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This paper presents the results of a comprehensive experimental study of the desiccation of fine-grained soils. Air drying of initially saturated soil slabs in controlled conditions is investigated by performing three kinds of tests: free desiccation tests, constrained desiccation tests (prevented shrinkage), and crack pattern tests. Strains, suction, water content, and crack geometry are investigated. Results reveal that unconstrained drying exhibits two stages: a domain with large, mostly irrecoverable deformations and degree of saturation close to 100%, followed by a domain with lower deformations at a decreasing degree of saturation. Homogeneous soil macroscopic cracking is possible only in the presence of boundary constraints and (or) moisture gradients, inducing the build-up of tensile stresses. Results also show that, for the initially saturated remoulded soils tested here, in the whole sample and near a crack initiation point, the degree of saturation remains very close to 100% until cracking, while cracking onset, the air-entry suction, and the shrinkage limit are close to each other. Cavitation nuclei and the formation of an irregular drying front at cracking initiation are commented upon in light of this observation. Finally, results suggest that the crack pattern geometry is the result of energy redistribution. A quantification of the process is proposed.Résumé : Cet article présente les résultats d'une étude expérimentale complète de la dessiccation de sols fins. Le séchage de sols initialement saturés est étudié dans des conditions contrôlées et à température ambiante. Trois types d'essais ont été effectués : essais de dessiccation libre, essais de dessiccation avec contraintes (le retrait est empêché dans une direction) et essais de plans de fissures. Les déformations, la succion, la teneur en eau et la géométrie des fissures ont été investiguées. Les résultats démontrent que le séchage sans contrainte présente deux étapes : un domaine durant lequel les déformations sont grandes et irréversibles et le degré de saturation reste près de 100 %, suivi d'un domaine durant lequel les déformations sont plus faibles et le degré de saturation diminue. Pour un sol homogène, des fissures macroscopiques apparaissent seulement lorsqu'il y a des contraintes aux frontières et (ou) des gradients d'humidité, ce qui induit des contraintes en tension. Les résultats montrent aussi que pour les silts et argiles remaniés initialement saturés utilisés dans cette étude, le degré de saturation reste très près de 100 % jusqu'à la fissuration. De plus, la fissuration, l'entrée d'air et la limite de retrait sont proches les unes des autres. Suite à ces observations, l'existence de germes de cavitation et la formation d'un front de séchage irrégulier lors de l'initiation des fissures sont discutées. Finalement, les résultats suggèrent que la géométrie des fissures résulte d'une redistribution de l'énergie à travers le corps. Une quantification du processus est proposée.

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The effect of temperature on the volumetric strains of the clay skeleton and clay – water system has been examined experimentally. Heating tests at (various) constant effective stress and isotropic loading tests at (various) constant temperatures were performed on low-porosity clays under drained conditions. The results indicate that the thermal expansion of adsorbed water in these clays is significantly lower than that of free water. Depending on the effective pressure, the clay skeleton may undergo either compression or expansion. Key words: nuclear waste isolation, clay, low porosity, thermal strain, adsorbed water.

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Failure conditions in soils at elevated temperatures appear to be strongly dependent on the history of the application of stress and temperature. Four cases of such history leading to various modes of failure are identified and interpreted in terms of thermal Cam-clay models. Particular attention is given to the influence of thermal variability on the coefficient of the critical state, M, or the angle of internal friction. A detailed analysis of the material history offers an explanation of an apparent confusion about whether the soil strength is decreased or increased by temperature. In a companion paper, numerical analysis of the development of axisymmetric thermal and stress fields around a cylindrical heat source suggests that thermal failure may arise in conditions that are far from any mechanically critical situation.

Chemically active saturated homoionic clays are considered in a two-phase framework. The solid phase contains the clay particles, absorbed water and salt. The fluid phase, or pore water, contains free water and salt. Water, and possibly salt, can transfer between the two phases. In addition free water may diffuse through the porous medium. A global understanding of the phenomena involved, namely deformation, transfer and diffusion, is proposed. Emphasis is laid on the chemo-mechanical constitutive equations in an elastic-plastic setting. Elastic chemo-mechanical coupling is introduced through a potential, in such a way that the tangent poro-elasticity matrix remains symmetric. Material parameters needed to quantify the coupling are calibrated from specific experiments available in the recent literature. The elasto-plastic behaviour aims at reproducing qualitatively and quantitatively the typical experimental responses observed on almost pure Na-Montmorillonite clays during chemical and mixed chemo-mechanical loadings. Increase of the salinity of pore water at a constant confinement stress leads to a volume decrease, so-called chemical consolidation. Subsequent exposure to a distilled water solution produces swelling: however, the latter is smaller than the chemical consolidation so that the chemical loading cycle results in a net contractancy, the amount of which increases with the confinement. In fact, plastic yielding takes place at low salinities of pore water, and when it stops, chemical preconsolidation is generated.Natural clays which contain cations of different species are considered in a companion paper, Gajo et al. [Int. J. Solids Struct., this issue], as they require to account for electro-chemo-mechanical couplings. Ó