Article

CASM: A Unified State ParameterModel for Clay and Sands

International Journal for Numerical and Analytical Methods in Geomechanics (Impact Factor: 1.56). 08/1998; 22(8):621 - 653. DOI: 10.1002/(SICI)1096-9853(199808)22:8<621::AID-NAG937>3.0.CO;2-8

ABSTRACT The purpose of this paper is to present a simple, unified critical state constitutive model for both clay and sand. The model, called CASM (Clay And Sand Model), is formulated in terms of the state parameter that is defined as the vertical distance between current state (v, p′) and the critical state line in v–ln p′ space. The paper first shows that the standard Cam-clay models (i.e. the original and modified Cam-clay models) can be reformulated in terms of the state parameter. Although the standard Cam-clay models prove to be successful in modelling normally consolidated clays, it is well known that they cannot predict many important features of the behavior of sands and overconsolidated clays. By adopting a general stress ratio-state parameter relation to describe the state boundary surface of soils, it is shown that a simple, unified constitutive model (CASM) can be developed for both clay and sand. It is also demonstrated that the standard Cam-clay yield surfaces can be either recovered or approximated as special cases of the yield locus assumed in CASM.The main feature of the proposed model is that a single set of yield and plastic potential functions has been used to model the behaviour of clay and sand under both drained and undrained loading conditions. In addition, it is shown that the behaviour of overconsolidated clays can also be satisfactorily modelled. Simplicity is a major advantage of the present state parameter model, as only two new material constants need to be introduced when compared with the standard Cam-clay models. © 1998 John Wiley & Sons, Ltd.

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    • "As compared with metal plasticity (Hill, 1950), the most significant progress in the model is the introduction of the soil voids ratio into constitutive modelling of soil behaviour and the modelling of the stress-ratio-dependent volumetric deformation. Since then a large number of models have been developed within the theoretical framework of Cam Clay, characterised by the existence of a critical state of deformation as the final failure state and volumetricdeformation-dependent hardening (e.g., Gens and Potts, 1988; Yu, 1998; Liu and Carter, 2000a). These models generally successfully represent the behaviour of laboratory reconstituted soils under various circumstances. "
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    • "The choice of the yield function is an important step, because the strain rates depend essentially on its derivatives with respect to stress. The sandy silt used in the current experimental studies is predominately fine-grained cohesionless soil, and therefore the following yield function, that has been successfully employed for fine sand [42] [51], was also adopted in this study: "
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    • "As compared with metal plasticity (Hill, 1950), the most significant progress in the model is the introduction of the soil voids ratio into constitutive modelling of soil behaviour and the modelling of the stress-ratio-dependent volumetric deformation. Since then a large number of models have been developed within the theoretical framework of Cam Clay, characterised by the existence of a critical state of deformation as the final failure state and volumetricdeformation-dependent hardening (e.g., Gens and Potts, 1988; Yu, 1998; Liu and Carter, 2000a). These models generally successfully represent the behaviour of laboratory reconstituted soils under various circumstances. "
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