The study of bifurcation criteria for non-isothermal processes in geomaterials requires approaches that deviate from the classical material bifurcation approach. Indeed, in a quasi-static stress state of the medium, the admissible equilibria are of steady-creep-type and are governed also by the energy balance. Furthermore the steady-state temperature profiles are far from homogeneous as shown by [3], leading the analytical stability analysis to be rather complex. In this contribution, we adopt an overstress plasticity approach and present approximations for the criteria of the onset of localisation of deformation in a plane strain setting, using numerical continuation methods.