2573
Appliedmethod:
Back-Analysis Experimental
Analytical
FEM Assumed
Intact Zone
Smear
Zone
R
r
s
r
w
Intact Zone
Smear
Zone
R
r
s
r
w
Intact Zone
Smear
Zone
R
r
s
r
w
Vertical
drain
S
k
s
k
s
k
s
Intact Zone
Smear
Zone
R
r
s
r
w
k
s
k
h
k
v
k
h
k
v
k
h
k
v
k
h
k
v
Vertical
drain
Vertical
drain
Vertical
drain
0 1 2 3 4 5 6 7 8
Barron(1948)
Casagrandeand Poulos (1969)
Holtzand Holm (1973)
Akagi(1976)
Hansbo(1981)
Hansboet al.(1981)
Jamiolkowski etal. (1983)
Bergadoet al. (1991)
Onoueet al. (1991)
Bergadoet al. (1993)
Almediaet al. (1993)
Hansbo(1994)
Mesriet al. (1994)
Hansbo(1997)
Indraratnaand Redana(1998)
Chaiand Miura (1999)
Eriksson et al. (2000)
Sharma and Xiao (2000)
Hird and Moseley (2000)
Boet al. (2003)
Indraratnaetal. (2005b)
Indraratnaet al. (2005c)
Sathananthanand Inraratna
…
Sathananthanetal. (2008)
Ghandeharioonet al. (2009)
Kimet al. (2010)
Tran-Nguyen and Edil(2011)
Ghandeharioonet al. (2012)
Extent ratio(r
s
/r
m
)
Lowerbound
Upperbound
0 1 2 3 4 5 6 7 8
Barron (1948)
CasagrandeandPoulos(1969)
Holtzand Holm(1973)
Akagi(1976)
Hansbo (1981)
Hansbo etal.(1981)
Jamiolkowskietal.(1983)
Bergado etal.(1991)
Onoue etal.(1991)
Bergado etal.(1993)
Almediaetal.(1993)
Hansbo (1994)
Mesrietal.(1994)
Hansbo (1997)
IndraratnaandRedana(1998)
ChaiandMiura(1999)
Erikssonetal.(2000)
SharmaandXiao (2000)
Hirdand Moseley(2000)
Bo etal.(2003)
Indraratnaetal.(2005b)
Indraratnaetal.(2005c)
SathananthanandInraratna(2006)
Sathananthanetal.(2008)
Ghandeharioonetal.(2009)
Kimetal.(2010)
Tran-NguyenandEdil(2011)
Ghandeharioonetal.(2012)
Extentratio (r
s
/r
m
)
Lowerbound
Upperbound
Numerical Analysis to Quantify the Influence of Smear Zone Characteristics on
Preloading Design in Soft Clay
Analyses
numériques pour quantifier l’
influence des caractéristiques de la zone endommagée sur
la conception de préchargement dans les argiles molles
Parsa-Pajouh A.
,
Fatahi H., Khabbaz B.
School of Civil and Environmental Engineering, University of Technology Sydney (UTS), Sydney, Australia
ABSTRACT: In this paper, the effects of uncertainties of smear zone characteristics induced by installation of prefabricated vertical
drains on the preloading design are numerically investigated. FLAC 2D finite difference software with additional developed
subroutines has been employed to conduct the numerical simulations. The finite difference analyses have been verified using a case
study. Furthermore, a comprehensive parametric study is conducted to investigate the influence of smear zone permeability and extent
on the model predictions. Results of this study indicate that the assumptive properties for smear zone characteristics may result in
inaccurate predictions of ground deformations and pore water pressures. This may lead to early removal of the surcharge in the
construction process causing excessive post construction settlement. It is recommended to practising engineers to use results of trial
preloading to back calculate the required smear zone characteristics in the early stages of embankment construction to optimize the
design.
RÉSUMÉ : Dans cet article, les effets des incertitudes des caractéristiques de la zone endommagée induites
par l’
installation des
drains verticaux préfabriqués sur la conception du préchargement sont étudiés par une méthode numérique. Le logiciel de différences
finis FLAC2D avec sous-programmes additionnels a été utilisé afin de réaliser les simulations numériques. Les analyses de
différences finis ont été vérifiées à l’aide d’une étude de cas. Par ailleurs, une étude paramétrique approfondie est effectuée afin
d’
investiguer
l’influence de la perméabilité de la zone endomm
agé sur les prédictions du modèle. Les résultats de cette étude
montrent que les propriétés supposées pour les caractéristiques de la zone endommagé peuvent entrainer des prédictions incorrectes
de déformations du sol et de pressions interstitielles. Cela peut conduire à un retrait précoce de la surcharge dans le processus de
construction engendrant un tassement post-construction excessive. Il est recommandé aux ingénieurs
d'utiliser les résultats de l’
essai
de préchargement afin de calculer les caractéristiques requises de la zone endommagée pour optimiser la conception.
KEYWORDS: FLAC, numerical analysis, preloading, smear zone, vertical drain
1 INRODUCTION
Finding efficient ground improvement techniques to modify the
soft soil properties, considering the project time limitation and
the construction cost has been a continuous challenge for the
construction companies. Various ground improvement methods
have been proposed to improve the strength properties of the
soft soil. In the last two decades, employing prefabricated
vertical drain (PVD) assisted preloading has been recognised as
a very efficient ground improvement method for sites with deep
soft soil deposits (Holtz et al. 1991; Shang et al. 1998;
Indraratna et al. 2005). Installation of the prefabricated vertical
drains using mandrel, induces disturbance of the soil
surrounding the drain, resulting in a smear zone of reduced
permeability adversely affecting the consolidation process.
Predicting the soil behaviour surrounding the drain requires an
accurate estimation of the smear zone properties. Generally, two
major parameters are proposed to characterise the smear zone;
the permeability (
k
s
), and the extent (
r
s
) of the smear zone.
Figure 1 illustrates the cross section of prefabricated vertical
drains surrounded by smear zone, which are installed in
rectangular pattern. Determining both the smear zone extent
and its permeability is a challenging task. According to
literature,
very diverse values are reported for the
permeability
ratio (
k
h
/k
s
) and extent ratio (r
s
/r
m
), which are illustrated in
Figure 2. The proposed range shows that the extent of the smear
zone (
r
s
) may vary between 1.6 to 7 times of the drain radius
(
r
w
) or, 1.0 to 6 times of mandrel equivalent diameter (
r
m
). The
proposed range for the permeability ratio (
k
h
/k
s
) is 1.3 to 10,
where k
h
is the horizontal permeability of the intact soil.
Figure 1. Cross section of PVD surrounding by smear zone
Figure 2. Proposed values for smear zone characteristics
It can be observed that wide ranges are proposed for
k
h
/k
s
and
r
s
/r
m
and there is no definite method to predict these parameters
precisely to be used by practising engineers. The assumptive
properties for smear zone characteristics may result in
