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A simplified model for collapse using suction controlled tests
Un modèle simplifié d’effondrement, basée sur des essais de succion controlée
Vázquez M., Justo J.L., Durand P.
University of Seville, Seville, Spain
ABSTRACT: Alonso et al. (1990) have presented the most comprehensive theory for partly saturated soils. Their constitutive
equations present very complex formulations that rely on a large number of parameters, which are difficult to achieve unless advanced
laboratory tests are performed. This paper presents a simple model for predicting the oedometric collapse of soils compacted with low
density. The model has a minimum complexity, only needs two parameters, and establishes a linear relationship between log suction
and volume change for different vertical pressures, until the moment when suction reaches the field capacity; then volume change
remains at a constant value. This linear relationship is controlled by the Instability Index,
Ipt
. Suction controlled oedometer test have
been carried out, and the results agree with sufficient degree of accuracy with the proposed model.
RÉSUMÉ : Alonso et al. (1990) on présenté la théorie la plus avancée pour des sols partiellement saturés. Les équations constitutives
présentent des formulations très complexes qui dépendent d’un grand nombre de paramètres, qui sont difficiles à évaluer sans des
essais de laboratoire très avancés. Cet article présente un modèle simple pour prédire l’effondrement oedométrique du sol compacté
avec une faible densité. Le modèle a une complexité minimale, nécessite seulement deux paramètres, et établit une relation linéaire,
entre le logarithme de la succion et le changement de volume pour différentes pressions verticales, jusqu’au moment où la succion
atteint la capacité de champ ; à partir de ce moment le changement de volume resteconstante. La relation linéaire est contrôlée par
l’Indice d’Instabilité,
Ipt
. Les essais oedométriques avec succion contrôlée ont été réalisés, et les résultats sont en accord avec le
modèle proposé.
KEYWORDS: unsaturated soil, model, collapse, suction, oedometer.
1 INTRODUCTION.
Expansive and collapsing soils have, generally, the common
condition of being partly saturated.
In partially saturated soils with an open structure, the
increase in the degree of saturation resulting from
environmental or manmade changes can produce irrecoverable
volume reductions without any change in the external forces.
This phenomenon receives the name of collapse.
The first technical description of the collapse phenomenon
may be the one made by Terzaghi and Peck (1948) when they
describe the loss of strength and increase in compressibility of
loess upon saturation. The word
collapse
to designate this
phenomenon was already used by Jennings and Burland (1962).
In general terms, a soil will swell or collapse after being
flooded, depending upon whether the external pressure is
smaller or larger than the swelling pressure. That is the reason
why significant collapse may occur in a wide variety of open-
structure soils ranging from well-graded sand, gravel and
rockfill to plastic clay under high pressure, as long as the degree
of saturation is low enough (Justo and Saetersdal 1979). In these
soils and under these conditions, collapse is produced as suction
decreases. As the external pressure increases, collapse increases
up to a maximum; then the particles are so tight that any further
decrease in suction produces volume expansion (Booth 1975,
Yudhbir 1982, Maswoswe 1985).
During the wetting process in suction-controlled oedometric
tests, sometimes swelling has been followed by collapse
reached at very low suction values (Escario and Sáez 1973, Cox
1978, Alonso et al. 1987).
Oedometric cells similar to the ones described by Escario
(1969), and Escario and Sáez (1973) have been used to
investigate swelling, shrinkage and collapse under constant
vertical net stress, as well as the loading and unloading
behaviour under a constant matrix suction (Balmaceda 1991;
Yuk Gehling 1994; Vilar 1995 and Romero 1999).
In the deformational behaviour of partly saturated soils, the
soil is sometimes considered elastic and isotropic. Fredlund and
Morgenstern (1976), and Justo et al. (1984 a and b) use different
elastic moduli with respect to the external stresses and suction.
Justo and Saettersdal (1979) present an analysis of expansive
and collapsing soils and a revision of calculation methods,
including the elastic methods.
Alonso et al. (1987) analyse the volumetric deformations of
these soils in the space of net stress and suction. Other authors
have continued in this line, generating models that agree with a
good approximation to the behaviour of partly saturated, non
expansive soils (Josa et al. 1992, Cui et al. 1995, Wheeler and
Sivakumar 1995, Habibagahi and Mokhberi 1998, Sheng et al.
2004).
The lattest tendencies in the study of partly saturated soils
are addressed to coupling in the same model the expansive and
collapsing behaviour of soils (v. Justo and Saetersdal 1979),
generating the so called consistent models (Li and Fang 2011).
Along this line, it is proposed in this paper a model for open
structure collapsing soils based upon two parameters obtained
from the relationship between the volumetric deformation
(under oedometric conditions), the suction and the vertical
pressure.
From the results obtained, it will be observed that the model
describes, with sufficient precission, the behaviour of a
collapsing mixture of clay, when subject to oedometric
conditions, and to a wetting stress path under constant vertical
stress.
