1291
Long-term performance of preloaded road embankment
Compo
rtement à long terme d’un remblai routier préchargé
Islam M.N., Gnanendran C.T.
School of Engineering and Information Technology, UNSW Canberra, Australia
Sivakumar S.T.
Queensland Department of Transport and Main Roads,
Australia
Karim M.R.
School of Engineering and Information Technology, UNSW Canberra, Australia
ABSTRACT: The results from an investigation into the long-term performance of the preloaded Nerang-Broadbeach Roadway (NBR)
embankment near the Gold Coast in Queensland, Australia, are presented in this paper. The soil profile along this roadway consists of
deep Cainozoic estuarine alluvial soft clay deposit. To predict the performance of the preloaded embankment, two fully coupled
nonlinear Finite Element Analyses (FEA) were conducted adopting an elasto-viscoplastic (EVP) and an elasto-plastic Modified Cam
Clay (MCC) model to represent the soft clay using the UNSW Canberra modified version of the nonlinear stress analysis program
AFENA. It was found that the MCC model under-predicted the ultimate settlement while the creep-based EVP model captured it well
but over-predicted the pore pressure response. Observational approaches using the Asaoka and Hyperbolic methods were also applied
from which it was observed that, when the soft soil exhibited creep, after a certain cutoff time increment (
∆t)
, the Asaoka plot became
parallel to the 45
line and the settlement prediction was unrealistic compared with the field measurement. After a modification was
introduced into the Asaoka method for creep-susceptible soil, the predicted settlement was found to be in good agreement with that
obtained from Hyperbolic method and the details are presented in the paper.
RÉSUMÉ : Les résultats d'une enquête sur le comportement à long terme de la route Nerang-Broadbeach (NBR), en remblai ayant fait
l’objet d’
un préchargement près de la Gold Coast dans le Queensland (Australie) sont présentés dans ce papier. Le profil de sol le long
de cette chaussée se compose de
dépôts profonds d’argile molle marine de l’horizon alluvial «
Cainozoic ». Afin de prédire la
performance du remblai préchargé, deux analyses couplées aux élément finis non linéaires (FEA) ont été réalisées en adoptant un
modèle élasto-viscoplastique (EVP) et un modèle élasto-plastique modifié du type Cam Clay (MCC) ; ces modèles ont été choisis
pour r
eprésenter l’argile molle en utilisant la version modifiée de l’analyse en contrainte non linéaire du programme AFENA.
Il a été
constaté que le modèle MCC a sous estimé les tassements ultimes, tandis que le modèle EVP basé sur le fluage a bien évalué ces
tassements mais a surestimé la réponse en terme de pression interstitielle. Les approches observationnelles utilisant la méthode
d’Asaoka
ou la méthode hyperbolique ont été aussi utilisées ; il a été constaté que lorsque le sol exhibe du fluage, après un certain
seuil dans l’incrément de temps (
∆t
), la courbe d’Asaoka devient parallèle à la droite à 45° et la prédiction de tassement devient
irréaliste par rapport aux mesures sur le terrain. Après qu’une modification ait été introduite dans cette méthode d’Asaoka pour les
sols sujets au fluage, le tassement prédit a été trouvé en bon accord avec celui tiré de la méthode hyperbolique.
KEYWORDS: Preload, soft clay, Modified Cam Clay model, Elasto-viscoplastic model, creep, Asaoka method, Hyperbolic method.
1 INTRODUCTION
The Nerang-Broadbeach Roadway (NBR) was constructed by
the Queensland Department of Transport and Main Roads
(QDTMR) and completed in 2001. It is located closer to the
Gold Coast Highway in the South part of Surfers Paradise, Gold
Coast, Queensland, Australia. It was constructed to
accommodate the
region’s
transport network and enhance road
safety. The roadway embankment was founded on deep
Cainozoic estuarine alluvial, soft sensitive deposits of
thicknesses from 5 to 21 m overlaying greywackes and argillite
bedrock. This estuarine deposit is highly compressible, exhibits
low bearing capacity and undergoes extensive time-dependent
settlement when subjected to extrinsic loads. Although there are
several techniques for accelerating the ongoing settlement of
estuarine clay and to mitigate post-construction damage,
preloading in conjunction with surcharging has been proven to
be one of the most efficient ground improvement techniques for
estuarine clay in the Queensland region (Islam et al. 2012,
2013). The NBR was divided into five distinct preloading
embankment sections:
North of Main Drain; Main Drain to
Meadow Drive; Meadow Drive to Witt Ave Drain; South of Witt
Avenue Drain; and Gin House Creek (Fig. 1)
. Performance of
the embankment section located in between
Gin House Creek
and
Witt Avenue Drain
and nearer to settlement plate SP18 that
had a preloading height of 3 m is examined in this paper.
Field monitoring data (measured settlement and excess pore
water pressure) of this embankment was compared with the
corresponding predicted responses obtained from Finite
Element Analysis (FEA). In particular, nonlinear fully coupled
FEAs were carried out adopting a creep-based elastic-
viscoplastic (EVP) model and Modified Cam Clay (MCC)
elasto-plastic model for the foundation soil from which it was
found that the creep-based EVP model captured the field
settlement of the embankment better than the MCC model but
over-estimated the excess pore water pressure. The ultimate
settlement was estimated using Asaoka
’s
and Hyperbolic
observational methods in this study. Since the foundation soft
soil exhibited creep, after a certain cutoff time increment (
∆t)
,
Asaoka plot became parallel to the 45
line and the predicted
settlement was unrealistic compared to those obtained from
FEA using the creep-based EVP model as well as Hyperbolic
method. Therefore, some modification was necessary for the
Asaoka method for capturing the ultimate settlement of creep-
susceptible foundation soil and it is the focus of this paper.
2 SUBSURFACE CONDITIONS OF SITE
To delineate the subsurface conditions of the NBR, two subsoil
investigations were carried out by the QDTMR, in 1991 (Main
Roads 1991) and 1999 (Main Roads 1999), from which a poor
subsoil strata was identified. This led to further investigations of
QDTMR in 2000 (Main Roads 2000) and 2001 (Main Roads
2001) which included six borehole tests, twenty electric cone
penetrometer tests (CPT) and four piezocone dissipation tests
(CPT-u). The reasons behind the boreholes were to obtain
undisturbed soil samples for laboratory testing and to conduct
further in-situ field testing. The CPT and CPT-u tests were
-term performance of pr loaded ro d embankment
