To testify the Mogi-Coulomb criterion in triaxial unloading test and based on available unloading testing data, the Mogi-Coulomb strength criterion with a linear fitting method was used in the conventional triaxial unloading tests and the true triaxial unloading tests, respectively. A comprehensive comparative analysis was presented for the related strength parameters according to the fitting results and combination of the failure characteristics of rock specimens. The results showed that the Mogi-Coulomb strength criterion has a good applicability to rock samples under conventional triaxial unloading conditions. However, under true triaxial unloading tests, even though the fitting degree is very good, the fitting parameter a is negative which leads to a negative cohesion of rock samples. This is impossible in practice. Since the Mogi-Coulomb strength criterion is based on the assumption that rock fails in shear, it cannot describe the failure of rock under true triaxial unloading conditions in which the failure modes of rocks specimens may change from shear to extensional slabbing or rock burst. Therefore, for combined shear and tensile failure processes of rocks under true triaxial unloading conditions, it is necessary to build a new strength criterion to describe its constitutive relationships, which is important for deep rock mechanics.

Mogi-Coulomb strength criterion  /  triaxial unloading test  /  failure mode  /  rockburst  /  data fitting