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This study investigates the energy dynamics of sandstone subjected to failure in conditions typical of deep underground construction. Research was conducted using both standard triaxial compression and cyclic loading–unloading techniques at six distinct confining pressures, with the objective of elucidating the deformation and failure processes of rock materials. The tests demonstrated that, regardless of the stress path, sandstone primarily fails through shear under different confining pressures, which also reduces the formation of secondary cracks. The energy transformation observed during cyclic loading and unloading processes exhibits a distinctive peak-like distribution, marked by an inflection point that indicates changes in energy distribution. In the initial stages of the loading cycle, the energy profile of the rock increases, characterized by a condition in which the energy stored elastically exceeds the energy dissipated. Nevertheless, subsequent to reaching peak stress, there is a rapid transmutation of elastic strain energy into other forms, culminating in a pronounced elevation in the ratio of dissipated energy, which ultimately achieves a state of equilibrium influenced by the confining pressures. The study introduces the energy consumption ratio (Ke) as a metric for assessing rock damage accumulation and stability, noting a critical pattern where Ke decreases and then spikes at the rock's failure point, with K = 1 identified as the critical threshold for failure. This comprehensive analysis illuminates the intricate relationship between energy distribution patterns and the stability of rock structures, thereby enhancing our understanding of failure mechanisms from an energetic perspective. [ABSTRACT FROM AUTHOR]

The exploitation and utilization of coal resources are closely related to sustainable social and economic development. To uncover the deformation and seepage patterns of coal on the mining process, this study devised a new stress program with simultaneous changes in axial and confining pressures, then performed coal seepage experiments at various gas pressures. The results show that the residual deformation exhibited a stepwise change, the relative residual deformation at the same level decreased gradually, and the increase in gas pressure led to a reduction in residual deformation. In each stress grade, the absolute permeability damage rate increased gradually, while the relative permeability damage rate decreased with the number of cycles, and the growth of gas pressure could decrease the permeability damage rate. The higher gas pressure led to a lower average energy dissipation ratio at each stress level and increased the rate of growth of elastic energy before destruction of the specimens. A higher gas pressure led to a quicker rate of change in damage variables at high stress levels. The findings have implications for the effective mining and sustainable development of coal resources. [ABSTRACT FROM AUTHOR]

In order to study the mechanical properties and strain field evolution of gas-bearing coal under cyclic loading, the mechanical properties of gas-bearing coal under cyclic loading and unloading conditions were tested with the Rock Mechanics Test (MTS816) system and the independently developed gas-bearing coal gas-solid coupling device. The strength and deformation characteristics of coal samples under cyclic loading and unloading were analyzed, and the strain field evolution of gas-bearing coal was investigated with digital image correlation (DIC) technology. The results indicated that: ① Under the action of cyclic loading and unloading, the loading and unloading curves did not coincide with each other, forming a hysteresis loops. With the increase in the number of cycles, the area of the hysteresis loops gradually increased, and gradually moved towards the direction of increasing strain. Coal samples with varying gas pressures exhibited significant brittle failure under cyclic loading. ② Under the action of cyclic loading and unloading, the peak strength of coal samples decreased, and the loading and unloading deformation modulus increased with the increasing gas pressure. The unloading deformation modulus was always greater than the loading deformation modulus. As the number of cycles increased, the difference in deformation modulus gradually decreased and eventually fell within the range of 0 to 0.1 GPa. ③ Under cyclic loading and unloading conditions, the trend between the irreversible strain of coal samples and the number of cycles was a three-stage pattern of “initial, stable and accelerated expansion”, and the overall curve progressed from an L-shape to a U-shape. The trend between the cumulative irreversible strain and the number of cycles was rapidly increasing, slowly increasing and rapidly increasing, and both irreversible strain and cumulative irreversible strain increased with increasing gas pressure. ④ Under low gas pressure, the strain concentration area of coal samples was mainly a single vertical strain concentration zone. As the gas pressure increased, the local strain concentration zone gradually transitioned from vertical single to disordered and complex. The higher the gas pressure, the more pronounced the strain field fluctuation and the intensity was concentrated in the central region. At high gas pressures, the greater the number of peak points, the greater the strain with increasing gas pressure.

