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Experimental research is conducted on a compacted sample of silt using a triaxial equipment modified for testing unsaturated soils at elevated temperatures. An extensive array of isothermal and non-isothermal tests is conducted, including temperature-controlled soaking and desaturation tests, temperature- and suction-controlled isotropic consolidation tests, and suction-controlled thermal loading and unloading tests. Temperature values applied ranged from 25°C to 60°C, and the suction values varied from 0 to 300 kPa. Baseline data are presented on the influences of temperature and suction on critical-state parameters, yield surface, preconsolidation pressure, strength properties, and water retention characteristics of the soil. All data are carefully analysed and prevailing trends are highlighted.

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SUMMARYA unified constitutive model for unsaturated soils is presented in a critical state framework using the concepts of effective stress and bounding surface plasticity theory. Consideration is given to the effects of unsaturation and particle crushing in the definition of the critical state. A simple isotropic elastic rule is adopted. A loading surface and a bounding surface of the same shape are defined using simple and versatile functions. The bounding surface and elastic rules lead to the existence of a limiting isotropic compression line, towards which the stress trajectories of all isotropic compression load paths approach. A nonassociated flow rule of the same general form is assumed for all soil types. Isotropic hardening/softening occurs due to changes in plastic volumetric strains as well as suction for some unsaturated soils, enabling the phenomenon of volumetric collapse upon wetting to be accounted for. The model is used to simulate the stress-strain behaviour observed in unsaturated speswhite kaolin subjected to three triaxial test load paths. The fit between simulation and experiment is improved compared to that of other constitutive models developed using conventional Cam-Clay-based plasticity theory and calibrated using the same set of data. Also, the model is used to simulate to a high degree of accuracy the stress-strain behaviour observed in unsaturated Kurnell sand subjected to two triaxial test load paths and the oedometric compression load path. For oedometric compression theoretical simulations indicate that the suction was not sufficiently large to cause samples to separate from the confining ring.

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A new bounding surface constitutive model for sands is presented and is suited to a wide range of stresses, including those sufficient to cause particle crushing. The basic concepts of critical state soil mechanics are shown to be valid, and a uniquely shaped critical state line is defined to capture the three modes of plastic deformation observed across a wide range of stresses, including particle rearrangement, particle crushing, and pseudoelastic deformation. A limiting isotropic compression line is separated from the critical state line in the υ − ln ′ p plane by a constant shift along an elastic unload-reload line. In the deviator stress -mean effective stress (q-p′) plane, the loading and bounding surfaces are homologous about the origin and defined by a simple and versatile function. Isotropic hardening and softening of the loading and bounding surfaces are controlled by plastic volumetric strains. A commonly used nonassociative flow rule is adopted. Experimental results of monotonically loaded drained and undrained triaxial tests, isotropic compression tests, and oedometric compression tests are presented for a quartz sand and used to calibrate the model. Membrane penetration is accounted for in the model simulations of the test results. A single set of material parameters is introduced enabling rigorous and accurate predictions of stress-strain behaviour in sands. Résumé :On présente un nouveau modèle constitutif de surface limite pour les sables qui convient à une large plage de contraintes incluant celles suffisantes pour produire le broyage des grains. On montre que les concepts de base de la mécanique des sols de l'état critique sont valides et on définit une ligne d'état critique de forme unique pour couvrir les trois modes de déformation plastique observés sur une large plage de contraintes incluant le réarrangement de particules, le broyage de particules, et la déformation pseudo-élastique. Une ligne limite de compression isotrope est sé-parée de la ligne d'état critique dans le plan υ − ln ′ p par un déplacement constant le long de la ligne déchargement-rechargement élastique. Dans le plan q-p′ les surfaces de chargement et surfaces limites sont homologues autour de l'origine et sont définies par une fonction simple et versatile. Le durcissement-ramollissement isotrope du chargement et les surfaces limites sont contrôlés par des déformations plastiques volumétriques. On a adopté une loi d'écoulement non-associative communément utilisée. On présente les résultats expérimentaux d'essais de compression monotonique triaxiale isotrope drainée et non drainée et d'essais de compression oedométrique pour un sable de quartz utilisé pour calibrer le modèle. On tient compte de la pénétration de la membrane dans les simulations du modèle des résultats d'essais. On introduit un simple ensemble de paramètres de matériau permettant des prédictions rigoureuses et précises du comportement contrainte-déformation des sables.

SUMMARYA comprehensive framework to de"ne the constitutive behaviour of unsaturated soils is developed within the theory of mixtures applied to three-phase porous media. Each of the three phases is endowed with its own strain and stress. Elastic and elastic}plastic constitutive equations are developed. Particular emphasis is laid on the interactions between the phases both in the elastic and plastic regimes. Nevertheless, the clear structure of the constitutive equations requires a minimal number of material parameters. Their identi"cation is provided: in particular, it incorporates directly the soil}water characteristic curve. Crucial to the formulation is an appropriate de"nition of the e!ective stress. The coupled in#uence of this e!ective stress and of suction makes it possible to describe qualitatively many of the characteristic features observed in experiments, e.g. for normally consolidated soils, a plastic behaviour up to air entry followed by an elastic behaviour at increasing suctions, and, on the way back, an elastic behaviour, unless compression is applied in which case plastic collapse occurs.