Data from a series of controlled suction triaxial tests on samples of compacted speswhite kaolin were used in the development of an elasto–plastic critical state framework for unsaturated soil. The framework is defined in terms of four state variables: mean net stress, deviator stress, suction and specific volume. Included within the proposed framework are an isotropic normal compression hyperline, a critical state hyperline and a state boundary hypersurface. For states that lie inside the state boundary hypersurface the soil behaviour is assumed to be elastic, with movement over the state boundary hypersurface corresponding to expansion of a yield surface in stress space. The pattern of swelling and collapse observed during wetting, the elastic–plastic compression behaviour during isotropic loading and the increase of shear strength with suction were all related to the shape of the yield surface and the hardening law defined by the form of the state boundary. By assuming that constant–suction cross–sections of the yield surface were elliptical it was possible to predict test paths for different types of triaxial shear test that showed good agreement with observed behaviour. The development of shear strain was also predicted with reasonable success, by assuming an associated flow rule. Les données obtenues dans une série d'essais triaxiaux à succion contrôlée sur des échantillons de kaolin speswhite compactés ont servi au développement d'un modèle élastoplastique à l'état critique pour les sols non saturés. Ce Modèle est défini par quatre variables d'état: la contrainte moyenne, le déviateur des contraintes, la succion et le volume spécifique. II inclue les hyperlignes des compressions normales isotropes et des états critiques et l'hypersurface d'état. Pour les états situés dans l'hypersurface d'état, le comportement du sol est supposé élastique. Tout déplacement au dessus de cette surface correspond à une expansion de la surface limite dans l'espace des contraintes. Les gonflements et effondrements observés lors de l'humidification, le comportement élastoplastique en compression lors des chargements isotropes et l'augmentation de la résistance au cisaillement avec la succion, ont tous ét&acute corrélés avec la forme de la surface limite ainsi qu'avec la loi de consolidation défine par la forme de la surface d'état. Si l'on suppose que les intersections succion constante–surface limite sont elliptiques, il est possible de calculer, pour différents types d'essais de cisaillement triaxiaux, des chemins qui se révèlent en bon accord avec les comportements observés. En choisissant une loi d'écoulement appropriée, le développement des déformations en cisaillement a également été prédit de manière fiable.

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An anisotropic elastoplastic model for soft clays is presented. Experimental data from multistage drained triaxial stress path tests on Otaniemi clay from Finland provide support for the proposed shape of the yield curve and for the proposed relationship describing the change of yield curve inclination with plastic straining. Procedures are proposed for determining the initial inclination of the yield curve and the values of the two additional soil constants within the model. Comparisons of model simulations with experimental data demonstrate significant improvements in the performance of the new model over the Modified Cam Clay model. The remaining discrepancies are mainly attributable to the important role of destructuration in the sensitive Otaniemi clay

Citation for published item:qllipoliD hF nd heelerD F tF nd urstunenD wF @PHHQA 9wodelling the vrition of degree of sturtion in deformle unsturted soilF9D q¡ eotehniqueFD SQ @IAF ppF IHSEIIPF Further information on publisher's website: httpXGGwwwFthomstelfordFomGjournlsGstrtFspctournlitleaq¡ eotehniqueertileshaIQIQtournlwenuatruetourn Publisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.

Consideration of the different roles of pore air pressure, pore water pressure within bulk water and pore water pressure within meniscus water suggests that the degree of saturation will have a significant influence on the stress–strain behaviour of an unsaturated soil, in addition to any influence of suction. This suggestion is supported by experimental evidence. In the light of this, a new elasto-plastic framework for unsaturated soils is proposed, involving coupling of hydraulic hysteresis and mechanical behaviour. Within the proposed framework, plastic changes of degree of saturation influence the stress–strain behaviour, and plastic volumetric strains influence the water retention behaviour. A specific constitutive model for isotropic stress states is proposed, and model predictions are compared with experimental results, in order to demonstrate some of the capabilities of the new framework. Forms of behaviour that can be represented include proper transitions between saturated and unsaturated types of response, the occurrence of irreversible compression during the drying stages of wetting–drying cycles, and the influence of a wetting–drying cycle on subsequent behaviour during isotropic loading.

