Journal of Geo-Engineering Sciences

The mechanical behaviour of natural clays is significantly affected by their in situ or initial structure in the form of cementation or interparticle bonding. This behaviour can differ substantially from the behaviour of reconstituted clays. Suction as well as plastic volumetric strains drive isotropic hardening/softening as this is a simple way to account for the phenomenon of volumetric collapse upon wetting and the stiffening effect that suction has on the soil skeletal response. A model that combines unsaturated and structured behavior is presented and then used to simulate stress strain behaviour observed for an unsaturated natural clay subjected to isotropic load paths. A parametric analysis is performed to observe the influence suction hardening has on mobilized strengths. It is also shown that the model can predict the maximum of collapse of unsaturated soils.

The paper presents an experimental study on the residual shear strength of different clayey soils at varying total suctions. In a first part of the paper some insight into the behaviour of residual shear strength of a filled rock discontinuity is presented to bring into light the sensitivity to hydraulic states of a clayey gouge material subjected to shearing along large displacements. An adapted Bromhead ring shear apparatus, in which the sample is enclosed in a controlled relative humidity chamber, has been used to control total suction during shearing. Afterwards, results on different types of clayey materials are presented to discuss the importance of the plasticity of the clay on the residual friction angle changes. These changes show an important increase in the residual friction angle with imposed total suction that may reach 15° for medium plastic clay and without cohesion component. The reasons for this increase have been explained by the more granular character of the dry clay as a result of aggregate densification and desaturation during strong drying. The changes have been interpreted using a simple model inspired in both macroscopic and microstructural observations, which is based on the stiffness of the soil (or aggregates) and depends on the product of the total suction applied and the degree of saturation. A microstructural model, already developed to take into account microstructural aspects on water retention and following an equivalent behavioural response to the macroscopic shrinkage curve, has been used to estimate the degree of saturation at the microstructural level (inside aggregates). To highlight its applicability in geotechnical practice, the different parameters used in the water retention and residual shear strength models have been shown to depend on the plasticity index of the different clayey soils studied.

The paper reviews a series geomechanical approaches for interpreting instabilities in unsaturated geomaterials, a class of solids involved in numerous geo-engineering problems, such as hazard forecasting, infrastructure management, underground disposal of by-products and energy technologies. The review details the connection between second-order work input, loss of controllability and material failure. Hydro-mechanical problems are addressed, focusing on a specific class of environmental perturbations that can cause sharp changes in both mechanical and hydrologic variables. A procedure to define the second-order work input to an unsaturated soil volume is discussed first. It is pointed out that this energy measure motivates the incremental form of the constitutive laws for stability analyses. It is then discussed how to link the theory of hydro-mechanical controllability with constitutive approaches for unsaturated soils, which are typically based on the framework of strain-hardening plasticity with extended hardening. In doing so, the crucial role of the properties that govern the interactions between soil skeleton and fluid-retention processes is emphasized. The implications of these findings are commented with reference to a specific application: the forecasting of landslide triggering in natural slopes. It is shown that the use of suitable stability indices allows one to differentiate between frictional slips and volumetric collapses turning into flows. These results suggest that geomechanical theories calibrated for site-specific properties can support the quantitative assessment of landslide susceptibility, as well as a number of other engineering applications involving the instability of unsaturated porous media.

Coupled flow-deformation analysis of consolidation in unsaturated lumpy clays is presented. The governing differential equations are discretised using the Galerkin method and the finite difference technique for space and time, respectively. Particular attention is given to the cross coupling coefficients arising from the pore scale deformation compatibility between micro and macro voids. Two examples are analysed: a one-dimensional soil column, and a strip footing acting on a two-dimensional medium of lumpy clay. A range of sensitivity analyses are preformed using different values of degree of saturation and volume fraction of macro voids. All results are carefully analysed and salient features of consolation in lumpy clays are highlighted. The significance of the cross coupling coefficients in the numerical results obtained are also examined.

The use of centrifuge modeling where unsaturated soils are involved is more limited than its use in problems involving dry sand or saturated clays. This limitation is certainly due to the well-known experimental complexities related with unsaturated soils that increase in centrifuge modeling; on the other hand, few data is available about the scaling laws for unsaturated soils. In this paper, physical modeling of unsaturated soil problems in centrifuge is evaluated considering some phenomena involved in the behavior of those materials such as water migration, expansion and collapse. An overview of the scaling laws that has to be used is presented, with a selection of geotechnical problems studied on small-scale models in centrifuge.