Uniaxial step cyclic loading and unloading tests on siltstone were conducted to investigate the mechanisms and evolution characteristics of rock deformation, including elastic, viscoelastic, and plastic aspects. This study proposes a method for separating dissipated energy into damage energy, which is used for particle slippage and structural fractures, and plastic energy, which remains in cracks that do not open after unloading. Additionally, elastic energy is divided into particle elastic energy, released by particle rebound, and crack elastic energy, released by the reopening of compacted cracks. The results indicate that as the stress amplitude increases, the damage energy consumption, plastic energy consumption, particle elastic energy, and crack elastic energy increase. At peak stress, significant expansion and penetration of cracks within the rock sample occur, leading to a sharp increase in damage energy consumption and a dramatic decrease in the rock sample's mechanical properties, with the particle elastic energy dropping quickly. Plastic energy dissipation relates solely to cracks that do not reopen during unloading, with minimal change after reaching peak stress. The calculated damage variables, based on damage energy consumption, align with the deformation and energy characteristics of the rock, providing a reasonable description of the damage development process of the rock under cyclic loading and unloading. [ABSTRACT FROM AUTHOR]

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In order to study the effect of basic roof period pressure on the dynamic characteristics of impact pillar, one-dimensional dynamic and static combined loading SHPB experimental device is used to carry out dynamic impact experiment on the impact sandstone pre-treated by cyclic loading and unloading. In the experiment, three different axial cycle thresholds are pre-set: 6, 12, 18 MPa, and then carry out dynamic compression experiments under different impact pressure conditions. The dynamic mechanical response, energy evolution and fragmentation distribution characteristics of impact sandstone under different cycle thresholds and impact pressure are discussed. The results show that the dynamic stress-strain curve of rock samples can be divided into elastic stage, yield stage and failure stage. The initial stage of dynamic stress-strain curve is almost straight line, and there is no fracture compaction stage in static uniaxial compression. The average dynamic compressive strength, the average dynamic elastic modulus and the average dynamic deformation modulus first increase and then decrease with the increase of the cyclic threshold. The average dynamic peak strain decreases first and then increases with the increase of cycle threshold. The average dynamic compressive strength, average dynamic elastic modulus and average dynamic deformation modulus of rock samples at the critical threshold are the largest, and the average dynamic peak strain is the smallest. The density of reflected energy and dissipated energy decreases first and then increases with the increase of the cycle threshold. The transmission energy increases first and then decreases with the increase of the cycle threshold. At the critical threshold, the density of reflected energy and dissipated energy is the smallest and the transmitted energy is the largest. There are four kinds of failure modes of rock samples under impact load: crushing failure, rock fragmentation, rock side spalling and rock splitting. Under each impact pressure, the fractal dimension of rock samples decreases first and then increases with the increase of cycle threshold, and increases nonlinearly with the increase of dissipated energy density. The densification of impact rock is better, the intensity of impact failure is weaker, the fragmentation degree is larger, the dissipated energy density is smaller, and the fractal dimension is smaller. The research results show that when the basic roof period pressure is less than the damage threshold value of the impact pillar, the periodic pressure can improve the rock density and affect the damage degree of the dynamic impact.

Exploring the energy evolution of fractured sandstone under true triaxial cyclic loading is helpful to clarify the law of rock damage and failure. The evolution characteristics of total absorbed energy, elastic energy, dissipated energy and plastic deformation energy during the test were systematically analyzed. A rock damage evaluation model was established, and the rock damage failure law was discussed in combination with the mesoscopic damage characteristics of the fracture surface. The results show as follows: with the increase of crack angle, the greater the peak strength of the sample during failure, the greater the total absorbed energy, they were 0.238, 0.276, 0.307 and 0.332 MJ/m3, respectively; the total absorbed energy, elastic energy and dissipated energy increase gradually during the whole test process, while the plastic deformation energy decreases first and then increases; based on hysteretic loop energy and total dissipated energy, an evaluation index was established to describe rock damage, and the results of the two evaluation methods are consistent, and it is found that the rock damage degree changes nonlinear with loading and unloading during the test; the number of cyclic loading and unloading during the failure of the sample has a significant influence on the internal damage of the sample. The more times of cyclic loading and unloading, the higher the degree of extrusion friction and the more complex the micromorphology of the fracture surface.

