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Red sandstone is selected for freeze–thaw cycles, nuclear magnetic resonance, and triaxial compression tests to study the changes in macro-meso physical and mechanical properties under freeze–thaw cycles. Based on the expansion characteristics of meso structure, the porosity and fractal dimension are introduced to determine initial and freeze–thaw damage variables. During loading process, considering the dynamic evolution process from non-damage to damage, the load damage variable is determined. Based on the impact of reducing the effective bearing area on each damage, the total damage variable and constitutive prediction model of rock under freeze–thaw cycles are established. The results show that with the increase of freeze–thaw cycles, the meso structure undergoes penetration and expansion, and the porosity increases by nearly 30% when freeze–thaw cycles reaches 60 times. From a macro perspective, it shows that the decreases of compressive strength and deformation resistance. With the increase of confining pressures, the pores are compacted, and the lateral deformation is limited. At the same time, the bonding force between particles is strengthened, so the damage is suppressed. Macroscopically, it shows that the resistance to failure is enhanced. Under freeze–thaw cycles and confining pressure, the predicted mechanical parameters have a small deviation from test obtained mechanical parameters, generally not exceeding 5%. So the prediction model can describe the entire process of deformation and failure of rock under freeze–thaw and load, and can effectively reduce mechanical parameters data required to determine model parameters, making model more adaptable, so as to provide a new idea for the theoretical research of rock mechanics in cold regions. [ABSTRACT FROM AUTHOR]

This study investigates the structural characteristics of SFRC with different amounts of steel fibers following exposure to freeze–thaw cycles, while taking into account various levels of confinement pressure and rates of deformation. The focus of the research is to examine the dynamic mechanical properties of SFRC exposed to freeze–thaw cycles. The inclusion of steel fibers improves the strength of concrete during freeze–thaw cycles, with 1% steel fiber content being the most effective. Strain rate and confining pressure significantly impact the strength and failure mode of SFRC. The strength of concrete increases linearly with the strain rate. With no confining pressure, the cracks in the concrete specimen align with the direction of the applied stress. And with confining pressure, concrete exhibits diagonal shear failure. Microstructural analysis results from scanning electron microscopy are consistent with the macroscopic properties. [ABSTRACT FROM AUTHOR]

This study investigates the structural characteristics of SFRC with different amounts of steel fibers following exposure to freeze–thaw cycles, while taking into account various levels of confinement pressure and rates of deformation. The focus of the research is to examine the dynamic mechanical properties of SFRC exposed to freeze–thaw cycles. The inclusion of steel fibers improves the strength of concrete during freeze–thaw cycles, with 1% steel fiber content being the most effective. Strain rate and confining pressure significantly impact the strength and failure mode of SFRC. The strength of concrete increases linearly with the strain rate. With no confining pressure, the cracks in the concrete specimen align with the direction of the applied stress. And with confining pressure, concrete exhibits diagonal shear failure. Microstructural analysis results from scanning electron microscopy are consistent with the macroscopic properties.

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In this study, the dynamic mechanical properties of steel fiber reinforced concrete under the influence of freeze–thaw cycles were studied. The studied parameters include steel fiber content (0%, 1% and 2%), confining pressures (0, 5 and 10 MPa) and strain rates (10−5/s, 10−4/s, 10−3/s and 10−2/s). Performance was also evaluated, including triaxial compressive strength, peak strain, the relationship between stress and strain, failure mode and microstructure. The results show that with the increase in F–T cycles, the compressive strength and energy absorption capacity of concrete gradually decrease. The mechanical properties of concrete increased with the addition of steel fibers during F–T cycles, and the optimum amount of steel fiber to enhance resistance to F–T cycles is 1% within the evaluation range. In this study, the effects of strain rate and confining pressure on the strength and failure mode of concrete after fiber addition are studied. Both the dynamic increase factor and the concrete strength increase linearly with the increase of strain rate, the dynamic increase factor is characterized by an increase in intensity caused by strain rate. When there is no confining, the crack direction of the concrete specimen is parallel to the stress loading direction, and when there is confining, it is manifested as oblique shear failure. The results of scanning electron microscopy analysis of the microstructure demonstrate the performance results at the macroscopic level (compressive strength and peak strain). [ABSTRACT FROM AUTHOR]

