Pierre Delage

The relationship between suction and swelling properties in a heavily compacted unsaturated clay

In order to study the hydro-mechanical behaviour of highly compacted unsaturated clays used in engineered clay barriers for nuclear waste disposal, some experimental techniques for controlling suction were extended into the range of high suctions. The technique of control by imposing a given relative humidity was considered for very high suctions (several hundreds of MPa), and calibration data from the literature were examined. Some corrected values were proposed, based on a calibration made with sodium chloride. The extension of the osmotic technique to higher suctions was made, and a value of 10 MPa was attained, enlarging considerably the range of this method. The study of the water retention and swelling properties of the FoCa7 clay under controlled suction and zero applied stress is presented. A good continuity between the two methods of suction control was observed. Fairly reversible responses to suction cycles were observed, in terms of water content and volume change. During these changes, it was observed that the air volume remained constant. The reversibility is related to the predominant role of a saturated microstructural level, strongly influenced by the physico-chemical bonds existing between water and the active clay minerals.

Temperature effects on the volume change behaviour of Boom clay

In this paper, an experimental study intended (1) to identify the effect of temperature on the preconsolidation pressure of Boom clay and (2) the effect of the over consolidation ratio (OCR) on the thermal volume changes is presented. In this study, OCR is defined in terms of isotropic stresses instead of the commonly used vertical stresses in soil mechanics. Tests were carried out in a temperature-controlled isotropic compression cells able to sustain high pressures. Based on the experimental results obtained, existing thermoplastic models are examined. In order to fully account for the effects of OCR on the volume change behaviour, an additional plastic mechanism is proposed.

A model for the volume change behavior of heavily compacted swelling clays

Abstract Engineered clay barriers used for isolating nuclear waste disposal at great depth are made of dense compacted swelling clays. Hydration tests carried out on the French FoCa7 clay showed some limitations of existing elastoplastic models for swelling soils, which are developed for looser swelling soils [Canadian Geotechnical Journal 29 (1992) 1013]. In dense compacted swelling clays, wetting–drying and loading–unloading tests showed a reversible volume change behavior, related to the absence of collapsible macropores. Based on experimental results, an elastic non-linear model is developed. The concept of critical swelling curve (CSC) is introduced, and constitutes the base of the model. This CSC curve and the model developed account for the couplings existing between hydraulic and mechanical effects, and provide satisfactory predictions of the volume change behavior of heavily compacted swelling clays.

Determining the unsaturated hydraulic conductivity of a compacted sand-bentonite mixture under constant-volume and free-swell conditions

Abstract Highly compacted sand–bentonite mixtures are often considered as possible engineered barriers in deep high-level radioactive waste disposals. In situ, the saturation of these barriers from their initially unsaturated state is a complex hydro-mechanical coupled process in which temperature effects also play a role. The key parameter of this process is the unsaturated hydraulic conductivity of the barrier. In this paper, isothermal infiltration experiments were conducted to determine the unsaturated hydraulic conductivity according to the instantaneous profile method. To do so, total suction changes were monitored at different locations along the soil specimen by using resistive relative humidity probes. Three constant-volume infiltration tests were conducted showing, unexpectedly, a decrease of the hydraulic conductivity during infiltration. One test performed under free-swell conditions showed the opposite and standard trend. These observations were interpreted in terms of microstructure changes during wetting, both under constant-volume and free-swell conditions.

Investigating the time-dependent behaviour of Boom clay under thermomechanical loading

Boom clay, a stiff clay, has been selected as a potential host formation for the geological disposal of radioactive waste in Belgium. The underground research facility HADES has been constructed to enable various in situ experiments to be performed on Boom clay so as to study the feasibility of high-level radioactive waste disposal, and to provide reliable data on the performance of Boom clay as a host formation. Among the various laboratory studies performed on samples extracted from the HADES facility to investigate the thermo-hydro-mechanical behaviour of Boom clay, relatively few were devoted to the time-dependent behaviour, limiting any relevant analysis of the long-term behaviour of the disposal facility. The present work aims at investigating the time-dependent behaviour of Boom clay under both thermal and mechanical loading. High-pressure triaxial tests at controlled temperatures were carried out for this purpose. The tests started with constant-rate thermal and/or mechanical consolidation and end...

