A framework for unsaturated soil behaviour

Abstract

A framework is put forward which explains the shear behaviour of an unsaturated soil in terms of the total stresses and suctions acting within the soil. It is based on the critical state model for saturated soils, but incorporates the additional variables which are needed to formulate unsaturated soil behaviour. The effects of total stress and suction are considered separately, thereby avoiding the dangers of treating the two stress components as equivalent. The importance of the soil fabric is particularly emphasized, as in unsaturated soils the initial fabric is not destroyed by shearing to large strains. The contributions of total stress and suction to shear strength can be expressed as two individual stress ratios, both of which are heavily dependent on the degree of saturation. No unique void ratio condition holds for the critical state in unsaturated soils since the initial fabric is not destroyed by shear. However, a consistent set of laws can be defined, for a compacted soil, which represents possible states in which the soil can exist at ultimate conditions. L'article présente un cadre pour expliquer le comportement au cisaillement d'un sol non-saturé sous l'action de l'ensemble des contraintes et des succions agissant à l'intérieur du sol. Il est basé sur le modèle de l'état critique pour des sols saturés mais il inclut les variables supplémentaires qui sont nécessaires pour caracteriser le comportement des sols non-saturés. Les effets des contraintes et des succions sont considérés séparément pour éviter les risques encourus en traitant ces deux constituants de la déformation comme équivalents. On accentue l'importance de la structure du sol, car dans des sols non-saturés la structure initiale n'est pas détruite par le cisaillement jusqu'à des déformations élevées. Le rôle joué par l'ensemble des contraintes et des succions dans la résistance au cisaillement peut s'exprimer dans la forme de deux rapports séparés de contrainte qui dépendent beaucoup du degré de saturation. Il n'existe pas d' indice des vides unique pour l'etat critique dans des sols non-saturés, puisque la structure initiale n'est pas détruite par le cisaillement. Dans le cas d'un sol compact il est du reste possible de définir un groupe cohérent de lois représentant les états possible dans lesquels le sol peut exister à des conditions limites.

An elasto-plastic model for unsaturated soil incorporating the effects of suction and degree of saturation on mechanical behaviour

“…The first part of this section analyses the data from isotropic virgin compression tests at constant suction (Sivakumar, 1993;Sharma, 1998). At the end of the section, the analysis of further experimental data from triaxial shear tests on compacted Speswhite kaolin (Sivakumar, 1993;Wheeler & Sivakumar, 2000) and on compacted Kiunyu gravel (Toll, 1990) demonstrates that the conclusion achieved for isotropic stress states can also be extended to non-isotropic stress states. Sivakumar (1993) and Sharma (1998) performed isotropic virgin compression of soil samples at different values of suction-100 kPa, 200 kPa and 300 kPa-as well as of saturated samples.…”

“…The validation of the assumption introduced in the previous section has involved the analysis of different sets of data from laboratory tests performed on compacted Speswhite kaolin (Sivakumar, 1993;Wheeler & Sivakumar, 2000), on a compacted mixture of bentonite and kaolin (Sharma, 1998), and on compacted Kiunyu gravel (Toll, 1990). The first part of this section analyses the data from isotropic virgin compression tests at constant suction (Sivakumar, 1993;Sharma, 1998).…”

“…Finally, the analysis of the data from undrained (with respect to the water phase) triaxial shear tests performed by Toll (1990) on compacted samples of a lateritic gravel from Kenya (Kiunyu gravel) is presented. Samples were compacted by Toll (1990) at different values of water content ranging from 17·0 to 27·7 and then sheared in axial compression to critical state while preventing the flow of water.…”

“…Samples were compacted by Toll (1990) at different values of water content ranging from 17·0 to 27·7 and then sheared in axial compression to critical state while preventing the flow of water. The author reports the measured values of the void ratio, degree of saturation, suction and net stress state at the end of the tests when the unsaturated samples have attained critical state conditions.…”

The strength of unsaturated bentonite-enhanced sand

“…More recently there has been signi®cant progress in describing the stress±strain behaviour of unsaturated ®ne-grained soil within a critical-state framework (e.g. Alonso et al, 1990;Toll, 1990;Wheeler & Sivakumar, 1995). Notably, Alonso et al (1990) proposed an elasto-plastic work-hardening constitutive model for unsaturated soils based on the stress variables of the mean net stress and suction.…”