Jean Vaunat

Modelling the mechanical behaviour of expansive clays

A simple formalism is presented to model the behaviour of expansive clays. Two levels of structure are considered. The behaviour of the macrostructure follows the model developed for unsaturated materials by Alonso et al. (Geotechnique 40 (3) (1990) 405-430). The behaviour of the microstructure is adapted from the work of Gens and Alonso (Can. Geotech. J. 29 (1992) 1013-1032) in order to include the possibility of the micropores being partially saturated. Mechanical coupling between both levels of structure are defined through two functions, one for wetting and the other for drying. They express the change in macrostructural void ratio due to a change in microstructural void ratio, and their value depends on the state of compaction of the macrostructure. The general shape of these curves is discussed on the basis of experimental evidence. Phenomena such as the dependency of strain on stress- suction path, accumulation of expansion strain during suction cycles at low confining stress, accumulation of compression strain during suction cycles at high confining stress, strain fatigue during drying-wetting cycles, macropore invasion by expanded microstructure and development of macroporosity during strong drying can be represented. A mathematical formulation of the model is described and its performance finally assessed by comparison with laboratory tests.

A stress point algorithm for an elastoplastic model in unsaturated soils

Abstract Two stress fields, combination of total stresses, liquid pressure and gas pressure have to be considered to explain the deformational behaviour of unsaturated media. Elastoplastic models developed for these materials consider generally two yield surfaces, each one associated to a stress field, and whose intersection produces a corner in the space of generalised stress components. In this paper, a stress point algorithm is proposed to cope with the integration of such constitutive laws, which can be seen as non smooth multisurface plastic models in the space of the two stress fields. The basic model developed by Alonso et al. (Alonso, E.E., Gens, A., 1990. A constitutive model for partially saturated soils. Geotechnique 40 (3), 405–430), which will be used to test the algorithm, is first described. Generalised stress and strain variables are then defined. Implementation of the return mapping algorithm, based on an implicit integration scheme, is presented with special attention devoted to the problem of mixed control imposed by the F.E. formulation generally used to analyse the hydromechanical behaviour of unsaturated media. Validation results on distinct generalised stress paths are given at the end.

Analysis of post-failure slope movements within the framework of hazard and risk analysis

The paper first recalls briefly a methodological framework to assess landslide hazard and risk analysis in terms of predisposition, triggering andrevealing factors. This framework, that reflects the mechanisms involved in the landslide, is based on the Geotechnical Characterisation of slope movements proposed by Vaunat et al. (1994) and Leroueil et al. (1996). The Geotechnical Characterisation can be schematized by a 3-D matrix having the following axes: types of movement; types ofmaterial; and the four stages of movement: pre-failure, failure, post-failure andreactivation. For each relevant element of this 3-D matrix, there is a characterisationsheet including: the controlling laws and parameters, the predisposition factors, thetriggering or aggravating factors, the revealing factors and the consequences of the movement. The paper focuses afterwards on the post-failure stage, which generallyis the most destructive, and on the mobility index. It is shown that this laterindex can be described as the product of sub-indices associated with failure, brittlenessof the material, ability of the soil to develop pore pressures, geometry of the moving soil mass and characteristics of the terrain. It is also shown how these aspectscan be incorporated into the Geotechnical characterisation of slope movements. This seems to provide a rational basis for examining slope movements at the post-failure stage and assessing associated risks.

Thermal Conductivity of Argillaceous Rocks: Determination Methodology Using In Situ Heating Tests

This study focuses on the characterisation of thermal conductivity for three potential host rocks for radioactive waste disposal. First, the heat conduction process is reviewed on the basis of an analytical solution and key aspects related to anisotropic conduction are discussed. Then the existing information on the three rocks is summarised and a broad uncertainty range of thermal conductivity is estimated based on the mineralogical composition. Procedures to backanalyse the thermal conductivity on the basis of in situ heating tests are assessed and a methodology is put forward. Finally, this methodology is used to estimate the impact of experimental uncertainties and applied to the four in situ heating tests. In the three potential host rocks, a clear influence of the bedding planes was identified and anisotropic heat conduction was shown to be necessary to interpret the observed temperature field. Experimental uncertainties were also shown to induce a larger uncertainty on the anisotropy ratio than on the equivalent thermal conductivity defined as the geometric mean of the thermal conductivity in the three principal directions.

