Abstract：The Hoek-Brown failure criterion and the generalized Hoek-Brown failure criterion are widely used in rock engineering. But the criteria need improvements as they do not consider the effect of the intermediate principal stress. A couple of twin shear stress models are presented to take account of the twin shear stresses by Yu Maohong，two normal stresses and two or one smaller principal stress. Based on these models，the unified strength theory by Yu Maohong，the nonlinear unified strength criteria and generalized nonlinear unified strength criteria proposed by Zan Yuewen，Yu Maohong and Wang Sijing can be established. Meanwhile the Hoek-Brown failure criterion is one special case of them. Rock’s failure mode transfers from brittle failure to ductile failure when its stress state develops from low level to high level. The proposed criteria are of clear physical concept and wide prospect of application