In this study, the dynamic mechanical properties of steel fiber reinforced concrete under the influence of freeze–thaw cycles were studied. The studied parameters include steel fiber content (0%, 1% and 2%), confining pressures (0, 5 and 10 MPa) and strain rates (10−5/s, 10−4/s, 10−3/s and 10−2/s). Performance was also evaluated, including triaxial compressive strength, peak strain, the relationship between stress and strain, failure mode and microstructure. The results show that with the increase in F–T cycles, the compressive strength and energy absorption capacity of concrete gradually decrease. The mechanical properties of concrete increased with the addition of steel fibers during F–T cycles, and the optimum amount of steel fiber to enhance resistance to F–T cycles is 1% within the evaluation range. In this study, the effects of strain rate and confining pressure on the strength and failure mode of concrete after fiber addition are studied. Both the dynamic increase factor and the concrete strength increase linearly with the increase of strain rate, the dynamic increase factor is characterized by an increase in intensity caused by strain rate. When there is no confining, the crack direction of the concrete specimen is parallel to the stress loading direction, and when there is confining, it is manifested as oblique shear failure. The results of scanning electron microscopy analysis of the microstructure demonstrate the performance results at the macroscopic level (compressive strength and peak strain).

Understanding the creep behaviours of rocks is essential for the long-term stability of underground excavations in mining engineering. Creep behaviours are more important when the mining depth is greater, which leads to the emergence of weak rock masses and high in situ stresses. In this study, the creep behaviours of argillaceous sandstone (AS) were systematically investigated. For the experimental investigation, creep tests were conducted on AS with different confining pressures (3, 6, 9, 12, 15, and 18 MPa) using an MTS815.02 rock mechanics test system. The mechanical characteristics of AS were analysed. For the numerical study, a nonlinear creep model of AS under equal and different confining pressures was established based on rock creep theory and plastic theory. The results showed that confining pressure could effectively improve the creep failure strength of AS, accelerating its creep deformation rate and process and reducing the final expansion volume. The nonlinear creep model was embedded in the FLAC3D software, and the experimental and numerical results agreed well. The experimental investigation and proposed creep model can provide important guidance in underground mines for safe long-term stability of underground excavations. [ABSTRACT FROM AUTHOR]

The dynamic weakening of rock is frequently observed when the rock is subjected to a series of repetitive impact loads. Sandstone is utilized in our experiment on the dynamic performance under impact loading. The split Hopkinson pressure bar system with a confining pressure device is used for the experiments. Observations on strain waves recorded by a transmission bar indicate that sandstone demonstrates an evident nonlinear response under impacting loads which accompany a dynamic weakening effect. To quantify the damage induced by impact loads, we introduce a damage factor to study the weakening degree induced by impact loading and determine that sandstone breaks when the damage factor exceeds 0.52. Detailed statistics on the broken characteristics of the explored sandstone samples show that the damage evolution mainly extends along the maximum shear stress face, thus resulting in shear failure. Based on nonlinear wave propagation theory, we determine a shock wave distance for the explored sandstone. Result shows that the shock wave distance is larger than the sample length. Moreover, from a uniaxial compression test on specimens after impact tests, we found that failure patterns develop from split failure to shear slip failure with the increase in damage factor. Moreover, experimental observations comprise numerous dynamic regimes which can be extended to other granular rocks. [ABSTRACT FROM AUTHOR]

Understanding the creep behaviours of rocks is essential for the long-term stability of underground excavations in mining engineering. Creep behaviours are more important when the mining depth is greater, which leads to the emergence of weak rock masses and high in situ stresses. In this study, the creep behaviours of argillaceous sandstone (AS) were systematically investigated. For the experimental investigation, creep tests were conducted on AS with different confining pressures (3, 6, 9, 12, 15, and 18 MPa) using an MTS815.02 rock mechanics test system. The mechanical characteristics of AS were analysed. For the numerical study, a nonlinear creep model of AS under equal and different confining pressures was established based on rock creep theory and plastic theory. The results showed that confining pressure could effectively improve the creep failure strength of AS, accelerating its creep deformation rate and process and reducing the final expansion volume. The nonlinear creep model was embedded in the FLAC3D software, and the experimental and numerical results agreed well. The experimental investigation and proposed creep model can provide important guidance in underground mines for safe long-term stability of underground excavations.

