Study of the Mechanical Properties and Fracture Evolution of Sandstone with Different Moisture Contents Under True Triaxial Stress

The typical sandstone of the Shendong mining area, China, was considered as the research object in order to determine the mechanical properties and fracture evolution characteristics under true triaxial stress and different moisture contents by X-ray diffraction, true triaxial mechanical test, and CT scanning. The results show that the main mineral components of the sandstone are quartz, plagioclase, potassium feldspar, siderite, and clay minerals (kaolinite, illite, and chlorite), with clay minerals accounting for 25.7%. When σ2 > σ3, after sandstone failure, compressive deformation occurs in the ε2 direction, while expansion deformation occurs in the ε3 direction. A larger σ3 corresponds to a higher moisture content and a greater ductility of sandstone. σ3 influences the sandstone failure mode more significantly, compared with σ2 and moisture content. For a constant moisture content, the sandstone’s compressive strength and the Young’s modulus increase with increase in σ3. On the contrary, as σ2 increases, the compressive strength and the Young’s modulus first increase and then decrease for all moisture contents. Additionally, when the stress state is kept the same, as the moisture content increases, the compressive strength and the Young’s modulus of the sandstone decrease. Similarly, for the same moisture content, with increase in σ2, the smaller the number of fractures after sandstone failure and the more regular the fracture distribution. Moreover, under the same stress conditions, dry sandstone has the least regular fracture development after failure, followed by saturated sandstone, while the sandstone with natural moisture content has the most regular fracture development after failure.