Experimental Study of Mechanical Behavior and X-Ray Micro CT Observations of Sandstone Under Conventional Triaxial Compression

This paper reports a series of triaxial compression experiments and X-ray observations carried out to explore the mechanical behavior and internal damage mechanism of sandstone material. The results show that the Young's modulus of sandstone increased nonlinearly with increasing confining pressure, but the Poisson's ratio remained unaffected. The nonlinear Hoek-Brown criterion can better reflect the peak strength properties than the linear Mohr-Coulomb criterion. However, the residual strength of sandstone exhibits a clear linear relationship with the confining pressure, which can be best described by the linear Mohr-Coulomb criterion. The sensitivity of the crack damage threshold on the confining pressure was clearly lower than that for the peak strength. After unaxial and triaxial compression failure, the sandstone specimens were analyzed using a 3D X-ray micro CT scanning system. Based on the horizontal and vertical cross-sections of sandstone specimens, we found that under uniaxial compression and lower confining pressure, the sandstone specimen is dominated mainly by axial splitting tensile cracks; however, under higher confining pressure, the sandstone specimen is mainly dominated by a single shear crack. To quantitatively evaluate the internal damage of sandstone material, crack area and aperture extent for each horizontal cross-section were calculated by analyzing the binarized pictures. The system of crack planes under uniaxial compression is much more complicated than that under triaxial compression, which is also testified by the evolution behavior of crack area and aperture extent. Finally, the fracture mechanism of sandstone during uniaxial and triaxial compression is discussed in detail and simplified models are proposed.