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In recent years, more and more unique volumetric behaviors, including the nonlinear compression and unloading curves in the e-lnp plane, and different types of swelling and shrinkage curves obtained from suction-controlled swelling and shrinkage tests, etc., which are significantly different from those of nonexpansive clays, have been reported on compacted bentonite. More importantly, limitations were encountered when describing the unique behavior with existing constitutive models for unsaturated expansive soils. In this study, a new concept of critical saturation state (CSS) was proposed for compacted bentonite, in which, as the saturated state was reached, the matric suction still remained nonzero in the soil. Incorporated with this concept, a constitutive model was proposed. In this model, the CSS curve was defined in the suction-isotropic stress plane, delimiting the unsaturated zone. To validate the model, the volume change tests conducted on compacted GMZ bentonite under different stress paths were simulated. Good agreement confirmed that the proposed model could well describe the volume change behavior, including the swelling on hydration, shrinkage during drying, nonlinear compression, and unloading behavior. Key words: compacted bentonite, constitutive model, critical saturated state (CSS) curve, nonlinear compression and unloading behavior, swelling and shrinkage behavior Ces dernieres annees, de plus en plus comportement volumetrique unique, y compris les courbes non lineaires de compression et de dechargement dans le plan e-lnp, differents types de courbes de gonflement et de retrait obtenues par des essais de gonflement et de retrait controles par aspiration, etc., qui sont sensiblement differents de ceux des argiles non expansives, ont ete rapportes sur la bentonite compactee. Plus important encore, des limites ont ete rencontrees lors de la description de ce comportement unique avec les modeles constitutifs existants pour les sols expansifs non satures. Dans cette etude, un nouveau concept d'etat de saturation critique (CSS) a ete propose pour la bentonite compactee. Dans lequel, lorsque l'etat de saturation est atteint, la succion matricielle reste non nulle dans le sol. Incorpore a ce concept, un modele constitutif a ete propose. Dans ce modele, la courbe CSS a ete definie dans le plan de la contrainte de succion-isotropique, delimitant la zone non saturee. Pour valider le modele, les essais de changement de volume effectues sur la bentonite GMZ compactee sous differentes trajectoires de contrainte ont ete simules. Les bons accords ont confirme que le modele propose pouvait bien decrire le comportement de changement de volume, y compris le gonflement lors de l'hydratation, le retrecissement lors du sechage, le comportement non lineaire de compression et de dechargement. [Traduit par la Redaction] Mots-cles : bentonite compactee, modele constitutif, courbe d'etat de saturation critique (CSS), compression et dechargement non lineaires de dechargement, comportement de gonflement et de retraction, Introduction Compacted bentonite has been selected as a buffer/backfill material for constructing deep geological repositories for disposal of high-level radioactive waste in many countries (Komine and Ogata 1996; Villar 2006; [...]

The interlayer soil identified in conventional French rail track corresponds to a mixture of fine soil and coarse grains. To investigate the role of fines in the soil-water retention property of such a mixture, different coarse grain contents [f.sub.v] and dry densities of fine soil [[rho].sub.d-f] were considered. The filter paper method was applied to measure matric suction. Mercury intrusion porosimetry tests were performed to observe the microstructure of the fine soil. In terms of the gravimetric water content of fine soil [w.sub.f] with matric suction [psi], the soil-water retention curve (SWRC) was significantly affected by [[rho].sub.d-f] for [psi] < 715 kPa, while independent of [[rho].sub.d-f] for [psi] > 715 kPa. Interestingly, this threshold [psi] corresponded to a delimiting pore diameter of the bi-modal microstructure of fine soil, which separated micropores from macropores. In terms of degree of saturation [S.sub.r] with [psi], the SWRC was significantly affected by [[rho].sub.d-f] in the full suction range, while it was independent of [f.sub.v]. These findings help to better understand the results for samples where dry density of mixture [[rho].sub.d] is kept constant: an increase in [f.sub.v] resulted in a decrease in [[rho].sub.d-f] and the suction changed accordingly. In this case, both [f.sub.v] and [psi] affected the mechanical behavior. Key words: soil-water retention, matric suction, pore size distribution, dry density, coarse grain content. La presence d'un sol intercouche identifie dans la voie ferree conventionnelle franqaise represente un melange de sol fin et de gros grains. En vue d'etudier le role des fines dans la propriete de retention d'eau d'un tel melange, differentes teneurs en gros grains [f.sub.v] et densites seches de sol fin [[rho].sub.d-f] ont ete considerees. On a applique la methode du papier filtre pour mesurer la succion matricielle. On a effectue des tests de porosimetrie par intrusion de mercure pour l'observation de la microstructure du sol fin. La courbe de retention d'eau dans le sol (SWRC) a ete significativement affectee par [[rho].sub.d-f] pour [psi] < 715 kPa, alors qu'elle etait independante de [[rho].sub.d-f] pour [psi] > 715 kPa, en termes de teneur en eau gravimetrique du sol fin [w.sub.f] avec la succion matricielle [psi]. Il est interessant de noter que ce seuil [psi] correspondait a un diametre de pore delimitant la microstructure bimodale du sol fin, qui separait les micro-pores des macro- pores. En termes de degre de saturation [S.sub.r] avec C, le CRSW a ete significativement affecte par [[rho].sub.d-f] dans la gamme complete de succion, tout en etant independant de [f.sub.v]. Ces constatations permettent de mieux comprendre les resultats obtenus sur des echantillons dont la densite seche du melange [[rho].sub.d] est maintenue constante : une augmentation de [f.sub.v] a entraine une diminution de [[rho].sub.d-f] et la succion a change en consequence. Dans ce cas, [f.sub.v] et [psi] ont tous deux affecte le comportement mecanique. [Traduit par la Redaction] Mots-cles: retention d'eau dans le sol, succion matricielle, distribution de la taille des pores, densite seche, teneur en gros grains., Introduction In conventional French rail track, an interlayer was naturally created in the substructure under long-term traffic loading, which corresponded to a mixture of ballast grains and subgrade fine soil. [...]