The study on impact tendency of coal body containing gas under unloading damage plays a key role in exploring the formation mechanism of coal and gas combined rock burst in deep underground mine. In order to explore the influence of unloading action on impact tendency of coal body containing gas, this paper carried out cyclic loading and unloading of coal body containing gas with varying upper limit under different unloading amplitude conditions. The test results show that: gas has a deterioration effect on mechanical properties of coal. Under 9 MPa confining pressure, the average compressive strength of coal specimens under 1 MPa and 2 MPa methane action decreases by 11.2% and 18.7%, respectively. Under the unloading amplitude of 3 MPa and 6 MPa, the compressive strength σc decreases by 12.6% and 36.5%, and the residual elastic energy index CEF decreases by 10.4% and 16.7%, respectively. For coal containing 1 MPa gas, σc decreases by 13.5% and 39.8%, and CEF decreases by 15.9% and 43.6%, respectively. For coal containing 2 MPa gas, σc decreases by 16.7% and 47.2%, and CEF decreases by 20.3% and 48.5%, respectively. With the increase of unloading range, the mechanical property of coal specimen decreases continuously, and the gas action will further aggravate the unloading deterioration degree, and the deterioration degree will gradually deepen with the increase of gas pressure. In addition, pressure relief can effectively reduce the impact tendency of coal body. Meanwhile, the deterioration degree of impact tendency index of coal body containing gas after unloading is higher than that of original coal specimen, and with the increase of gas pressure, the reduction degree of residual elastic energy index is higher.

As the deep coal mining tends to be “normal”, it is of great significance to understand the mechanical properties and damage evolution of coal under high ground stress and high ground temperature environment in deep coal mining. The coal body of the Ji16-17-31030 working face in the Pingmei 12th Coal Mine was selected as the research object. The triaxial cyclic loading and unloading tests on the coal samples were carried out under different temperature conditions to examine the impact of temperature on their fundamental mechanical parameters, including peak strength, deformation modulus, and Poisson’s ratio. The stress-strain problems associated with different loading and unloading rates were reformulated into time-stress-strain problems, and a fractional viscoelastic-plastic constitutive equation considering the coupling effect of thermal and force was proposed by introducing the fractional derivative theory and the continuum damage theory. The results show that the deformation modulus of coal decreases linearly with the increase of cycle times, while the Poisson ratio of coal shows a deceleration increase, reaches the extreme value at the peak stress, and then decreases, indicating that the cyclic loading and unloading has a deterioration effect on the basic mechanical properties of the coal. With the increase of temperature, the peak strength of coal decreases nonlinearly, the damage accumulation slows down, and the corresponding strain increases gradually, which indicates that heating could enhance the ductile deformation ability of coal and accelerate the development of coal damage. In the process of cyclic loading and unloading, the input energy density, elastic energy density and dissipated energy density decay obviously decay with the increase of temperature in the stage of coal failure and instability, indicating that high temperature intensifies the damage inside the coal, reduces the coal strength, and reduces the external input energy required for failure. The fractional viscoelastic-plastic model considering the effect of temperature can better describe the mechanical behavior of deep coal body under cyclic loading and unloading conditions. The model is suitable for the complex stress state of deep coal body under a thermal-mechanical coupling, and provides an important reference for the research on the deformation and stability of deep coal body.

A series of freeze–thaw cycling tests, as well as cyclic loading and unloading tests, have been conducted on nodular sandstones to investigate the effect of fatigue loading and freeze–thaw cycling on the damage evolution of fractured sandstones based on damage mechanics theory, the microstructure and sandstone pore fractal theory. The results show that the number of freeze–thaw cycles, the cyclic loading level, the pore distribution and the complex program are important factors affecting the damage evolution of rocks. As the number of freeze–thaw cycles rises, the peak strength, modulus of elasticity, modulus of deformation and damping ratio of the sandstone all declined. Additionally, the modulus of elasticity and deformation increase nonlinearly as the cyclic load level rises. With the rate of increase decreasing, while the dissipation energy due to hysteresis increases gradually and at an increasing rate, and the damping ratio as a whole shows a gradual decrease, with a tendency to increase at a later stage. The NRM (Nuclear Magnetic Resonance) demonstrated that the total porosity and micro-pores of the sandstone increased linearly with the number of freeze–thaw cycles and that the micro-porosity was more sensitive to freeze–thaw, gradually shifting towards meso-pores and macro-pores; simultaneously, the SEM (Scanning Electron Microscope) indicated that the more freeze–thaw cycles there are, the more micro-fractures and holes grow and penetrate each other and the more loose the structure is, with an overall nest-like appearance. To explore the mechanical behavior and mechanism of cracked rock in high-altitude and alpine areas, a damage model under the coupling of freeze–thaw-fatigue loading was established based on the loading and unloading response ratio theory and strain equivalence principle. [ABSTRACT FROM AUTHOR]

