At present, the understanding of structural failure and energy analysis of sandstone under hydrofracture is still insufficient. This time, we selected the sandstone in Chongqing Three Gorges Reservoir Area as the main analysis object to discuss and compare the characteristics of conventional uniaxial (triaxial) compression strength test and uniaxial (triaxial) compression hydraulic fracturing strength test and conduct instability analysis from the perspective of energy. Based on the mechanical characteristics and parameters about the uniaxial (triaxial) compressive strength test and uniaxial (triaxial) compressive hydrofracture strength test, we focus on the analysis of the evolution rules of the hydraulic pressure-strain curve, analyzing the criteria of crack expanding and instability by using energy principle. According to the viewpoint of fracture mechanics, the fracture morphology and failure type of sandstone under hydrofracture were discussed. The test results show that the deformation evolution rules of rock hydrofracture can be divided into four stages, including the characteristics of pore fissure water injection stage (OA), the elastic deformation stage (AB), the volume expansion stage (BC), and global rupture stage (CD). Using the P/C modulus (the ratio of hydraulic pressure and cohesive force, abbreviated as P/C), the ability of hydraulic pressure overcoming cohesive force can be evaluated during hydrofracture. Using energy variables (expressed by E k ) about crack expanding from beginning to end, the unexpanded state, critical state, unstable state, and unstable failure of crack can be estimated. There is an interaction between tensile deformation and shear deformation from the crack initial stage to crack expanding stage.
Considering the increasing frequency of geological disasters related to groundwater activities, it is important to study the relationship between geological dislocation and groundwater flow for the safety assessment of engineering rock mass stability. To elucidate the non-linear seepage characteristics at rock discontinuities during shearing, a custom-made device was used to conduct seepage tests at discontinuities that exhibit varying undulation angles and different shear displacements. The results show that as the shear displacement increases, the shear stress at a structural plane involving different undulation angles fluctuates with an increasing trend. Based on an identical shear displacement condition, the shear strengths of the structural planes increase as the undulation angle increases, and this enhances the shear expansion. Concerning an identical fluctuation angle and hydraulic gradient, the seepage flow at a structural plane increases as the shear displacement increases. By contrast, both the linear term coefficient a and non-linear term coefficient b in the Forchheimer fitting equation decrease as the shear displacement increases. In addition, the critical Reynolds number initially increases, followed by stabilisation as the shear displacement increases, and this number varies between 9.65 and 1758.52. The shear fracture morphology of the structural plane exhibits obvious anisotropy. During shearing, the roughness coefficient decreases in all but the vertical direction. The dominant seepage channel is perpendicular to the shear direction. The findings can provide a valuable reference for the stability research and analysis of rock slopes with structural planes.
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