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Abstract The strength characteristics of deep rock mass exhibit distinctly nonlinear response to excavation. Based on triaxial compressive tests and unloading confining pressure tests results under high stress in laboratory，the behaving nonlinear strength property is validated. Moreover，the nonlinear strength parameters¢ change laws by loading and unloading paths of rock failure are brought into research. The quadratic parabola，hypobola，and power types of envelope functions are employed to present nonlinear strength characteristics；and the results reveal that the power type of Mohr criterion can favourably describe the strength property of rock under high geostress in loading path as well as unloading path. The triaxial unloading failure strength is usually less than that under triaxial compressive strength at low stress level，and reversed at higher stress level. According to relationship of internal friction angle with equivalent normal stress，the intrinsic friction angle is not constant，which takes on power functional type of nonlinear characteristics and attenuates gradually to angle of π/4.
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Received: 10 August 2009
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