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An extensive programme of research into the influence of undissolved gas bubbles on the behaviour of fine-grained onshore soils is reviewed. The programme has been based on the development of a laboratory technique for the preparation of reconstituted soil samples containing a uniform distribution of gas bubbles. The structure of these samples is similar to that observed in sediment recovered from the sea bed, and consists of large gas-filled cavities surrounded by a matrix of saturated soil. It is found that surface tension effects limit the difference between gas pressure and pore water pressure, and that the overall void size is effectively a function of the strength of the matrix, so that changes in void volume may be modelled by cavity expansion and contraction in an ideal plastic medium, leading to limits on the difference between gas pressure and mean total stress. A new parameter, operative stress, is shown to influence both the consolidation and the strength of these gassy soils. Thus, during consolidation, the gas volume is controlled by the total stress and the water volume by the operative stress; the undrained shear strength may be increased or decreased by the presence of the gas, depending on the specific values of total stress and operative stress. The operative stress for a gassy soil may therefore be seen as being analogous in its definition (total stress minus pore water pressure) to the effective stress for a saturated soil, but different from the effective stress in that it does not, on its own, control the strains and strength. Gas bubbles are shown to have a major influence on the acoustic behaviour of fine-grained offshore soils. L'article passe en revue un programme de grande envergure pour étudier l'influence exercée par des bulles de gaz non-dissoutes sur le comportement des sols à grains fins en mer. Le programme a été basé sur le développement d'une technique de laboratoire pour la préparation d'échantillons de sols reconstitués comprenant une distribution uniforme de bulles de gaz. La structure de ces échantillons ressemble à celle observée dans des sédiments obtenus au fond de la mer, comprenant de grandes cavités remplies de gaz entourées d'une matrice de sol saturé. Il a été trouvé que des effets de tension superficielle limitent la différence entre la pression du gaz et la pression de l'eau interstitielle et aussi que la valeur totale des vides dépend effectivement de la résistance de la matrice, de sorte que des changements dans le volume des vides peuvent être modelesés par l'expansion et la contraction des cavités dans un milieu idéal plastique, conduisant à des limites sur la différence entre la pression du gaz et la contrainte totale moyenne. On démontre la façon dont cette différence représente un nouveau paramètre (contrainte effective) qui influence à la fois la consolidation et la résistance de ces sols gazeux. Par exemple, pendant la consolidation le volume de gaz est contrôlé par la contrainte totale, tandis que le volume d'eau est contrôlé par cette contrainte effective. Selon les valeurs spécifiques de la contrainte totale et de la contrainte effective la résistance au cisaillement non-drainé peut être augmentée ou diminuée par la présence du gaz. On peut alors considérer que cette définition de la contrainte effective dans le cas d'un sol gazeux (contrainte totale moins pression d'eau interstitielle) est analogue à la contrainte effective pour un sol saturé mais diffère de celle-ci en ce qu'elle seul ne contrôle pas les déformations ni la résistance. On démontre que les bulles de gaz exercent une influence très importante sur le comportement acoustique des sols à grains fins en mer.

Existing elasto-plastic critical state constitutive models for unsaturated soil provide no information on the variation of water content or degree of saturation. These models cannot therefore, for example, be used to predict unsaturated soil behaviour during undrained loading, when the variation of suction is determined by the requirement that water content remains constant. This problem has been tackled by extending an existing elasto-plastic model to include relationships describing the variation of specific water volume (the volume of water and solids in an element of soil containing unit volume of solids). The proposed form of the variation of specific water volume was based on consideration of the soil fabric, resulting in a coupled form of elasto-plastic behaviour. Predictions from the elasto-plastic model showed good agreement with the experimental results from suction-controlled triaxial tests on unsaturated samples of compacted speswhite kaolin. Normal compression lines for specific water volume at different values of suction were well predicted, as was the variation of specific water volume during wetting. Critical state values of specific water volume were slightly underestimated, but test paths for both drained and undrained shearing were predicted with reasonable success. Key words: compacted clays, constitutive model, critical state, elasto-plasticity, triaxial tests, unsaturated.