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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. [...]

In this study, a series of suction-control oedometer tests were conducted on unsaturated and saturated MX80 bentonite pellet/powder mixtures. The mixture samples were imposed with various suctions from 0 to 113 MPa under a low vertical stress (0.1 MPa). Then, the mixture samples were loaded in steps with controlled suction until reaching three different target stresses σv (0.1, 3.2, and 12.8 MPa) and instantaneously unloaded. The microstructure of unloaded samples was observed with mercury intrusion porosimetry (MIP) and microcomputed tomography (μCT), together with the determination of suction, water content, void ratio, and saturation degree. Results showed that the water-retention behavior of bentonite pellet/powder mixture was rather characterized by pellets instead of powder. Due to the stress release, the measured suction after unloading sme was larger than the imposed suction sim. The suction difference Δs between sme and sim depended on both suction and load. Under a given σv , the suction effect was characterized by an increase of Δs when sim decreased from 113 to 4.2 MPa, but by a noticeable decrease of Δs when sim further decreased to zero. This was found to be related to the changes of the slopes of soil-water retention curves (SWRCs): the slope was decreasing with suction increases after 4.2 MPa suction, but was increasing with suction increases before 4.2 MPa suction. As far as the stress effect was concerned, a larger Δs was identified at a higher σv for a given sim. The narrowing difference between Δs and the reduction of mean stress of pellet Δppellet with increasing load was because of the decrease of larger pores due to loading. The total mixture void ratio eTmixture was highly dependent on suction and vertical stress, whereas the total pellet void ratio eTpellet was only dependent on suction in the stress range considered. [ABSTRACT FROM AUTHOR]

Metakaolin has been widely used as pozzolanic additive to improve the pozzolanic activity of lime-based products. In this study, the normal standard Proctor compaction test was performed on metakaolin with (5% lime) and without (0% lime) lime addition. The changes in stiffness, suction, and microstructure with remoulding water content were investigated on statically compacted samples. Results show that lime-treated metakaolin exhibits a 1.5-peak compaction curve, while untreated metakaolin exhibits common one-peak compaction curve. The uncommon shape of the compaction curve of the treated metakaolin can be explained by the non-fully developed soil suction when water is not continuous. Treated and untreated samples compacted at both dry and wet of optimum show uni-modal pore-size distribution characteristics, indicating the absence of aggregates. This is related to the specific thermal treatment, forming separate metakaolin platelets and leading to a modified uniform structure with diffuse platelets. The soil stiffness is rather dominated by the number of particle contacts or soil dry density, the effect of suction being insignificant. For the suction changes, on the dry side, the effect of pore-size distribution prevails facing the effect of water content, while on the wet side it is the effect of water content that becomes dominating. Key words: soil stabilization, compaction, microscopy, stiffness, suction. Le metakaolin a ete largement utilise comme additif pouzzolanique pour ameliorer l'activite pouzzolanique des produits a base de chaux. Dans cette etude, l'essai de compactage Proctor standard normal a ete effectue sur le metakaolin avec (5 % de chaux) et sans (0 % de chaux) ajout de chaux. Les changements de rigidite, de succion et de microstructure avec la teneur en eau de remoulage ont ete etudies sur des echantillons compactes statiquement. Les resultats montrent que le metakaolin traite a la chaux presente une courbe de compactage a un pic et demi, alors que le metakaolin non traite presente une courbe de compactage courante a un pic. La forme peu courante de la courbe de compactage du metakaolin traite peut s'expliquer par la succion du sol non entierement developpee lorsque l'eau n'est pas continue. Les echantillons traites et non traites, compactes du cote sec et du cote humide de l'optimum, presentent des caracteristiques uni-modales de la distribution de la taille des pores, indiquant l'absence d'agregats. Ceci est lie au traitement thermique specifique, formant des plaquettes de metakaolin separees et conduisant a une structure uniforme modifiee avec des plaquettes diffuses. La rigidite du sol est plutot dominee par le nombre de contacts des particules ou la densite du sol sec, l'effet de la succion etant insignifiant. Pour les changements de succion, du cote sec, c'est l'effet de la distribution de la taille des pores qui prevaut face a l'effet de la teneur en eau, tandis que du cote humide, c'est l'effet de la teneur en eau qui prevaut. [Traduit par la Redaction] Mots-cles: stabilisation des sols, compactage, microscopie, rigidite, succion., 1. Introduction Highway and railway embankments with clayey soils often encounter problems of stability or excessive settlement under the effects of water content change and mechanical loading. Lime stabilization is [...]

Water retention curve (WRC) is an important tool to study the hydraulic and mechanical properties of unsaturated soils, such as permeability, strength and deformation properties. Most of the existing WRC models fail to reflect the water retention mechanisms of unsaturated soils or they are complex in form, and these models are hard to give good performance on modelling the bimodal and multimodal WRCs. In this study, based on analyzing the water retention mechanisms of unsaturated soils, the WRC was divided into two domains that are governed by adsorption and capillary mechanisms, respectively. An adsorption water retention curve model (WRCM) was developed based on micro-pore filling theory and Kelvin’s law. A capillary WRCM was established based on the capillary condensation theory and Young-Laplace equation. Then, a new water retention curve model over the full suction range was built by superposing the adsorption and capillary WRCMs. Finally, the new model was validated through modelling the experimentally measured WRCs of six representative unsaturated soils, including Shanghai soft clay, Xi’an loess, Nanyang expansive soil, Guilin lateritic clay, Western Liaoning aeolian soil and Inner Mongolia Gaomiaozi (GMZ) bentonite. Results showed that the proposed model, which was simple in form with definite physical meaning parameters and successfully reflected the adsorption and capillary mechanisms of water retention, was able to simulate WRCs with different shapes for different types of soils under different conditions.