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Effect of Smear on Strength Behavior of SCP-Reinforced Soft Ground
Effet de comportement de l’étalement de force du SCP- Sol mou renforcé
Mir B.A.
Deptt. of Civil Engineering, National Institute of Technology Srinagar- 190006, Kashmir, India
Juneja A.
Deptt. of Civil Engineering, Indian Institute of Technology Bombay, Mumbai-400076, MH, India
ABSTRACT: Sand columns traditionally known as sand compaction piles-(SCPs) have been used to increase the load carrying
capacity of soft clays and accentuate consolidation during preloading. Installation of SCPs is known to cause disturbance due to
smear in a limited zone of the soil surrounding the SCP. In this study, conventional triaxial tests have been performed on 200mm
long and 100mm-diameter clay samples installed with SCP to simulate the strength behaviour of composite ground under different
confining pressures ranging from 50kpa to 575kpa. The SCPs were prepared using area replacement ratio of 6.25 to 64% and
compacted using pneumatic compactor. The smear zone was created by using a rough casing to drill the hole. The results seem to
suggest that the stress-strain behaviour of the clay was influenced by the presence of smear zone. The effect of smear zone on SCP
was investigated by observing the change in pore pressure during undrained shear strength of the composite ground. The natural
fabric of the soil was destroyed adjacent to the SCPs and the shear-induced pore pressures were less in composite specimens with
smear-effect. In addition, as the area replacement ratio was increased, both the stiffness and the strength of the specimen increased.
RÉSUMÉ : Des colonnes de sable traditionnellement connues comme piles de compactage (SCPs) de sable ont été utilisées pour
augmenter la capacité portante des argiles molles et accentuer la consolidation au cours du préchargement.
L’
installation de MCS est
connue pour causer des perturbations dues au frottis dans une zone limitée du sol entourant le SCP. Dans
cette étude, les essais
triaxiaux conventionnels ont été réalisés sur les échantillons d’argile, de 100 mm de long et de 200 mm de diamètre, installés avec
SCP pour simuler le comportement de la résistance du terrain composite sous différentes pressions de confinement allant de 50 kPa à
575 kPa. Les MCS ont été préparés à l’aide du coefficient de remplacement de 6,25 à 64 %, et compactées au pneumatique. La zone
de souillure a été créée en utilisant une enveloppe rugueuse pour percer le trou. Les résultats donnent à penser que le comportement de
contrainte-déformation de l’argile a été influencée par la présence de la zone de souillure. L’effet de zone de souillure sur SCP a été
examiné en observant le changement de pression interstitielle au cours de la consolidation et de la résistance au cisaillement du sol
composite. Le tissu naturel du sol a été détruit adjacent à la SCP et les pressions interstitielles induites par cisaillement étaient
inférieures dans les échantillons composites avec un effet de maculage. En outre, lorsqu’on augmente le ratio de remplacement du
frottis, la rigidité et la résistance de l’échantillon augmentent.
KEYWORDS: Sand compaction pile, installation effects, smear, soft ground
MOTS-CLÉS : Sable tas de compactage, les effets de l'installation, les frottis, sol mou
1 INTRODUCTION
Soft ground is widely distributed especially along the coastal
area, having large potential for settlement with low inherent
shear strength. In the recent years, improvement of soft soils
has been extensively implemented for the various development
projects all over the world due to extremely limited stable
construction sites. Granular piles such as sand compaction piles
(SCPs) are considered as cost-effective and alternative solution
to the problem of stability and settlement posed by construction
on soft ground. The insertion of SCPs into soft clay has been
shown to have a positive effect on the load carrying capacity of
the clay, resulting in a composite soil mass that has greater
shear strength and improved stiffness compared to the
unreinforced clay. Sand compaction pile (SCP) is a method of
constructing large diameter sand column in the ground. This
method of ground improvement has been widely used for rapid
improvement of soft ground, and also in near-shore regions for
land reclamation works (e.g. Aboshi and Suematsu 1985,
Bergado and Balasubramniam 1994). In India, the granular
columns have been used to improve ground for container freight
station at Navi Mumbai and the construction of dry dock at
Pipavav shipyard (Raj and Dikshith 2009). Many researchers
(e.g. Bergado et al. 1991, Juneja and Mir 2011) have
investigated the effect of SCP installation on disturbance to the
surrounding soil. The extent of the disturbed or smear zone can
affect the engineering behaviour of the composite ground. The
disturbance in this zone depends upon the column diameter and
the tools used in the installation (e.g. Singh and Hattab 1979,
Madhav et al. 1993). Shear induced pore pressures were found
to be less in specimens which had the smear zone surrounding
the sand column. However, pore pressures began to increase
close to failure due to rearrangement of soil particles (Mir
2010). Laboratory and field tests previously conducted to
determine the extent of the disturbance caused by pile driving
into soft clay deposits have demonstrated that the natural
structure of the clay around the pile is excessively disturbed
(Randolph et al. 1979, Xu et al. 2006). It was observed that the
diameter of the severely disturbed or remoulded ground around
a driven closed-ended casing was about 1.4 times the diameter
of the casing. Recently, Weber et al. (2010) compared the
smear zone around model SCPs to that observed around driven
piles. It was observed that the smear zone around SCPs
installed on the centrifuge extended up to 1.2 to 1.4 times the
SCP diameter. Dissipation of the excess pore pressures often
results in increase in the shear strength. Aboshi et al. (1979)
observed up to 50% increase in the undrained strength in about
one month after the SCP installation at test sites in Japan.
Matsuda et al. (1997) also reported an increase in strength of the
composite SCP ground within three months of the SCPs
installation.
