Characterizing the temperature-dependent shear mechanical responses of rock masses under constant normal stiffness (CNS) boundary conditions is of crucial importance for evaluating the stability and performance of deep underground projects. This paper experimentally analysed the shear mechanical properties and dilatancy deformation of sandstone exposed to high temperature with respect to various initial normal stresses under a constant normal stiffness. The results indicate that the developed thermally induced defects cause the porosity of sandstone to increase by 42.48% in a temperature range of 25–800 °C, while the P-wave velocity, unit weight and fractal dimension of pores are reduced. A typical shear failure process including a fracture surface generation process and a shear slipping process of surface asperities is identified. Due to the formation of fracture surfaces, both the normal displacement and normal stress curves show notable sudden drops. The peak shear strength, residual shear strength and terminal normal stress all display an exponential variation with temperature, i.e., initial fluctuations or a slight increase, then a dramatic decrease, achieving a threshold temperature of 400 °C. The secant peak shear stiffness declines by 43.79–70.48% in a temperature range of 400–800 °C due to enhanced ductility and decreasing peak shear strength. With increasing initial normal stress, both shear strength and terminal normal stress increase, but the terminal normal displacement decreases by 52.68–57.37% due to weakened dilation effects. The normal stress–shear stress variation paths are plotted, and the apparent internal friction angle decreases with temperature. Two representative failure patterns, including shear off of surface asperities and edge spalling of the rock matrix, are identified. Both the shear area and mass loss ratios of the sheared rock samples increase with both temperature and initial normal stress due to weakened shear strength and strong shear dilation inhibition effects.

Shear mechanical responses of sandstone exposed to high temperature under constant normal stiffness boundary conditions

Compression-shear failure properties and acoustic emission (AE) characteristics of rocks in variable angle shear and direct shear tests

Fractal permeability model for a complex tortuous fracture network

The research analyzes and improves upon the concept of ventilation safety subregions in coal mines and proposes a partition method based on the breadth-first search algorithm for assessing the quality of the ventilation. This involves the analysis of the function of ventilation subregions and of ventilation gas-monitoring data. Then, using the so-called ventilation sensitivity matrix as the analysis method, we confirm the consistency of considering safety subregions, which are associated with air consumption places as the core concept and the objective positioning of the safety subregions within the ventilation analysis. This allows us to establish the validity and practicability of employing regional characteristic information of mine ventilation and gas concentrations in the mine’s air. Finally, based on the characteristics of the ventilation subregions, ventilation air quantity network maps with safety subregions are proposed and their application is demonstrated. The advantages and characteristics of using these maps to replace ventilation network maps for ventilation analysis are demonstrated.

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Research on Safety Subregion Partition Method and Characterization for Coal Mine Ventilation System

When the deep tunnel is excavated, the pressure of the confined water is relatively high, causing the water inrush to have a hydraulic fracturing effect. The method of theoretical analysis was adopted to study this effect. A mechanical model for fracturing water inrush under blasting excavation conditions was established. The water inrush under this condition is the result of the combined action of static load (water pressure and in situ stress) and dynamic load (explosive stress wave). According to whether the normal stress on the hydraulic crack surface was tensile stress or compressive stress, two types of water inrush were proposed: water inrush caused by tensile-shear damage and water inrush caused by compression-shear damage. These two types of critical water pressures were calculated separately. The relationship between critical water pressure, in situ stress, and blasting disturbance load was given, and a pore water pressure splitting factor was introduced in the calculation process. The theoretically obtained critical water pressure had been verified in the case of water inrush in a deep-buried tunnel. The established theory can guide field practice well.

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Mechanical Mechanism of Hydraulic Fracturing Effect Caused by Water Inrush in Tunnel Excavation by Blasting

The permeability evolution of fractal-based two-dimensional discrete fracture networks (DFNs) during shearing under constant normal stiffness (CNS) boundary conditions is numerically modeled and analyzed based on a fully coupled hydromechanical (HM) model. The effects of fractal dimension, boundary normal stiffness and hydraulic pressure on the evolutions of mechanical behaviors, aperture distributions and permeability are quantitatively investigated. The results show that with increasing confining pressure from 0 to 30 MPa, the permeability decreases from the magnitude of 10−13 m2 to 10−16 m2, which is generally consistent with previous models reported in the literature. With the increment of shear displacement from 0 to 500 mm, the variations in shear stress, normal stress and normal displacement exhibit the same patterns with the conceptual model. As shear advances, the permeability evolution exhibits a three-stage behavior. In the first stage, the permeability decreases due to the compaction of fractures induced by the increasing shear stress from 0 to the peak value. In the second stage, the permeability holds almost constant values under constant normal load (CNL) boundary conditions, whereas that under CNS boundary conditions decreases by approximately one order of magnitude. Under CNS boundary conditions, although the aperture of shearing fracture increases enhancing its own permeability, the apertures of surrounding fractures are compacted due to the simultaneous increases in the normal and shear stresses, which result in the decrease in the total permeability of DFNs. When the fractal dimension increases from 1.4 to 1.5, the permeability increases following exponential functions in the early stage of shear, which fail to characterize the permeability in the residual stage due to the complex flow path distributions. At the start of shear, the ratio of permeability perpendicular to the shear direction to that parallel to shear decreases approximately from 1.0 to 0.5 and then gradually decreases from 0.5 to 0.3 in the residual stage. The hydraulic pressure tends to open up the fractures and enhances the permeability. The magnitude in permeability enhancement is of approximately the same order with the increase in the hydraulic pressure.

Ye Wang

Papers

Permeability Evolution of Two-Dimensional Fracture Networks During Shear Under Constant Normal Stiffness Boundary Conditions