Influence of the Intermediate Principal Stress on Sandstone Failure

This paper reports the results of fracture testing of sandstone under constant minor principal stress (20 MPa) and various intermediate principal stresses. The results show that when the minor principal stress is constant, as the intermediate principal stress increases, the ratio of the octahedral shear stress (τ_oct) to the octahedral normal stress (σ_oct) decreases. The strength criterion of τ_oct/σ_oct = f(σ₂) is obtained. This criterion reflects not only the hydrostatic stress and intermediate principal stress effects but also the Lode angle effect. This criterion reveals the reason why the rock strength increases and then decreases with increasing intermediate principal stress. The decreasing trend is fitted by linear, logarithmic and Boltzmann equations. The applicability of the three fitting equations for strength prediction and the π plane strength envelopes is analysed, and the results of the Boltzmann fitting equation are the best. The deformation characteristics of rock during the failure process are analysed. The changing process of the tangential deformation modulus of the rock is found to be divided into three stages during the loading process: an increasing stage, an initial decreasing stage and a rapidly decreasing stage. Based on an analysis of computed tomography (CT) images of the internal fractures of rock and photographs of the fracture surfaces, the internal fractures are found to be clear and smooth, and the shear stresses in the fracture surfaces are strengthened with increasing intermediate principal stress. The dominant shear stress in the process of failure is considered to cause these phenomena.