Accurate continuum modelling of granular flows is essential for predicting geohazards such as flow-like landslides and debris flows. Achieving such precision necessitates both a robust constitutive model for granular media and a numerical solver capable of handling large deformations. In this work, a novel unified phase transition constitutive model for granular media is proposed that follows a generalized Maxwell framework. The stress is divided into an elastoplastic part and a viscous part. The former utilizes a critical-state-based elastoplasticity model, while the latter employs a strain acceleration-based mu(I) rheology model. Key characteristics such as nonlinear elasticity, nonlinear plastic hardening, stress dilatancy, and critical state concept are incorporated into the elastoplasticity model, and the non-Newtonian mu(I) rheology model considers strain rate and strain acceleration (i.e., a higher-order derivative of strain) to capture changes in accelerated and decelerated flow conditions. A series of element tests is simulated using the proposed unified phase transition model, demonstrating that the novel theory effectively describes the transition of granular media from solid-like to fluid-like states in a unified manner. The proposed unified model is then implemented within the material point method (MPM) framework to simulate 2D and 3D granular flows. The results show remarkable consistency with results from experiments and other numerical methods, demonstrating the model's accuracy in capturing solid-like behaviour during inception and deposition, as well as liquid-like behaviour during propagation.
