Abstract:Based on experiments of fine sandstone specimens,it is found that the strength is in relation to average modulus when strength is lower,and the modulus becomes a constant when specimen is compressed under higher confining pressure. Rock specimen with sedimentary weak planes may be destroyed in shear or combination of shear and tensile. The strengths and confining pressure can not be directly used to regress for the Coulomb criterion. The cohesion and internal friction coefficient of sound rock and weak plane can be determined after classification of failure specimens under pseudo-triaxial compression results.