Accurate characterisation of the three-dimensional (3D) fracture network of coal is of vital significance to enhancing coal seam permeability during simultaneous extraction of deep coal and methane resources. The limitations of traditional experimental methods prevent direct in situ observation and characterisation of the 3D fracture network and its evolution during loading processes. This study presents a novel approach that incorporates computed tomography and servo-controlled triaxial loading techniques to accomplish the laboratory in situ observation of the continuous evolution of 3D fracture networks inside coal samples which were subject to confining pressures and axial compressive loads. Spatial growth and morphologies of the interior fractures at various loading stages were captured in situ and extracted using imaging processing algorithms. The 3D fracture networks observed at different loading stages were quantitatively characterised using fractal theory and compared to evaluate the influences of confining pressures and vertical loads on their evolution. The results indicated that the original existing fractures of coal closed when the specimens were subject to confining pressures and vertical compressive deformation were in the linear elastic stage. Load-induced fractures expanded notably only when the axial compressive load reached the maximum value. The fractal dimension of the 3D fracture network tended to decrease initially and subsequently increased during the loading process, which reflects the evolutionary characteristics of coal fractures from a closed to an expanded state. [ABSTRACT FROM AUTHOR]

In this research, a series of laboratory tests have been performed to investigate the effects of cement and polypropylene (PP) fiber on the triaxial behavior of sand. The cement contents were 0 and 5% by weight of the dry sand. Fiber length and diameter were 18 and 0.023 mm, respectively, were added at 0 and 0.6% by weight of dry sand–cement. Triaxial compression tests were performed at confining pressures of 0.1, 0.25, 0.5, and 1 MPa. The results of the study indicate that the inclusion of PP fiber increases the shear strength and the peak axial strain. The elastic modulus of specimen decreased with increase in fiber content and increased with the increase in cement content. Moreover, the initial stiffness and peak strength increased by increasing cement content. [ABSTRACT FROM AUTHOR]

The article discusses the study on a water-rich mudstone under cyclic train loading with regards to its accumulated deformation behavior and computational model. It states that numerous railroads in China have been built on or across soft rocks and soft soils which lead to various problems including accumulated deformation of the tracks. It details how the study was conducted and explores the characteristics of deformation under cyclic loading.

Stability control of surrounding rock of underground caverns in deep resources development must face the post-peak deformation and failure problems of cataclastic rock mass, but the research of the post-peak deformation and strength characteristics is insufficient at the moment, so often leading to the large volume collapse, large deformation and other major projects accident. The conventional triaxial compression test of pre-existing transfixion jointed rock cylindrical standard specimen was made by using RLW-1000 type triaxial servo rigid machine in order to study the post-peak strength and deformation characteristics of fractured rock masses in different confining pressures. The results show that the peak strength, residual strength and peak strain of the fractured specimens essentially increases with the confining pressure; The smaller the confining pressure, the more obvious post-peak expansion of fractured rock, rocks expansion with confining pressure decreases, and the combined of rock plasticity and confining pressure effect rock expansion. Combined with the test results analysis found that the post-peak discontinuous deformation control in the initial stage of the residual strength, and relatively low support resistance can effectively control the excessive rupture expansion deformation, to ensure the stability of tunnel surrounding rock. [ABSTRACT FROM AUTHOR]

This study experimentally analysed the influence of temperature levels (200, 300, 400, 500, 600, and 800 °C) on the permeability of granite samples. At each temperature level, the applied confining pressure was in the range of 10–30 MPa, and the inlet hydraulic pressure varied below the corresponding confining pressure. The results are as follows: (i) With an increase in the temperature level, induced micro-fractures in the granites develop, and the decrement ratios of both the P-wave velocity and the density of the granite increase; (ii) The relationship between the volume flow rate and the pressure gradient is demonstrably linear and fits very well with Darcy’s law. The equivalent permeability coefficient shows an increasing trend with the temperature, and it can be best described using the mathematical expression K0 = A × 1.01T; (iii) For a given temperature level, as the confining pressure increases, the transmissivity shows a decrease, and the rate of its decrease diminishes gradually. [ABSTRACT FROM AUTHOR]