Abstract：The twin-shear theory with a series of equations has not been compared directly with other true triaxial strength criteria on misfits for test data，although it has been studied thoroughly and applied widely in engineering. Linear and nonlinear unified strength theories are constructed with principal stresses. Parameters in various strength criteria are determined using true triaxial strengths，conventional compression and extension strengths as well. The theories present high mean misfits for test data，and cannot describe nonlinear increasing of conventional compression and extension strength with the minor principal stress，and the effect of the intermediate principal stress on strength. The nonlinear theory with two implicit equations of principal stresses is not friendly in calculation. The unified strength theories are not available in determining geostresses from borehole collapses and wellbore stability，as strength of rocks are extremely over-estimated at low minor principal stress.
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2013, Vol. 32 
