A thermomechanical model for saturated clays is proposed within the framework of recent extensions of the Cam-Clay model. The results of some tests found in the literature are analyzed, and the main features of the thermomechanical behaviour of clays are identified. The effect of the overconsolidation ratio (OCR) on the volume change of a soil (expansion-contraction) submitted to heating is well established using experimental data obtained for selected soils by various authors. However, existing models need to be modified to correctly model this feature. For this reason, a new volumetric thermal plastic mechanism is developed that allows for the prediction of plastic strains at higher OCR values. The overconsolidation effect observed when heating a normally consolidated soil is also modelled. Particular attention is paid to the coupling and hardening phenomena related to the combined effects of stress and temperature. A qualitative validation is made by examining the response of the model under a given thermomechanical path. Comparison with existing thermomechanical experimental results shows that the model can provide satisfactory predictions.Key words: clays, constitutive modelling, temperature effects, deformation, elastoplasticity, radioactive waste disposal.

: When a mass of saturated clay is heated, as in the case of host soils surrounding nuclear waste disposals at great depth, the thermal expansion of the constituents generates excess pore pressures. The mass of clay is submitted to gradients of pore pressure and temperature, to hydraulic and thermal flows, and to changes in its mechanical properties. In this work, some of these aspects were experimentally studied in the case of Boom clay, so as to help predicting the response of the soil, in relation with investigations made in the Belgian underground laboratory at Mol. Results of slow heating tests with careful volume change measurements showed that a reasonable prediction of the thermal expansion of the clay-water system was obtained by using the thermal properties of free water. In spite of the density of Boom clay, no significant effect of water adsorption was observed. The thermal consolidation of Boom clay was studied through fast heating tests. A simple analysis shows that the hydraulic and thermal transfers are uncoupled. Experimental results from fast heating tests showed that the consolidation coefficient does not change significantly with increased temperature, due to the opposite effect of increasing permeability and decreasing porosity. The changes of permeability with temperature were investigated by running constant head measurements at various temperatures. An indirect analysis, based on the estimation of the m v consolidation parameter, showed that the indirect method of estimating the permeability from consolidation tests should be considered carefully. Intrinsic permeability values were derived by considering the change of the viscosity of free water with temperature. A unique relationship between the intrinsic permeability and the porosity was observed, with no dependence on temperature, confirming that the flow involved in the permeability test only concerns free water.Keywords : Clays, Thermal consolidation, Adsorbed water, Permeability, Temperature effects, Radioactive waste disposal RESUME : Quand un massif d'argile saturée est soumise à une élévation de température, comme dans le cas des sols hôtes pour les stockages nucléaires à grande profondeur, la dilatation thermique des constituants engendre des surpressions interstitielles. Leur dissipation implique des gradients de pression et de température, des flux d'eau et de chaleur, et une modification des propriétés mécaniques du sol. Certains de ces aspects sont étudiés ici dans le cas particulier de l'argile de Boom, afin de contribuer à l'étude de la réponse du sol, en relation avec les recherches effectuées au laboratoire souterrain de Mol en Belgique. Les résultats d'essais de chauffage lent avec une mesure soigneuse des variations de volume ont montré qu'une prévision raisonnable de la dilatation thermique de l'eau interstitielle était obtenue en utilisant les coefficients thermiques de l'eau libre, avec donc un effet négligeable de l'eau adsorbée contenue dans cette argile plutôt dense. La consolidation thermique de l'argil...

A series of empirical expressions for predicting gas hydrate stability, its volume fraction out of pore space and gas hydrate mass-density were established in different systems in consideration of gas composition (CH 4 , C 2 H 6 , H 2 S) and salinity (NaCl, seawater), and pore size at temperature between 273.15 and 300 K, based on our gas hydrate thermodynamic model (Sultan et al., 2004b, c). Six of the developed expressions for predicting gas hydrate stability were validated against the available published experimental data and they were also compared with other models. At temperature 273.15 to 290.15 K, the ARDPs (Average Relative Deviation of Pressures between the prediction and the experimental data) have shown that these empirical expressions are in agreement with the experimental data as well as other models, indicating their reliability of predicting gas hydrate stability for these systems. At higher temperatures, the empirical predictions for gas hydrate stability do not well reproduce the experimental data, because they are based on van der Waals model. The empirical expressions for predicting gas hydrate stability in the systems of CH 4 + H 2 S + H 2 O, CH 4 + seawater + poresize, CH 4 + H 2 S + NaCl and CH 4 + CO 2 + NaCl, and for evaluating gas hydrate fraction and its density need further validation due to lack of available published experimental data. However, the empirical expressions for gas hydrate fraction and its density show that the effects of pore size and salinity are negligible; gas hydrate fraction will increase if methane concentration continuously increases relatively in excess of methane solubility and decreases with pressure within gas hydrate stability zone, which is well consistent with data of natural gas hydrates in Cascadia; gas hydrate density tends to increase with ethane percentage and decrease with pressure. Keywords: gas hydrate, stability law, fraction out of pore space, mass-density, empirical expressionsIn recent years, numerous theoretical models are proposed about the gas hydrate stability (Handa, 1990;

Based on the field‐investigated gas geochemistry, the modeling of gas hydrate formation conditions is conducted in the Qinghai‐Tibet plateau permafrost (QTPP) in combination with predecessors' data such as the permafrost ground temperature (T0), the thermal gradient within the frozen layer (G1) and the thermal gradient below the frozen layer (G2). The modeled results show that the permafrost characteristics generally meet the requirements for gas hydrate formation conditions in the study area. Gas composition, temperaturerelated permafrost parameters (e.g. T0,G1,G2) are the most important factors affecting gas hydrate formation conditions in the study area, whose spatial variations may cause the heterogeneity of gas hydrate occurrences. The most probable gas composition to form gas hydrate is the hybrid of methane and weight hydrocarbon gases (ethane and propane). In the predicted gas hydrate locations, the minimal upper depth of gas hydrate occurrence is less than one hundred meters and the maximum lower depth can reach one thousand meters with the thickness up to several hundred meters. Compared with Canadian Mallik gas hydrate field, the QTPP is favorable for gas hydrate formation in aspects of G1, G2 and gas composition, except for relatively thin permafrost, still suggesting great gas hydrate potentials.