Finite element implementation of the Hoek-Brown material model with general strain softening behavior

A numerical implementation of the Hoek–Brown criterion is presented, which is capable of modeling different post-failure behaviors observed in jointed rock mass. This is done by making the material parameters a function of the accumulated plastic strain. The implementation is for use in finite element calculations, and is based on the return mapping framework. The updated stress state together with the consistent constitutive matrix is found in principal stress space based on the principles of boundary planes. The implementation is verified through the simulation of a tunnel excavation for perfectly-plastic, brittle and strain softening material behavior and the results are compared with known solutions.