In order to investigate freeze–thawed red sandstone failure processes under cyclic loading and unloading conditions, real-time acoustic emission (AE) and scanning electron microscopy (SEM) techniques were used to reveal the fracture process of the saturated red sandstone after cyclic loading and unloading tests using uniaxial compression. The results show that the stress–strain curves of the freeze–thawed sandstones show signs of hysteresis and exhibit a two-stage evolution of "sparse → dense". In the cyclic loading and unloading process, the modulus of elasticity in the loading process is always larger than that in the unloading process, while the Poisson's ratio is the opposite, and the radial irreversible strain and cumulative irreversible strain are larger than those in the axial direction. As the number of freeze–thaw cycles increases, the rock specimens need more cycles of loading and unloading to make the crack volume compressive strain Δ ε c v + reach the maximum value and tend to stabilize, while the crack volume extensional strain Δ ε c v − tends to decrease gradually. This study also shows that the growth phase of the cyclic loading and unloading process has more ringing counts and a shorter duration, while the slow degradation phase has more ringing counts with loading and less with unloading. In addition, the F-T cycle gradually changes the internal microcracks of the red sandstone from shear damage, which is dominated by shear cracks, to tensile damage, which is dominated by tensile cracks. This study's findings contribute to our knowledge of the mechanical characteristics and sandstone's degradation process following F-T treatment, and also serve as a guide for engineering stability analyses conducted in the presence of multiphysical field coupling. [ABSTRACT FROM AUTHOR]

Coal gangue is a byproduct of coal mining and processing, and according to incomplete statistics, China has amassed a substantial coal gangue stockpile exceeding 2600 large mountains, which poses a serious threat to the ecological environment. Utilizing gangue as a coarse aggregate to produce gangue concrete (GC) presents a promising avenue for addressing the disposal of coal gangue; however, gangue concrete presents several challenges that need to be tackled, such as low strength and poor resistance to repeated loads. In this study, polypropylene fibers (PPFs) were incorporated into gangue concrete to enhance its utilization rate. Uniaxial compressive and repeated loading experiments were then conducted to investigate the uniaxial strength and fatigue properties of polypropylene fiber-reinforced gangue concrete (PGC) with varying gangue substitution rates (20%, 40%, and 60%) and different polypropylene fiber admixtures (0, 0.1%, 0.2%, and 0.3%). The findings indicate that incorporating gangue at a substitution rate of 40% could notably enhance the uniaxial compressive strength of PGC, resulting in a maximum increase of 19.4%. In the repeated loading experiments, the ductility of PGC was enhanced with the incorporation of PPFs, resulting in a reduction of 33.76% in the damage factor and 19.42% in residual strain for PGC-40-0.2 compared to PGC-40-0. A PPF content of 0.2% was found to be optimal for enhancing the fatigue performance of PGC. Scanning electron microscope (SEM) testing proved the improvement effect of polypropylene fiber on gangue concrete from a microscopic perspective. This study provides crucial experimental data and a theoretical foundation for the utilization of gangue concrete in complex stress environments. [ABSTRACT FROM AUTHOR]

In this study, a method was introduced to validate the presence of a Representative Elementary Volume (REV) within marine clayey sediment containing cracks during cyclic loading and unloading of confinement pressure. Physical testing provided the basis for this verification. Once the existence of the REV for such sediment was confirmed, we established a machine-learning predictive model. This model utilizes a hybrid algorithm combining Particle Swarm Optimization (PSO) with a Support Vector Machine (SVM). The model was trained using a database generated from the aforementioned physical tests. The machine-learning model demonstrates favorable predictive performance based on several statistical metrics, including the coefficient of determination (R2), mean residual error (MSE), mean relative residual error (MRSE), and the correlation coefficient R during the verification process. Utilizing the established machine-learning predictive model, one can effortlessly obtain the permeability tensor of marine clayey sediment containing cracks during cyclic loading and unloading of confinement pressure by inputting the relevant stress condition parameters. The original research cannot estimate the permeability tensor under similar loading and unloading conditions through REV. In this study, the physical model test was used to determine the REV of marine cohesive sediments with cracks by cyclic-constrained pressure loading and unloading. Referring to the results of physical tests, we developed a machine-learning prediction model that can easily estimate the permeability tensor of marine cohesive sediments with cracks under cyclic loading and constrained pressure unloading conditions. This method greatly saves time and computation and provides a direct method for engineering and technical personnel to predict the permeability tensor in this case. [ABSTRACT FROM AUTHOR]

