2965
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Aspects on the modelling of smear zones around vertical drains
Aspects de la modélisation de la zone remaniée autour des drains verticaux
R. Müller
Tyréns AB, Borlänge, Sweden / KTH Royal Institute of Technology, Stockholm, Sweden
S. Larsson
KTH Royal Institute of Technology, Stockholm, Sweden
ABSTRACT: The analytical design of vertical drains in soft clay requires knowledge of the coefficient of consolidation and also of
the disturbance effects induced during the installation of the drains. Several analytical models describing the disturbance effects in
different ways are proposed in the literature. The earliest and simplest models describe the disturbance effect in terms of concentric
cylinders around a drain where a reduced and constant permeability is assumed, while more recent models attempt to describe the
disturbance more realistically via more complex mathematical formulations. Although these new models describe the real in situ
behaviour more realistically than the early ones, they may not always be suitable for practical use as many of the required variables
are difficult to assess by standard investigation methods. This study investigates and discusses the difference between some of the
available models and evaluates the influences on the results of the variables incorporated in the models.
RÉSUMÉ: L’étude analytique des drains verticaux dans les argiles molles nécessite la connaissance du coefficient de consolida
tion et
des effets du remaniement produit par l’installation des drains. Ces e
ffets peuvent être modélisés de plusieurs façons. Les modèles les
plus anciens et les plus simples décrivent le remaniement à l’aide de cylindres concentriques autour d’un drain en supposant
que la
perméabilité est réduite et constante, tandis que des modè
les plus récents s’efforcent à décrire le remaniement de façon plus réaliste à
l’aide de formulations mathématiques avancées. Bien que ces modèles décrivent le comportement in situ de manière plus réalist
e que
leurs prédécesseurs, leur utilisation pratique est souvent limitée car plusieurs des paramètres requis sont souvent difficiles à évaluer à
l’aide de sondages, forages et essais classiques. Cette étude s’intéresse aux différences entre certains des modèles existant
s et évalue
l’influence des divers par
amètres sur les résultats.
KEYWORDS: Vertical Drains, Design, Modelling
1 INTRODUCTION
During the installation of prefabricated vertical drains (PVDs)
in soft clay, the original soil fabric is disturbed. The disturbance
occurs when the installation device, the mandrel, is pushed
through the clay displacing the soil material. According to e.g.
Hird and Moseley (2000) this results in a disruption of the
initial soil fabric, e.g. the destruction of any permeability
anisotropy (the ratio of horizontal to vertical permeability
⁄
), and causes excess pore pressures that trigger a
subsequent reconsolidation of the clay and an associated
decrease in void ratio that in turn decreases the permeability
(e.g. Tavenas et al. 1983). The nature of the disturbance is
highly complex and depends on many factors such as the
characteristics of the soil material, the shape, surface roughness
and size of the mandrel, the installation rate and the soil
movement after the mandrel has been removed (e.g. Onoue et
al. 1991, Hird and Moseley 2000). Laboratory studies
investigating the spatial characteristics of the disturbed zone
show that the degree of disturbance (i.e. the reduction in
) is
most pronounced in the vicinity of the drain where
approaches
and decreases with increasing radial distance
from the drain (Onoue et al. 1991, Bergado et al. 1991, Madhav
et al. 1993, Indraratna and Redana 1998, Hird and Moseley
2000, Sharma and Xiao 2000, Sathananthan and Indraratna
2006).
For the design of PVDs and the assessment of the average
degree of consolidation (
), several theoretical models
describing the characteristics of the disturbed zone have been
proposed over the years. The early rather simple models (Barron
1948, Hansbo 1979) assumed a unit cell soil cylinder dewatered
by one centric drain and a disturbed (smear) zone with a
constant and reduced horizontal permeability (Figure 1).
According to Basu et al. (2006), previous studies based on this
model suggest that the extent (diameter) of the smear zone (
)
is 2 to 4 times larger than the equivalent diameter of the PVD
(
) and that the reduced horizontal permeability (
) is 2 to
10 times lower than the undisturbed permeability (
), i.e.
⁄
2-4 and
⁄
2-10. However, the cited
laboratory studies have indicated that the extent of the disturbed
zone can be as large as
⁄
9 (where
is the equivalent
diameter of the mandrel).
More recent models attempt to capture the nature of the
smear zone more realistically, describing the variation of
within the disturbed zone (e.g. Walker and Indraratna 2006,
Basu et al. 2006, Chung et al. 2009). In addition, temporal
effects, such as the reconsolidation of the clay after drain
installation, affecting the characteristics of the disturbed zone
have been incorporated in the models presented by Indraratna et
al. (2005) and Walker et al. (2012).
To a practising engineer creating a design involving PVDs,
the choice of model and the widely varying suggestions
regarding the values of
and
may be confusing. This paper
investigates the differences between six of the analytical models
available in the literature and the influences of the involved
variables on the assessment of
. All the models investigated
can be written on the form:
(1)
where
⁄
is the time factor for horizontal
consolidation,
⁄
is the undisturbed horizontal
coefficient of consolidation in the clay (where
is the vertical
t t
li
f
ti l
i
Aspects de la modélisation de la zone remaniée autour des drains verticaux
Müller R.
Tyréns AB, Borlänge, S eden / KT oyal Institute of Technology, Stockhol , S eden
Larsson S.
KT oyal Institute of Technology, Stockhol , S eden
