A Three‐Surface Elastic‐Viscoplastic Cyclic Model of Clay

ABSTRACT Experimental evidence has shown that the cyclic behavior of clayey soils is strongly influenced by several factors, including the number of loading cycles, cyclic stress amplitude, loading frequency, and overconsolidation ratio (OCR). However, most existing models pay limited attention to the influence of loading frequency, particularly in scenarios involving a large number of cycles. To address this gap, this paper develops a three‐surface elastic‐viscoplastic cyclic model for clays. The proposed model incorporates several notable features: (1) a unified framework that integrates bounding surface theory and overstress theory through the introduction of a dynamic bounding surface, enabling the model to capture both viscous and cyclic behaviors; (2) a viscoplastic modulus associated with the dynamic loading surface, which incorporates the effect of accumulated viscoplastic strain, allowing for flexible control of axial strain and pore water pressure development under cyclic loading paths; and (3) consideration of anisotropy in both elastic and viscoplastic responses, together with a non‐associative flow rule, which improves the model's ability to simulate complex cyclic behavior. The model's performance is evaluated by comparing its predictions with results from four series of cyclic undrained triaxial tests, which vary in the number of cycles, cyclic stress amplitudes, loading frequencies, and OCRs. To maintain simplicity, the proposed formulation does not include destructuration, kinematic hardening, or mapping rules with relocatable projection centers. These features can be incorporated later if required by specific applications.