To investigate the dynamic response of fractured rock under cyclic loading and unloading, a WHY-300/10 microcomputer-controlled electro-hydraulic servo universal testing machine was used to conduct uniaxial cyclic loading and unloading tests. Simultaneously, acoustic emission (AE) and a CCD high-speed camera were employed to monitor the fracturing characteristics of sandstone. The mechanical properties, energy evolution, AE characteristics, and deformation of 45° sandstone were analyzed. The results indicate that as the load cycle level increases, both the elastic modulus and deformation modulus exhibit a "parabolic" increase, with a rapid rise initially and a slower rate of increase later. The damping ratio generally shows a decreasing trend but tends to rise near the peak load. The total energy, elastic energy, dissipated energy, damping energy, and damage energy all follow exponential function increases with the load level. The b-value fluctuates significantly during the stable crack propagation phase, unstable crack propagation phase, and peak phase. When the FR (Felicity ratio > 1), the rock is relatively stable; when the FR (Felicity ratio < 1), the rock gradually extends towards an unstable state. The Felicity ratio can be used as a predictive tool for the precursors of rock failure. Shear fractures dominate during the compaction and peak phases, while tensile fractures dominate during the crack propagation phase, ultimately leading to a failure characterized by tensile fracture. High-speed camera observations revealed that deformation first occurs at the tips of the prefabricated cracks and gradually spreads and deflects toward the ends of the sandstone. This study provides theoretical support for exploring the mechanical behavior and mechanisms of fractured rock under cyclic loading and unloading, and it has significant practical implications. [ABSTRACT FROM AUTHOR]

Abstract Low permeability sandstones are widely found in oil and gas reservoirs. To gain a better understanding of the mechanical behavior of low-permeability sandstones, the permeability evolution of sandstone is investigated experimentally under triaxial compression loading, especially with multiple loading/unloading cycles, using two groups of experimental tests. The one is the triaxial compression tests with and without loading/unloading cycles. The other one is gas permeability tests in triaxial compression. The test results show that the stress–strain curves exhibited obvious nonlinear behavior and softening characteristics. It was demonstrated that important strain hysteresis occurred during the loading and unloading test. The permeability evolution of sandstone exhibited three phases: slow decrease, slow increase and rapid increase. Meanwhile, the permeability evolution followed the evolution of damage. The permeability under different confining pressures was different when the damage variable was approximate to 0.1. As the confining pressure increases, the permeability and the damage variable tend to be constant. The total permeability change is 0.568 when the confining pressure is 30 MPa. Moreover, a deviatoric stress threshold 0.8 $${\sigma }_{\rm p}$$ σ p is observed in the process of permeability evolution. When the applied stress is less than the threshold, the influence of confining pressure doesn’t play a dominant role in the permeability evolution. When the applied stress is larger than the threshold, the permeability evolution depends on the combination of axial stress and confining pressure.

In coal mining, rock strata are fractured under cyclic loading and unloading to form fracture channels. Fracture channels are the main flow narrows for gas. Therefore, expounding the flow conductivity of fracture channels in rocks on fluids is significant for gas flow in rock strata. In this regard, graded incremental cyclic loading and unloading experiments were conducted on sandstones with different initial stress levels. Then, the three-dimensional models for fracture channels in sandstones were established. Finally, the fracture channel percentages were used to reflect the flow conductivity of fracture channels. The study revealed how the particle size distribution of fractured sandstone affects the formation and expansion of fracture channels. It was found that a smaller proportion of large blocks and a higher proportion of small blocks after sandstone fails contribute more to the formation of fracture channels. The proportion of fracture channels in fractured rock can indicate the flow conductivity of those channels. When the proportion of fracture channels varies gently, fluids flow evenly through those channels. However, if the proportion of fracture channels varies significantly, it can greatly affect the flow rate of fluids. The research results contribute to revealing the morphological evolution and flow conductivity of fracture channels in sandstone and then provide a theoretical basis for clarifying the gas flow pattern in the rock strata of coal mines.

