Clays are complex natural materials and their behavior is affected by many factors, including the nature of the constituents, anisotropy, stress history, time-dependent effects, among others. Traditionally, most clay models were developed based on a particular type of clay and the validation was rather limited. Therefore, a clay model that may be applicable to different types of clays is greatly needed.;In this study, by investigating a large number of anisotropic clays, a form of Dafalias' yield surface is adopted. The resulting clay model is an anisotropic critical state elastoplastic-viscoplastic bounding surface model with a nonassociative flow rule, and is capable of describing the following aspects of clay behavior in a hierarchical order: anisotropy, overconsolidated nature or cyclic loading behavior, and time-dependent effects. The model has 14 parameters associated with it. The parameters have been extensively explained, with the focus on the physical meaning, and a procedure for their calibration is presented. Due to the hierarchical feature of the model, not all parameters are necessary in all applications. The simplest form of anisotropic bounding surface model requires only 2 additional parameters compared to the Modified Cam-clay model.;The simulative capacity of the proposed model is validated against various types of clays under different test conditions. In all cases, the simulations showed very good agreement with the test results. This indicated that the proposed model has great potential to realistically describe different types of clays, whether strain-hardening or strain-softening, for time-independent or time-dependent response under general stress paths and under drained or undrained conditions. The general stress paths here may refer to different shearing modes (triaxial, plane strain, true triaxial, and torsional simple shear), monotonic or cyclic loading conditions, and varying overconsolidation ratios.
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