Given the contrasting behaviour observed for geomaterials, for example during landslides of the flow type, this contribution proposes an original constitutive model which associates both an elasto-plastic relation and a Bingham viscous law linked by a mechanical transition criterion. This last is defined as the second-order work sign for each material point, which is a general criterion for divergence instabilities. FEMLIP method (Finite Element Method with Lagrangian Integration Points) is chosen as a framework for implementing the new model because of its well-known ability to deal with both solid and fluid behaviours in large deformation processes. A first boundary model considering a sample of initially stable soil, a slope, and an obstacle is performed. The results show the power of the constitutive model since the consistent evolution of initiation, propagation and arrest of the mudflow is described. A parametric study is led on both plastic and viscous parameters to determine their influence on the flow development and arrest. Finally, forces against the obstacle are compared with good agreement with those of other authors for the same geometry and a pure viscous behaviour.