The tangential–subloading surface model can describe the normal-plastic strain rate due to the rate of stress inside the yield surface and the tangential-plastic strain rate due to the stress rate component tangential to the subloading surface. The localized and diffuse bifurcation modes of the rectangular soil specimen subjected to plane strain loading under the undrained conditions are analyzed by adopting the tangential–subloading surface model, exhibiting the characteristic regimes of the governing equations (elliptic, hyperbolic and parabolic) in this article. Further, the conditions for shear band formation, shear band inclination, diffuse bifurcation modes, and the long and short wavelength limit of diffuse modes are discussed in relation to material properties and the state of stress, i.e. the stress-ratio and the normal-yield ratio. The tangential-plastic strain rate makes the inception of bifurcation modes easier in not only normal-yield, but also subyield states.

Keywords