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Foundations of embankments using encased stone columns
Fondations de remblais avec des colonnes ballastées entourées de géotextile
Castro J., Sagaseta C., Cañizal J., Da Costa A., Miranda M.
University of Cantabria, Santander, Spain
ABSTRACT: Stone columns are a common improvement technique for foundations of embankments in soft soils. When the soft soil
does not provide enough lateral support, the columns are encased with a geosynthetic. This paper presents a closed-form solution to
study soft soil improvement, both reduction of settlement and consolidation time, by means of encased stone columns. An end-bearing
column and its surrounding soil, is modelled in axial symmetry under a rigid and constant load. Soil is assumed as elastic but plastic
strains are considered in the column. An elasto-plastic behaviour is also considered for the encasement by means of a limit tensile
strength. Parametric studies of the settlement reduction and stress concentration show the efficiency of encasing the columns, which is
mainly ruled by the encasement stiffness compared to that of the soil. The analytical results are in good agreement with numerical
analyses. Finally, the encasement length is analysed using the closed-form solution.
RÉSUMÉ: Les colonnes ballastées sont une technique d'amélioration de sol pour les remblais en sols mous. Lorsque le sol mou ne
fournit pas assez de soutien latéral, les colonnes sont entourées avec un géosynthètique. Cet article présente une solution analytique
pour étudier l'amélioration des sols mous, la réduction des tassements ainsi que le temps de consolidation, au moyen des colonnes
entourées en géotextile. Une colonne ne reprenant les efforts que par la pointe et le sol environnant sont modélisés en axisymétrie
sous une charge constant. Le comportement du sol est supposé élastique mais les déformations plastiques sont considérées dans la
colonne. Un comportement élasto-plastique est également pris pour le géosynthètique au moyen d'une résistance à la traction limite.
Des études paramétriques de la réduction du tassement et de concentration de contraintes montrent l'efficacité de l'enveloppe
géosynthètique des colonnes, ce qui est principalement régie par la rigidité de l’enveloppe géosynthètique par rapport à celle du sol.
Les résultats analytiques présentent une bonne concordance avec les analyses numériques. Finalement, la longueur de l’enveloppe
géotextile est analysée en utilisant la solution basée sur une cellule élémentaire constituée d’une colonne et d’un volume élémentaire
de sol.
KEYWORDS: soft soils, ground improvement, encased stone columns, analytical solution, numerical analyses.
1 INTRODUCTION
Stone columns, either by the vibro-replacement or vibro-
displacement methods, are one of the most common
improvement techniques for foundation of embankments or
structures on soft soils. The inclusion of gravel, which has a
higher strength, stiffness and permeability than the natural soft
soil, improves the bearing capacity and the stability of
embankments and natural slopes, reduces total and differential
settlements, accelerates soil consolidation and reduces the
liquefaction potential. Alteration of the natural soft soil caused
by stone column installation (Guetif et al. 2008, Castro and
Karstunen 2010) is not usually considered in their design.
Stone columns may not be appropriate in very soft soils that
do not provide enough lateral confinement to the columns. It is
generally accepted that those are soils with undrained shear
strengths below 5-15 kPa (Wehr 2006). To increase the lateral
confinement of the columns, and consequently their vertical
capacity, encasing the columns with geotextiles has proved to
be a successful solution in recent years.
A high tensile stiffness of the encasement is recommended
as it will be shown in this paper; and therefore, other
geosynthetics, such as geogrids, are also used to encase the
column (Sharma et al. 2004, Gniel and Bouazza 2009).
However, geogrids do not act as a filter and do not avoid
contamination of the column with fines.
The development of encased stone columns as a ground
improvement technique has come with an increasing number of
studies in the last decade. However, most of the research is done
using numerical methods (e.g. Murugesan and Rajagopal 2006,
Malarvizhi and Ilamparuthi 2007, Smith and Filz 2007, Yoo
2010, Lo et al. 2010) and there are very few analytical solutions
available in the literature (Raithel and Kempfert 2000, Pulko et
al. 2011). That recently motivated the authors to develop a new
closed-form solution to study the deformation and consolidation
around encased stone columns (Castro and Sagaseta 2011). That
solution is an extension of another previous analytical solution
developed for non-encased stone columns (Castro and Sagaseta
2009).
This paper analyses the main features of that closed-form
solution, showing its limitations and range of applicability, the
influence of the key parameters for routine design and a
comparison with numerical analyses.
2 CLOSED-FORM SOLUTION
2.1
Model
The vertical capacity of the columns is a fundamental issue
when the applied load is concentrated on the columns.
Therefore, column encasement is very useful in those cases
(Murugesan and Rajagopal 2010, Khabbazian et al. 2010); but
also under distributed loads, such as tanks or embankments,
because the increase of lateral confinement reduces the
settlement.
The authors' closed-form solution (Castro and Sagaseta
2011) is limited to distributed uniform loads because it is based
on a “unit cell” model, i.e. only one column and its surrounding
soil are studied in axial symmetry. Furthermore, the column is
assumed to be fully penetrating in the soft soil and the applied
load is considered as rigid, i.e. uniform settlement. The area of
soft soil,
A
l
, that is improved by each column,
A
c
, is generally
expressed by the area replacement ratio,
a
r
=
A
c
/
A
l
, but
sometimes is also defined in terms of the relation between
diameters or radii,
N
=
r
l
/
r
c
=1/√
a
r
.
The solution is developed for a horizontal slice at a depth
z
of the unit cell, and consequently, shear stresses between slices
at different depths are not considered (Figure 1). The overall
