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The water retention behaviour of compacted bentonites is strongly affected by multi-physical and multi-scale processes taking place in these materials. Experimental data have evidenced major effects of the material dry density, the imposed volume constraints, and the soil fabric. This paper presents a new water retention model accounting for proper retention mechanisms in each structural level of compacted bentonites, namely adsorption in the intra-aggregate pores and capillarity in the inter-aggregate ones. The model is calibrated and validated against experimental data on different bentonite-based materials, showing good capabilities in capturing the main features of the behaviour. The model is able to reproduce experimental data on compacted bentonites over a wide range of suction values, within a unified framework, and using a limited number of parameters. Some of the parameters introduced are shown to take approximately the same value for several bentonites, providing a significant basis for preliminary design when dedicated experiments are missing. Key words: water retention model, compacted bentonites, unsaturated soils, microstructure. Le comportement de retention d'eau des bentonites compactees est fortement affecte par des processus ayant plusieurs caracteres physiques et echelles qui ont lieu dans ces materiaux. Les donnees experimentales ont montre de principaux effets sur la densite seche de la matiere, sur les contraintes de volume imposees et sur le tissu de sol. Cet article presente un nouveau modele de retention d'eau pour tenir compte des bons mecanismes de retention dans chaque niveau structurel de bentonites compactees, notamment dans l'adsorption des pores intra-agreges et la capillarite dans ceux inter agrege. Le modele est etalonne et valide avec des donnees experimentales sur differents materiaux a base de bentonite, montrant une bonne capacite a capter les principales caracteristiques du comportement. Le modele est capable de reproduire les donnees experimentales sur des bentonites compactees sur une large plage de valeurs d'aspiration, dans un cadre unifie et a l'aide d'un nombre limite de parametres. Certains des parametres introduits sont indiques a prendre environ la meme valeur pour plusieurs bentonites, fournissant une base pour la conception preliminaire lorsque des experiences dediees sont manquantes. [Traduit par la Redaction] Mots-cles: modele de retention d'eau, bentonites compactees, sols non satures, microstructure., Introduction The water retention curve is one of the fundamental properties required for predicting the behaviour of unsaturated soils. Early water retention models (Gardner 1958; Brooks and Corey 1964; van [...]

In most of the concept designs for nuclear waste disposals, bentonite-based materials are used to seal underground galleries and shafts. In order to assess the safety of such engineered barriers and to provide a good prediction for the material behaviour, a number of experimental campaigns and constitutive models have been developed mainly focusing on bentonite compacted blocks. On the other hand, in recent years, the use of high density bentonite pellets combined with powdered bentonite has also been considered as an interesting alternative. The bentonite pellets mixtures present a prominent initial heterogeneity level as a result of the inter-pellets porosity, which evolves strongly during hydration. This paper aims to present the numerical modelling strategy adopted for a swelling pressure test on pellets mixture carried by CEA (France). Taking advantage of the finite element code LAGAMINE, the Barcelona Basic model is considered for the bentonite mechanical behaviour and the double porosity model proposed by [1] is used for the water retention behaviour and permeability evolution. Given the already-known complexity of the multiphysical and multiscale coupled processes taking place during bentonite-hydration, the obtained numerical results are in good agreement with experimental measurements. Especially, the non-monotonic evolution of the swelling pressure during the hydration phase is well captured by this model, which is always a challenge for this type of problem.

Due to climate change, soil desiccating became a serious concern in the agricultural area of Belgium. Knowing soil evaporation kinetic can help to elucidate and predict: the soil moisture regime, soil water retention and soil water content. Those parameters are vital for water use efficiency and sustainable agriculture. This research investigated the mechanism of soil evaporation using both laboratory experiment and numerical simulation. Soil samples (Luvisol) were collected from the agricultural field in Gembloux- Belgium, and tested in a small drying chamber. Sensors measured the chamber temperature and humidity, while digital camera monitored the soil surface throughout the experiment. HYPROP device recorded the water change, soil suction, and soil water retention curve. During three evaporation experiments, four periods were observed rather than three as commonly recorded in the theory of drying. The modelling considered thermo-hydro-mechanical framework for predicting the drying process of Luvisol. The model used the finite element code LAGAMINE created at the University of Liege. The software aims at assessing the mechanism of water transport between soil and atmosphere. The results of the simulation showed major domination of Darcean flow during desiccating, while some short vapour diffusion occurred only after the soil surface began to de-saturate.

