In this paper, generalised elastic-viscoplastic (EVP) theory combined with a power law is used to derive simplified equations relating undrained shear strength and preconsolidation pressure to strain rate. An additional equation is derived relating the strain-rate parameter in EVP theory to the secondary compression index. The derived equations are used to evaluate the rate-sensitive and drained creep response of 20 clays reported in the literature. The evaluation shows strong evidence that the rate-sensitivity and time-dependency of clay in compression can be described simultaneously using the generalised EVP theory and power law. In addition, the constitutive parameter governing rate-sensitivity in EVP theory appears to be unique for constant rate-of-strain tests such as undrained triaxial compression and drained oedometer compression as well as stress controlled tests such as drained oedometer creep. To conclude, an approach to select the viscous parameters for clay is provided.Dans la présente communication, on utilise une théorie élastique-viscoplastique (EVP) généralisée alliée à une loi exponentielle pour dériver des équations simplifiées permettant de calculer les rapports entre la résistance au cisaillement non drainé et la pression de préconsolidation d'une part, et la vitesse de déformation de l'autre. On dérive également une équation supplémentaire mettant en rapport le paramètre de vitesse de déformation dans la théorie EVP et l'indice de compression secondaire. On utilise les équations dérivées pour évaluer la réponse sensible à la vitesse et à la reptation drainée de vingt argiles mentionnées dans la documentation. Cette évalua-tion apporte des éléments démontrant que la sensibilité à la vitesse et la dépendance temporelle de l'argile sous compression peuvent être décrites simultanément avec la théorie élastique-viscoplastique (EVP) généralisée et la loi exponentielle. En outre, le paramètre constitutif régissant la sensibilité à la vitesse dans la théorie EVP semble être unique pour des essais de vitesse de déformation constante, comme la compression triaxiale non drainée et la compression à l'oedomètre drainé, ainsi que des essais contrôlés par les contraintes, comme la reptation à l'oedomètre drainée. Pour conclure, la communication pré-sente une méthode de sélection des paramètres visqueux pour l'argile.

This paper extends an existing elastic–viscoplastic (EVP) constitutive model using a state-dependent viscosity parameter to describe the engineering response of undisturbed structured clay. The term structure refers to the effects of fabric and weak cementation bonds between clay particles. The extended constitutive model is coupled with the Biot consolidation theory and is formulated to describe the intrinsic or unstructured response of clay using overstress viscoplasticity, an elliptical cap yield surface, Drucker–Prager failure envelope, and a hardening law from critical state theory. The clay structure is mathematically accounted for by assuming that the initial fluidity of structured clay at yield and failure is very low and that the fluidity increases with increasing plastic strain. This process is usually referred to as “destructuration.” The formulation is evaluated using Saint-Jean-Vianney (SJV) clay by comparing calculated and measured behaviour during consolidated isotropically undrained triaxial compression, triaxial creep, and constant rate-of-strain Ko′-consolidation tests. The comparisons indicate that the EVP constitutive model can describe most of the rate-sensitive behaviour of SJV clay during both drained and undrained laboratory tests involving either constant-volume shear or predominantly volumetric compression of the soil skeleton.

SUMMARYThis paper describes a constitutive approach to model the behavior of rate-dependent anisotropic structured clay. Rate-sensitivity is modeled using overstress viscoplasticity. Clay structure is treated as a viscous phenomenon whereby the viscosity of the undisturbed structured clay is initially very high and the viscosity degrades or decreases with plastic straining until the intrinsic or residual viscosity is reached. A microstructure tensor approach is used to make the structured viscosity anisotropic; whereas, the intrinsic viscosity is assumed to be isotropic. The behavior of the constitutive model is compared with the measured response of two clays (Gloucester and St. Vallier clay) from Eastern Canada during triaxial compression tests on specimens trimmed at different orientations to the vertical. The comparisons show that the constitutive framework is able to describe the anisotropic and rate-sensitive response of both clays. The response of the model is also examined for the more general case of anisotropic consolidated triaxial compression and extension.

The authors have decided to study the influence of microstructure on the time-dependent response of undisturbed and reconstituted London clay using drained and undrained triaxial compression tests (CIU and CID) with step changes in the applied strain rate. The paper presents interesting behaviour, and the authors should be commended for demonstrating the viscous response of London clay.The primary influence of microstructure on the engineering response of London clay can be seen in Fig. 18(a), which compares the stress-strain response in the undisturbed and reconstituted states. Fig. 18(b) shows similar behaviour from triaxial compression tests on Rosemère clay from Eastern Canada (Philibert, 1976). From Fig. 18, it can be seen that there are similarities in the relative stress-strain response of the two materials, in spite of their vastly different index properties (e.g. I L % 0 compared with I L % 1.2). The stress-strain response of both clays during triaxial compression is characterised by: (a) a peak shear strength followed by post-peak strength reduction with large strain; (b) a predominantly strain-hardening response of the reconstituted or disturbed materials; and (c) at large strain, a post-peak strength of the undisturbed clay that approaches that of the reconstituted and 'cut' materials respectively. The difference in behaviour (the shaded areas in Figs 18(a) and 18(b)) is typically attributed to the effects of microstructure or weak bonding between the clay particles and aggregates of clay particles. Such behaviour is analogous to that typically observed in oedometer consolation tests on undisturbed and reconstituted materials (Burland, 1990).Regarding the time dependence or rate sensitivity of London clay, the authors quantify viscous effects using the jump in deviatoric stress induced immediately after changing the axial strain rate. Although such an approach has merit, the following presents an alternative interpretation of the rate-sensitive response of London clay using the theory of overstress viscoplasticity (Perzyna, 1963). The current authors hope that this alternative interpretation will provide additional insight into the viscous response of undisturbed and reconstituted London clay.

This paper uses both two-dimensional (2D) and three-dimensional (3D) finite element (FE) analyses to examine three cases involving the construction of full-scale test embankments to failure on soft clay deposits. By comparing the calculated fill thickness at failure from 2D and 3D analyses, it is shown that 3D effects are significant for all test fills, despite the dramatically different locations, fill thicknesses, and underlying clay deposits. In addition, the calculated undrained displacement and extent of failure from 3D analysis agree well with those measured in each case. The risk of neglecting 3D effects is highlighted by the analyses, where it is shown that failure to account for 3D effects while interpreting the response of a test embankment can lead to unsatisfactory performance of the actual long embankment. Finally, by comparing FE analysis results with well-known bearing capacity factors, it is shown that test embankments with a base length to width ratio less than 2 are more strongly influenced by 3D effects than spread footings on similar soil profiles. The analyses presented in this paper provide practical insight into some factors that should be taken into account for the design and construction of embankments and test fills on soft clay deposits.