SANICLAY-RD: A model for rate-dependent effects under complex loading paths

The rate-dependent behaviours of clay, including creep, stress relaxation, and strain-rate dependency, play a critical role in predicting soil behaviour. However, existing constitutive models exhibit limitations in capturing these effects under complex stress paths, such as those resulting from loading-unloading cycles. Moreover, existing soil models are not yet equipped to reproduce transitions from stable creep to delayed failure. This study addresses these shortcomings by proposing four enhancements of the SANICLAY constitutive framework, here referred to as SANICLAY-RD. First, the projection center is allowed to evolve with the viscoplastic strain in a way that reflects the available evidence for delayed deformation. Second, an evolution law for the static loading surface quantifying the overstress at the core of the viscoplastic deformation kinetics is proposed. Third, a hybrid flow rule is used to adjust the viscoplastic flow during complex loading cycles by considering contributions from both the actual and the image stress states. Finally, the viscous constants are allowed to evolve dynamically with the stress state to capture accelerating creep. Validation against existing experimental results demonstrates the capability of the model to replicate the rate-dependent behaviour of clays subjected to a wide range of past stress histories and complex loading cycles. Additionally, the model is shown to effectively captures the transition from stable (i.e., primary) creep to unstable (i.e., tertiary) creep. This model provides a robust tool for predicting the long-term behaviour of soils in geotechnical applications, such as the stability of slow-moving landslides, as well as the long-term settlement of earthen infrastructures.