Experimental investigation on the creep behavior of an unsaturated clay

Abstract

To better understand the long-term deformation of landslides with consideration of the influence of water content variation, a series of triaxial creep tests with suction control was conducted on clay specimens taken from one large-scale landslide in the Three Gorges Reservoir area in China. Results indicate that, in the double-logarithmic coordinates, the axial strain increases linearly and the axial strain-rate decreases linearly with the elapsed time; the axial strain and strain rate increase with increasing deviator stress levels and decreasing matric suction. For theoretical analysis, based on the simulation of the test results by an empirical creep model developed for saturated soils, a linear relationship was established between suction and one of the parameters of the model. Then, a revised model with consideration of suction effects was developed. In the revised model, a power function was adopted for the description of the strain–time relationship and a hyperbolic function was employed for the stress–strain relationship. Verification indicated that the calculated results were in good agreement with the experimental ones.

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ARTICLE
Experimental investigation on the creep behavior of an
unsaturated clay
X.L. Lai, S.M. Wang, W.M. Ye, and Y.J. Cui
Abstract: To better understand the long-term deformation of landslides with consideration of the influence of water content
variation, a series of triaxial creep tests with suction control was conducted on clay specimens taken from one large-scale
landslide in the Three Gorges Reservoir area in China. Results indicate that, in the double-logarithmic coordinates, the axial
strain increases linearly and the axial strain-rate decreases linearly with the elapsed time; the axial strain and strain rate increase
with increasing deviator stress levels and decreasing matric suction. For theoretical analysis, based on the simulation of the test
results by an empirical creep model developed for saturated soils, a linear relationship was established between suction and one
of the parameters of the model. Then, a revised model with consideration of suction effects was developed. In the revised model,
a power function was adopted for the description of the strain–time relationship and a hyperbolic function was employed for the
stress–strain relationship. Verification indicated that the calculated results were in good agreement with the experimental ones.
Key words: clay, creep, triaxial tests, suction, unsaturated soils, creep model.
Résumé : Afin de mieux comprendre la déformation a
`long terme de glissements de terrain en lien avec l’influence des variations
de la teneur en eau, une série d’essais de fluage triaxiaux avec contrôle de la succion ont été réalisés sur des échantillons d’argile
provenant d’un grand glissement de terrain dans la région du réservoir des Trois Gorges en Chine. Les résultats indiquent que,
dans des coordonnées en double logarithme, la déformation axiale augmente linéairement et le taux de déformation axiale
diminue linéairement selon le temps écoulé; de même que la déformation axiale et le taux de déformation axiale augmentent
lorsque les niveaux de contraintes déviatoires augmentent et que la succion matricielle diminue. Pour l’analyse théorique, basée
sur la simulation des résultats des essais avec un modèle empirique de fluage développé pour des sols saturés, une relation
linéaire a été établie entre la succion et un des paramètres du modèle. Ensuite, un modèle révisé qui considère les effets de la
succion a été développé. Dans le modèle révisé, une fonction de puissance a été adoptée pour décrire la relation déformation–
temps et une fonction hyperbolique a été utilisée pour la relation contrainte–déformation. Des vérifications ont indiqué que les
résultats calculés correspondent bien aux résultats expérimentaux. [Traduit par la Rédaction]
Mots-clés : argile, fluage, essais triaxiaux, succion, sols non saturés, modèle de fluage.
Introduction
The time dependent stress–strain behavior of soils has been
actively investigated in the past decades in areas such as civil,
petroleum, and mining engineering. It is generally recognized
that the time dependency of clay is too significant to be ignored
(Bjerrum 1967;Vaid and Campanella 1977;Graham et al. 1983;Kim
and Leroueil 2001;Leroueil 2006).
Time effects are usually associated with creep, i.e., deformation
behavior under constant effective stresses. As summarized by
Augustesen et al. (2004) and Liingaard et al. (2004), a large number
of creep tests of soils have been conducted since the 1950s and a
great number of constitutive models have been proposed based
on the test results (Suklje 1957;Bjerrum 1967;Singh and Mitchell
1968;Mesri and Godlewski 1977;Vaid and Campanella 1977;Mesri
et al. 1981;Mesri and Castro 1987;Yin 1999;Yin and Graham 1999).
Generally, these models can be classified into three categories:
empirical models, rheological models, and general stress–strain–
time models. The empirical models are mainly obtained by fitting
experimental results from creep tests. Therefore, they only can be
applied to problems with specific boundary and loading condi-
tions. However, these models are usually simple in form. In con-
trast, they provide a basis for developing three-dimensional
constitutive models. Among the empirical models, the semi-
logarithmic functions in log (time) are usually adopted to describe
the creep behavior associated with one-dimensional compression
in oedometer tests (Mesri and Godlewski 1977;Yin 1999); the Singh
and Mitchell’s creep model (Singh and Mitchell 1968) and the
model developed by Mesri et al. (1981) are generally employed to
simulate the creep behavior in triaxial creep tests.
However, most of the creep tests in the literature have been
carried out on saturated materials and only few studies have fo-
cused on the creep behavior of unsaturated soils. In fact, it is of
significant practical interests to investigate the time dependency
of volume change behavior of unsaturated geomaterials (De
Gennaro et al. 2003,2009;Schwarz et al. 2006;Oldecop and
Alonso 2007;Priol et al. 2007;Deng et al. 2011). Deng and Tatsuoka
(2004,2007) observed significant viscous property in unsaturated
compacted clay. To interpret the time-dependent subsidence of
Received 16 February 2013. Accepted 3 February 2014.
X.L. Lai. Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Tongji University, Shanghai 200092, China.
S.M. Wang. Key Laboratory of Geological Hazards in Three Gorges Reservoir Area of Education Ministry, Three Gorges University, Yichang, Hubei, China.
W.M. Ye. Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Tongji University, Shanghai 200092, China; United
Research Center for Urban Environment and Sustainable Development, the Ministry of Education, Shanghai 200092, China.
Y.J. Cui. Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Tongji University, Shanghai 200092, China; Ecole des
Ponts ParisTech, UR Navier/CERMES, France.
Corresponding author: Weimin Ye (e-mail: ye_tju@tongji.edu.cn).
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Can. Geotech. J. 51: 621–628 (2014) dx.doi.org/10.1139/cgj-2013-0064 Published at www.nrcresearchpress.com/cgj on 5 March 2014.