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Geoencased columns: toward a displacement based design
Colonnes renforcée par géotextiles: vers une conception basée sur le déplacement
Galli A., Prisco di C.
Politecnico di Milano
ABSTRACT: As is largely testified by the scientific literature, in the last decade geoencased columns have become a quite common
alternative solution to standard stone columns. This is essentially due to the possibility of employing reinforcements to better the
mechanical response of the inclusions without reducing their drainage efficiency. Although GEC are often used to reduce settlements
induced by the construction of large embankments on soft soils, up to now a rational displacement based design approach has not yet
been introduced. This is thus the final objective of this paper, that, by starting from a critical review of the standards presently
available, will illustrate the results of a series of finite difference numerical analyses. The unit cell of an ideal reinforced soil
embankment placed on a soft soil stratum will be accounted for and the effect of the main geometrical/mechanical parameters, as well
as the response of the system during the construction stages, is discussed.
RÉSUMÉ : Comme il est largement connu dans la littérature de ces vingt dernières années, les colonnes en matériaux granulaires
renforcée par géotextiles (GEC) sont devenues une solution très utilisée par rapport aux colonnes ballastées standard. Cela est
essentiellement dû à la possibilité d'employer des renforts pour améliorer la réponse mécanique des inclusions sans réduire leur
efficacité de drainage. Bien que les GEC soient souvent utilisées pour réduire les tassements induits par la construction de remblais
importants sur sols mous, une approche rationnelle de conception basée sur le déplacement n'a, jusqu'à présent, pas encore été mise en
place. Cela est donc l'objectif final de cette étude, qui, en partant d'une analyse critique des normes actuellement disponibles, illustrera
les résultats d’analyses numériques aux différences finies. Une cellule élémentaire d'un remblai idéal de sol renforcé placé sur un sol
mou sera prise en compte et l'effet des principaux paramètres géométriques et mécaniques et la réponse du système au cours des
différentes étapes de la construction seront discutés.
KEYWORDS: geoencased granular columns, geotextiles, numerical analyses, displacement based design, earth reinforced structures.
1 INTRODUCTION.
As it is well documented in the literature (see e.g. Raithel et
al. 2005), since mid-nineties the use of geoencased granular
columns (GEC) as foundations of earth structures on soft and
very soft soils has been progressively increased. GECs have
both mechanical and hydraulic functions: they work not only as
reinforcement inclusions, capable of preventing the global
collapse of the foundation and reducing differential settlements
within the structure, but they work additionally as vertical
drains, thus reducing the consolidation time of the soft soil.
Figure 1. sketch of an earth embankment on GECs.
The GEC foundation system is composed of an array of
granular columns of length L and diameter D, placed at a
regular spacing S below an embankment of height H (Figure 1).
The columns are encased by a geotextile with the double aim of
reinforcing the column and filtering to prevent the clogging of
the column itself. At the base of the embankment, to redistribute
vertical stresses, several layers of geotextile are also inserted
during the construction. The effectiveness of this foundation
system has been clearly proved both on real scale data (see e.g.
Kempfert 2003, where the response of the system is analyzed
by varying the spacing among columns and of the stiffness of
the encasing geotextile), and by means of numerical and
experimental researches (Murugesan and Rajagopal 2006, di
Prisco and Galli 2011). The fundamentals of the mechanical
behavior of the system is therefore quite well understood.
Nevertheless, common design standards are still based on too
simplified approaches, unable of capturing the actual
mechanical complexity of the system in particular, the
interaction between embankment and columns (this point will
be tackled in further details in the following section by critically
reviewing the most used design standards). Conversely to
traditional deep foundation systems (like reinforced concrete
piles or jet-grouted columns which can be considered axially
rigid with respect to the surrounding soil), GECs are axially
deformable inclusions, whose axial deformability is strictly
coupled with the stiffness of the surrounding soil. Moreover,
since this latter is very often characterized by a very low
permeability and high deformability, its mechanical response
should be modeled by properly taking into account the hydro-
mechanical coupling (for the sake ofbrevity, this aspect will
however be disregarded in the following).
In the present paper the attention will be initially focused on
a critical review of the most common design standards. Then an
engineering displacement based approach will be briefly
introduced, and some numerical analyses, with the particular
aim of studying the distribution of differential settlement at the
top of the embankment, which are generally neglected by the
design approaches, will be presented.
