(State Key Laboratory for Geomechanics and Deep Underground Engineering，School of Mechanics and Civil Engineering，China University of Mining and Technology，Xuzhou，Jiangsu 221008，China)
Abstract：Rock material strength will degrade with the damage in rock failure process. This paper reports one self-designed method to test rock strength degradation and introduces the technical routine，sample preparation and its key technique in detail. Through self-designed direct shear tests，the relationships between shear stress and compressive stress of irregular damaged rock samples which are produced in common uniaxial and triaxial compressive experiments are obtained；and a Coulomb strength curve can be gained by the linear regression through compression-shear testing data. A comparison between the Coulomb strength curve and the ultimate Mohr circle envelope linearly-regressed by previous uniaxial and triaxial compressive experimental data is performed；and the evolution laws of material strength(cohesion and internal friction angle) in rock failure process are analyzed and discussed. The application scopes of two kinds of completely reverse evolution laws of cohesion and internal friction angle are clarified through the analysis and discussion. The research results show that the cohesion measured by uniaxial and triaxial experiments of intact rock samples is obviously greater than that obtained by direct shear tests of irregular rock blocks. This difference mainly resulted from the damage in rock failure processes of uniaxial and triaxial compressive experiments，not from the deviation between different testing methods. Cohesion reflects rock intrinsic strength characteristic，slightly influenced by different stress states. The internal friction angle obtained by uniaxial and triaxial experiments is smaller than that gained by direct shear tests of rock blocks. This difference mainly resulted from the influence of different stress states and rock flaw distribution. Internal friction angle increases rapidly to the maximum and decreases steeply to keep a certain stable trend in rock failure process. Internal friction angle reflects rock frictional strength characteristic，heavily influenced by different stress states. The sensitivity of cohesion to stress level is far lower than that of internal friction angle. The material strength itself degrades obviously before and after rock failure.
