The behavior of jointed rocks depends on the intrinsic structure, surrounding geological factors and prevailing stress conditions. Discontinuities are one of the effective parameters on strength reduction of rocks that cause anisotropic behavior in strength and deformability. Thus behavior and effect of planes of weaknesses must be considered in the design and analysis. The engineering response of the jointed rock mass under unconfined or very low confining pressure conditions is governed by both the properties of intact rock and joints. The object of this study was the investigation of anisotropic effect of weak planes (schistosity planes) on uniaxial strength and the resulted fracture mechanism. For this purpose, uniaxial compression test was carried out on schistose rock cores. Test specimens were taken from Angoran mine, Zanjan province, Iran. Maximum strength was obtained for samples with the weak planes inclination of 0 degree and minimum strength was obtained for samples with the weak planes inclination of 60 degree. It was observed that the dominant failure modes were changed based upon the applied load angle with respect to the weak planes orientation. These failure modes were of three types; tensile fracture across discontinuities (TM), sliding failure along discontinuities (SD) and tensile-split along discontinuities (TD).
Investigation of the engineering properties of rocks and determination of the mechanical specifications play a dominant role in the design of surface or underground structures in rock. Engineering properties of rock mass depend on several factors like its geological, environmental, lithological, hydrological and mechanical specification. Anisotropy is an important phenomenon that must be noticed in order to have a perception of strength and deformability of rock. Thus, the investigation on strength properties and fracture mechanism of anisotropic rocks are necessary in order to assess their behavior.