Abstract: The paper describes a proposed unified hydro-elasto-plastic model with a solid-fluid transition.The model associates a hydro-elasto-plastic model for partially saturated geomaterials, a Bingham’s viscous law, and the transition criterion between solid and fluid states.The model describes both solid and fluid behaviors of partially saturated geomaterials in a unified framework.In addition, this paper describes a novel Finite Element Method with Lagrangian Integration Points (FEMLIP) formulation for solving hydro-mechanical problems.Based on the equilibrium equation of momentum and the continuity equation of water flow, the formulation was developed and implemented in a FEMLIP tool.Bishop’s effective stress expression and proper water retention diagrams were taken into account.A heuristic column and real rainfall-induced mudflows were also simulated and analyzed in this study.The permeability effects of partially saturated soil and the effective cohesion were considered, which included consideration of the entire mudflow process.The results were proven satisfactorily in a qualitative fashion.