5
Figure 7. Numerical and experimental results of stress
efficacies.
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0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 1 2 3 4 5 6
Stressefficacy
Embankment height (m)
D (2R)
E (3R)
F (4R)
(2R)
MG
CE
Figure 8. Numerical results of stress efficacy on selected edge
piles.
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-2500
-2000
-1500
-1000
-500
0
0 1 2 3 4 5 6
Verticalstressonpilehead(kPa)
Embankment height (m)
A (2R)
D (2R)
Experimental final
results
Figure 9. Experimental and numerical results of vertical stress
applied to the heads of the center andedge piles.
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-0,30
-0,25
-0,20
-0,15
-0,10
-0,05
0,00
0 10 20 30 40 50 60 70
Settlement(m)
Length (m)
Global model results
Experimental results
T35-T36
T26-T28
T22T10
T14-T16
T1
T5-T7
Figure 10. Numerical and experimental results of
settlements.
Slope 1R 4R 3R 2R Slope
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0,00
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0 10 20 30 40 50 60
Settlementmeasuredons
oftsoil(m)
Stress applied on soft soil (kPa)
ECP 7
ECP 12
(a)
0,00
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0 10 20 30 40 50 60
Settlementmeasuredonsoft
soil(m)
Stress applied on LTP (kPa)
ECP 8
ECP 13
(b)
Figure 11. Experimental results.
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-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
0 100 200 300 400 500 600
Verticaldisplacement(m
m)
Axial load (kN)
MG
CE
2R
3R
4RToe Head
Embedded pile load test
(a)
0
0,2
0,4
0,6
0,8
1
0 0,2 0,4 0,6 0,8 1
Settlementefficacy,
ET
Stress reduction ratio, SRR
2R 3R 4R
MG CE
(b)
Figure 12. Experimental and numerical responses on the pile head
level.
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-12
-10
-8
-6
-4
-2
0
0 10 20 30 40 50
Depth(m)
Lateral displacement (mm)
Experimental
Analytical method g(z)
Numerical results
(a) Zone 1R
-12
-10
-8
-6
-4
-2
0
0 4 8 12 16 20
Depth(m)
Lateral displacement (mm)
Experimental
Analytical method g(z)
Numerical results
(b) Zone 2R
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-12
-10
-8
-6
-4
-2
0
0 4 8 12 16 20
Depth(m)
Lateral displacement (mm)
Experimental
Analytical method g(z)
Numerical results
(c) Zone 3R
-12
-10
-8
-6
-4
-2
0
0 4 8 12 16 20
Depth(m)
Lateral displacement (mm)
Experimental
Analytical method g(z)
Numerical results
(d) Zone 4R
Figure 13. Experimental, analytical and numerical results of
lateral displacements..
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TABLES
Table 1. Soil parameters.
Embankment
fill
Load transfer
platform
Silty clay
(dry crust)Clay Sandy clays 1 Sandy clays 2
Sand and gravel
alluvial deposits
Density, [kN/m3] 19.1 21 20 15 20 20 20
Thickness, z [m] 1.7 0.6 4.2 1.5 - 2.5
CC/(1+e0) [-] 0.1 0.2 0.06 0.08
CompressibilityIndices,
CC [-] 0.2 0.54 0.1 0.13
[-] 0.087 0.235 0.044 0.056
Swelling Indices,
CS [-] 0.03 0.05 0.01 0.01
[-] 0.013 0.022 0.005 0.004
Void ratio, e0 [-] 1 1.7 0.7 0.6
Friction ratio, [] 36.6 36 26 26 26 26 33
Cohesion, c [kPa] 17.3 61 0 0 0 0 0
Dilatancy, [] 6.6 3 3
Overconsolidation
pressure, p[kPa] 30
Pressuremeter
modulus, EM[MPa]
34.5
Pressuremeter
limit pressure, Pl[MPa]
2.3
Elastic modulus, E [MPa] 50 70 76.6
Poisson ratio, [-] 0.3 0.3 0.3
Parameter
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Table 2. Summary of the analytical and experimental results
2R 3R 4R 2R 3R 4R
Experimental 0.18 0.89 0.74
Combarieu (1988) 0.45 2.5
BS8006 (2D) 0.59 3.28
BS8006-1 (3D) 0.42 2.33
EBGEO 0.41 0.68 0.72 2.28 0.76 0.97
GeogridBridge 0.35 0.48 0.50 1.94 0.54 0.68
Combarieu (2007) 0.31 1.72
Final stress efficacy H=5m Analytical/experimental ratio
Table 3. Interface properties
Interface qsI qsII
Depth [m] 0 to 4 under 4
[] 0 0
c [kPa] 45 30
[] 0 0
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Table 4. Summary of stress efficacies
Zone Experimental CE MG CE MG
2R 0.18 0.46 0.50 2.55 2.77
3R 0.89 0.54 0.64 0.61 0.72
4R 0.74 0.53 0.66 0.72 0.89
CE=Elementary cell, MG=Global model.
Efficacy Ratio
Table 5. Numerical and experimental results of settlement
efficacies.
Zone Experimental CE MG CE MG
2R 0.60 0.87 0.76 1.45 1.27
3R 0.73 0.87 0.83 1.19 1.14
4R 0.75 0.87 0.81 1.16 1.08
Settlement Efficacy Ratio
CE=Elementary cell, MG=Global model.
Table 6. Ratios between settlement and lateral displacement.
Experimental Numerical g(z) Experimental Numerical g(z)
Experimental Numerical g(z)
1R 260 230 260 31 25 46 0.12 0.11 0.18
2R 105 67 105 16 10 16 0.15 0.15
3R 71 44 71 12 8 11 0.17 0.18 0.16
4R 64 48 64 14 11 10 0.22 0.23
Maximal settlement * Maximal lateral displacement**
RatioZone
* at the center of the embankment **at the toe of the embankment
s lope
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HIGHLIGHTS
Monitoring of a piled supported embankment built over soft
alluvial soil.Several analytical and numerical models were tested
to assess the arching effect.
Comparisons between the experimental data and the design
methods.Settlement efficacy is a reliable parameter to assess the
embankment performance.