A FORMULATION OF FULLY COUPLED THERMAL–HYDRAULIC–MECHANICAL BEHAVIOUR OF SATURATED POROUS MEDIA—NUMERICAL APPROACH

The theoretical aspects of fully coupled thermohydromechanical behaviour of saturated porous media are presented. The non-linear behaviour of soil skeleton is assumed. A new concept called ‘thermal void ratio state surface’ is introduced to include thermal effects, and the stress state level influence on volume changes. The fluid phase flows according to Darcys law and energy transport is assumed to follow Fouriers law classically. Variation of water permeability, water and solid unit weight due to thermal effects and pore pressure changes are included. A finite element package is developed based on final matrix form obtained from discretization of integral form of field equations by finite element method and integration in time. A very good agreement between the theoretical predictions and the experimental results was obtained for the several simple problems proposed by other authors.

An evaluation of the osmotic method of controlling suction

Experimental techniques of testing the mechanical properties of unsaturated soils are complex and difficult to conduct. As a consequence, complete sets of parameters that characterise the behaviour of unsaturated soils remain scarce and necessary. In this context, it has been found useful to gather the information obtained after some years of practice of the osmotic technique of controlling suction. As compared to the more documented axis-translation technique, the osmotic technique has its own advantages and drawbacks that are discussed in this paper, together with some potential future developments. The osmotic method has been developed by soil scientists in the 1960s and adapted to geotechnical testing in the early 1970s. This paper presents the osmotic technique and comments on its advantages (including suction condition close to reality and higher suctions easily attained) and drawbacks (including some concern with the membrane resistance and some membrane effects in the suction/concentration calibration). Various applications to geotechnical testing are presented such as the determination of the water retention curve, oedometer and triaxial testing procedures and the determination of the permeability of unsaturated soils. Recent developments, that include the extension of the method up to high suctions (10 MPa) are also described, together with some recent and novel applications such as data from high controlled suction oedometer compression test and the determination of the oil/water retention properties of oil reservoir chalks.

Clays in radioactive waste disposal

Clays and argillites are considered in some countries as possible host rocks for nuclear waste disposal at great depth. The use of compacted swelling clays as engineered barriers is also considered within the framework of the multi-barrier concept. In relation to these concepts, various research programs have been conducted to assess the thermo-hydro-mechanical properties of radioactive waste disposal at great depth. After introducing the concepts of waste isolation developed in Belgium, France and Switzerland, the paper describes the retention and transfer properties of engineered barriers made up of compacted swelling clays in relation to microstructure features. Some features of the thermo-mechanical behaviors of three possible geological barriers, namely Boom clay (Belgium), Callovo-Oxfordian clay (France) and Opalinus clay (Switzerland), are then described, including the retention and transfer properties, volume change behavior, shear strength and thermal aspects.

Udam: A powerful finite element software for the analysis of unsaturated porous media

Abstract In this paper, a brief review of the basic points of the infinitesimal deformation theory of unsaturated porous media is presented, including the main modifications due to nonlinear and suction-dependent behaviour. Spatial discretization of the field equations carried out by the finite element method and the dynamic integration in time domain are described, as well as the required formulae. The developed graphic software for the pre- and postprocessing steps of the finite element analysis runs on personal computers and it is also described. An illustrative numerical example of an earth dam analysis is presented and discussed.

Consequences of the Thermal Transient on the Evolution of the Damaged Zone Around a Repository for Heat-Emitting High-Level Radioactive Waste in a Clay Formation: a Performance Assessment Perspective

A proper evaluation of the perturbations of the host rock induced by the excavation and the emplacement of exothermic wastes is essential for the assessment of the long-term safety of high-level radioactive waste disposals in clay formations. The impact of the thermal transient on the evolution of the damaged zone (DZ) has been explored in the European Commission project TIMODAZ (thermal impact on the damaged zone around a radioactive waste disposal in clay host rocks, 2006–2010). This paper integrates the scientific results of the TIMODAZ project from a performance assessment (PA) point of view, showing how these results support and justify key PA assumptions and the values of PA model parameters. This paper also contextualises the significance of the thermal impact on the DZ from a safety case perspective, highlighting how the project outcomes result into an improved understanding of the thermo–hydro–mechanical behaviour of the clay host rocks. The results obtained in the TIMODAZ project strengthen the assessment basis of the safety evaluation of the current repository designs. There was no evidence throughout the TIMODAZ experimental observations of a temperature-induced additional opening of fractures nor of a significant permeability increase of the DZ. Instead, thermally induced plasticity, swelling and creep seem to be beneficial to the sealing of fractures and to the recovery of a very low permeability in the DZ, close to that of an undisturbed clay host rock. Results from the TIMODAZ project indicate that the favourable properties of the clay host rock, which guarantee the effectiveness of the safety functions of the repository system, are expected to be maintained after the heating–cooling cycle. Hence, the basic assumptions usually made in PA calculations so far are expected to remain valid, and the performance of the system should not be affected in a negative way by the thermal evolution of the DZ around a radioactive waste repository in clay host rock.