This study investigates the energy dynamics of sandstone subjected to failure in conditions typical of deep underground construction. Research was conducted using both standard triaxial compression and cyclic loading–unloading techniques at six distinct confining pressures, with the objective of elucidating the deformation and failure processes of rock materials. The tests demonstrated that, regardless of the stress path, sandstone primarily fails through shear under different confining pressures, which also reduces the formation of secondary cracks. The energy transformation observed during cyclic loading and unloading processes exhibits a distinctive peak-like distribution, marked by an inflection point that indicates changes in energy distribution. In the initial stages of the loading cycle, the energy profile of the rock increases, characterized by a condition in which the energy stored elastically exceeds the energy dissipated. Nevertheless, subsequent to reaching peak stress, there is a rapid transmutation of elastic strain energy into other forms, culminating in a pronounced elevation in the ratio of dissipated energy, which ultimately achieves a state of equilibrium influenced by the confining pressures. The study introduces the energy consumption ratio (Ke) as a metric for assessing rock damage accumulation and stability, noting a critical pattern where Ke decreases and then spikes at the rock’s failure point, with K = 1 identified as the critical threshold for failure. This comprehensive analysis illuminates the intricate relationship between energy distribution patterns and the stability of rock structures, thereby enhancing our understanding of failure mechanisms from an energetic perspective.

Horizontal well cavern completion and stress release is considered a potential technique for efficient development of coalbed methane in tectonically deformed coal (TDC). Pulsating loading and unloading is a key technique for the controlled expansion of caverns and broader stress release within the reservoir. However, current understanding of the mechanical characteristics and pore network structure evolution of TDC under cyclic loading and unloading conditions is still limited. This paper employs numerical simulation methods to study the mechanical behavior and damage characteristics of TDC under cyclic loading and unloading. After obtaining a set of micromechanical parameters reflecting the behavior of TDC samples under triaxial compression in high-stress states, the effects of different stress gradients and cyclic amplitudes on the stress–strain curve, porosity changes, and crack propagation in TDC samples were analyzed. The study results indicate that under various cyclic loading and unloading conditions, the mechanical response characteristics of TDC samples are broadly similar, primarily divided into compression, slow expansion, and accelerated expansion phases. Under low unloading level conditions, the volume expansion of TDC samples is minimal. Also, at the same unloading level, the strain increment decreases with an increasing number of cycles. Correspondingly, under these conditions, the porosity and microcrack expansion in TDC are less than in high-stress gradient scenarios. Under the same unloading level but different amplitudes, the volume expansion rate at 50% unloading amplitude is higher than at 1 MPa unloading amplitude for TDC, with an increased number of crack expansions. Therefore, under cyclic loading conditions, the sensitivity of crack propagation within TDC samples to amplitude is greater than that to unloading level. Under actual pulsating excitation conditions, a low-amplitude, low-stress gradient pulsation method should be used to maintain the stability of horizontal well caverns, and gradually increase the cyclic amplitude to achieve the efficient extraction of coalbed methane in TDC reservoirs. The findings of this study can serve as an important reference for optimizing process parameters in cyclic pulsating stress release engineering for TDC. [ABSTRACT FROM AUTHOR]

With the depletion of shallow resources, deep resource mining has become a trend. However, the high temperature and complex stress environment in deep mines make resource extraction extremely challenging. This paper developed a thermal insulation grouting material made of glazed hollow beads, sodium silicate, and cement and tested the compressive strength, gelation time, and stone rate under various curing days in light of the issue of high temperature heat damage in high ground temperature mines and the impact of mining on roadway grouting bolt support. Fatigue strength, fatigue deformation, load-residual strain, energy evolution and microscopic features were studied and analyzed in relation to the damage law of graded cyclic loading and unloading under the number of varying cycles. The findings demonstrate that cyclic loading and unloading strength is lower than uniaxial compressive strength. The fatigue strength is significantly decreased when the number of cycles reaches its limit. Residual strain is less sensitive to changes in stress than load strain. The fitting correlation coefficients of total output energy and elastic energy are higher than 0.71. [ABSTRACT FROM AUTHOR]