An Adapted Ring Shear Apparatus for Testing Partly Saturated Soils in the High Suction Range

The paper presents an adapted experimental setup to study the residual shear strength properties of clays under unsaturated state conditions. This issue is of geotechnical concern in stability analysis of slopes in fissured rocks with unsaturated clayey fills and also, in a more fundamental way, as an indicator of microstructural changes of clays undergoing drying paths. To this end, a commercial ring shear apparatus (Bromhead type) was instrumented and adapted to control total suction in a wide range by using vapor transfer technique. In addition, by using simulation-aided techniques, the time evolution of suction at local state and axial deformation was studied to overcome the experimental difficulty associated with the determination of the suction equalization time. The simulated equalization period agreed well with the experimental information based on a mechanical stabilization criterion (time required to attain vertical strain rates lower than 0.1%/day). Relevant test results showing the main capabilities of the adapted apparatus are presented, which bring up the important changes observed in residual friction angle when testing a clayey soil at different hydraulic states (total suctions ranging from nearly 0 up to 140 MPa).

COUPLED ANALYSIS OF A BACKFILL HYDRATION TEST

SUMMARY BACCHUS2 in situ isothermal wetting experiment has been analysed by means of a coupled flowdeformation approach. Backfill material, a mixture of Boom clay powder and high density pellets, has been extensively tested in the laboratory in order to determine its hydraulic and mechanical properties. Parameters of constitutive equations were derived from this experimental data base. Two mechanical constitutive models have been used in the simulation of the ‘in situ’ experiment: a state surface approach and an elastoplastic model. Calculations have shown several features of the hydration process which help to understand the behaviour of expansive clay barriers. Predictions using both models have been compared with each other and with actual measurement records. This has allowed a discussion of the comparative mertis of both approaches and the identification of some critical parameters of backfill behaviour. Overall agreement between calculations and field measurements is encouraging and shows the potential of the methods developed to model the behaviour of engineered clay barriers in the context of nuclear waste disposal. ( 1998 by John Wiley & Sons, Ltd.

Numerical modelling of slope–vegetation–atmosphere interaction: An overview

The behaviour of natural and artificial slopes is controlled by their thermo-hydro-mechanical conditions and by soil–vegetation–atmosphere interaction. Porewater pressure changes within a slope related to variable meteorological settings have been shown to be able to induce soil erosion, shrinkage–swelling and cracking, thus leading to an overall decrease of the available soil strength with depth and, ultimately, to a progressive slope collapse. In terms of numerical modelling, the stability analysis of partially saturated slopes is a complex problem and a wide range of approaches from simple limit equilibrium solutions to advanced numerical analyses have been proposed in the literature. The more advanced approaches, although more rigorous, require input data such as the soil water retention curve and the hydraulic conductivity function, which are difficult to obtain in some cases. The quantification of the effects of future climate scenarios represents an additional challenge in forecasting slope–atmosphere interaction processes. This paper presents a review of real and ideal case histories regarding the numerical analysis of natural and artificial slopes subjected to different types of climatic perturbations. The limits and benefits of the different numerical approaches adopted are discussed and some general modelling recommendations are addressed.

Residual Strength of Clays at High Suctions

This paper presents a study on the residual shear strength of a plastic clay (Boom clay) under high suction. The device, an adaptation of Bromhead ring shear apparatus to allow for suction control during shearing, is first described and the experimental procedure detailed. Then, results of shear strength measured under a suction equal to 70 MPa, several vertical stresses and different shearing history (first-shearing and shearing on pre-sheared samples) are presented and discussed. They evidence a huge increase in the residual friction angle whose value is

Applications of multiphysical geomechanics in underground nuclear waste storage

15^ˆ

Suction effects on the residual shear strength of clays

greater than that measured in saturated conditions and a highly dilatant response during first-shearing. ESEM micrographs performed on samples sheared in saturated and unsaturated conditions suggest that such a response can be explained by the character more granular of the dry material as a result of particle aggregation. Comparison with results obtained by \cite{JV:17} on a low-plastic clay indicate that this effect is enhanced by the plasticity of the clay.