In the context of underground exploitation, the behaviour of rocks near galleries and tunnels conditions their stability. Underground drilling generates deformations, damage, fracturing, and significant modification of flow characteristics in the surrounding rock. However, the influence of small-scale characteristics and behaviour on the rock deformations and damage at engineering scale remains a complex issue. Consequently, the multiscale behaviour of a clay rock is modelled starting from the large scale of the excavation damaged zone around galleries and then enriching the approach by considering microstructural characteristics from the scale of mineral inclusions. Lastly, a double-scale numerical framework is considered. It allows to relate small- to large-scale rock behaviour in terms of deformations and material rupture. In fact, the development of damage and cracking at microscale allows to predict large-scale fracturing. The developed method focuses on a claystone in the particular context of long-term management of high-level nuclear wastes by deep geological repository. The results highlight the possibilities of double-scale computing in the prediction of the behaviour of underground engineering structures.

Drying-induced shrinkage of geomaterials may have a strong effect on geostructure stability and deformation. The settlement of foundations and fracture openings in slopes, roads, and tunnel walls may be due to drying shrinkage. However, there is still a lack of knowledge concerning shrinkage evolution with time and shrinkage propagation within the material. In this study, the shrinkage of a specific clayey rock--Boom clay--under drying conditions is investigated experimentally. This rock is a deep geological formation, which is under study for high-level and long-term radioactive waste storage in Belgium. Two experimental campaigns are presented. The first, based on the vapour equilibrium drying technique and coupled with sample size manual measurement, aims to characterize the material shrinkage in balanced states. The second, based on the convective drying technique and coupled with shape monitoring using X-ray tomography, aims to analyse how shrinkage develops before reaching a steady state. Both approaches put in evidence the shrinkage anisotropy of this structurally bedded rock, with a ratio around 2 between the direction of maximum strains and the direction of minimum strains. However, the two drying techniques also provide complementary results as the relation between the amount of shrinkage and the retention curve (for uniform drying imposed with saline solutions) and the kinetics of shrinkage propagation inside the material (for nonuniform drying imposed with air convection). Key words: drying, Boom clay, shrinkage, hydromechanical coupling, tomography. Le retrait des geomateriaux par sechage peut beaucoup influer sur la stabilite et la deformation d'une geostructure. Le tassement de fondations et l'ouverture de fractures sur les pentes, sur les routes ou sur les parois des tunnels peuvent etre dus a ce phenomene de retrait. Cependant, revolution du retrait dans le temps et sa propagation au sein du materiau sont encore mal connues. Dans la presente etude, le retrait d'une roche argileuse specifique, l'argile de Boom, soumise au sechage est etudie de maniere experimentale. Cette roche provient d'une formation geologique profonde, qui fait l'objet d'etudes realisees a des fins de stockage de dechets a longue duree de vie et hautement radioactifs en Belgique. On presente ici deux campagnes experimentales. La premiere, basee sur la technique de sechage par equilibre liquide-vapeur et associee a des mesures manuelles de la taille d'echantillons, vise a caracteriser la contraction du materiau en condition d'equilibre. La seconde, basee sur la technique de sechage convectif et associee au suivi de la geometrie par tomographie aux rayons X, vise a analyser comment la contraction evolue avant de se stabiliser. Les deux methodes experimentales mettent en evidence l'anisotropie du retrait de cette roche litee, le ratio d'anisotropie entre la direction des contraintes maximales et celle des contraintes minimales etant environ egal a 2. Par ailleurs, les deux techniques de sechage fournissent des resultats complementaires, comme la relation entre l'amplitude de la contraction et la courbe de retention (dans le cas du sechage par solution saline) et la cinetique de propagation du retrait au sein du materiau (dans le cas du sechage convectif). [Traduit par la Redaction] Mots-cles: sechage, argile de Boom, retrait, couplage hydromecanique, tomographie., Introduction Geomaterial drying is an important phenomenon to deal with. On one hand, it is used as an industrial process for the production of some building materials, such as clay [...]

Bentonite-based materials are studied as potential barriers for the geological disposal of radioactive waste. In this context, the hydro-mechanical behaviour of the engineered barrier is first characterized by free swelling conditions followed by constant volume conditions. This paper presents an experimental study conducted in order to characterize the water retention behaviour of a compacted MX-80 bentonite/sand mixture. Then, based on observations of the material double structure and the water retention mechanisms in compacted bentonites, a new water retention model is proposed. The model considers adsorbed water in the microstructure and capillary water in the aggregate-porosity. The model is calibrated and validated against the experimental data. It is used for better understanding competing effects between volume change and water uptake observed during hydration under free swelling conditions.