The study of coal bump-prone property under confining pressure plays a key role in exploring the formation mechanism of coal burst in deep underground mines. In order to explore the influence of different confining pressures on bump-prone property of coal, this paper carried out cyclic loading and unloading of coal with different stress lower limits under multistage confining pressures. The test results show that: under the confining pressure environment of 3 MPa, 6 MPa and 9 MPa, the compressive strength of coal specimens increased by 3.1%, 4.7% and 6.2% respectively after cyclic loading and unloading, and the axial strain increased by 0.61%, 0.42% and 0.28% respectively in the last stage of cyclic loading and unloading. The volumetric strain increases by 0.32%, 0.24% and 0.17%, respectively, indicating that the larger the confining pressure is, the stronger the deformation constraint is. With the increase of confining pressure, the cumulative number of AE events gradually decreases, the cumulative number of AE after peak continuously decreases, and the cumulative energy also gradually decreases, indicating that with the increase of confining pressure, the coal specimen becomes more compact, and the failure mode gradually transitions from brittle failure to ductile failure. In addition, after the confining pressures of 3 MPa, 6 MPa and 9 MPa, the input energy \begin{document}$U_{{\rm{in}}}^{\rm{n}} $\end{document} and elastic strain energy \begin{document}$U_{\rm{e}}^{\rm{n}} $\end{document} increase continuously. Compared with the normal pressure condition, the residual elastic energy index (CEF) of coal specimens is increased by 21.76%, 42.92% and 71.69%, respectively, indicating that confining pressure has an enhanced effect on the bursting tendency of coal, and the residual elastic energy index (CEF) has a linear function relationship with confining pressure σ3.

This paper investigates mechanical behaviours of sandstone during post-peak cyclic loading and unloading subjected to hydromechanical coupling effect, confirming the peak and residual strengths reduction laws of sandstone with water pressure, and revealing the influence of water pressure on the upper limit stress and deformation characteristics of sandstone during post-peak cyclic loading and unloading. Regarding the rock strength, the experimental study confirms that the peak strength σp and residual strength σr decrease as water pressure P increases. Especially, the normalized strength parameters σp/σpk and σr/σre was negatively and linearly correlated with the P/σ3. Moreover, the Hoek-Brown strength criterion can be applied to describe the relationship between effective peak strength and effective confining stress. During post-peak cyclic loading and unloading, both the upper limit stress σp(i) and crack damage threshold stress σcd(i) of each cycle tend to decrease with the increasing cycle number. A hysteresis loop exists among the loading and unloading stress–strain curves, indicating the unloading deformation modulus Eunload is larger than the loading deformation modulus Eload. Based on experimental results, a post-peak strength prediction model related to water pressure and plastic shear strain is established.

Under the condition of roadway driving along goaf, slender coal pillar is affected by multiple mining-induced disturbances, and the permeability of coal and rock mass affected by mining will change due to the development and compaction of mining fractures and primary fractures. Determining the evolution of slender coal pillar permeability at different mining stages is the theoretical basis for the prevention and control of gas water disasters in adjacent goaf at the same layer. Taking the mining with slender coal gate pillar of the Carboniferous extra thick coal seam in Datong Mining Area as the engineering background, the distribution characteristics of the stress field for the slender coal gate pillar of the coal seam in different mining stages are comprehensively determined by the methods of geostress testing and numerical simulation, which provides a basis for the determination of the stress path for experimental research. The DJG - Ⅱ triaxial loading coal rock seepage testing equipment was used to conduct experimental research on the evolution of coal pillar permeability in different mining stages. The research results are as follows: The quantitative influence relationship between permeability and stress of slender coal gate pillar in different mining stages is established. The overall performance is that the permeability decreases with the increase of axial stress, and the permeability increases with the decrease of axial pressure in unloading stage; It reveals the evolution of stress strain permeability of the coal pillar in different mining stages. When loading and unloading in the first and second stages, the deformation of coal sample is still in the elastic deformation stage, and the change amplitude and rate of permeability are relatively gentle. In the third mining-impacted stage, the irreversible plastic failure of the specimen made the permeability increase sharply, and the rate of increase was also significantly greater than the first two mining stages. The permeability of slender coal pillar increased by 324.389 times compared with the initial permeability. In this stage, the slender coal pillar was damaged and lost its gas water barrier performance. It was clear that the 6 m small coal pillar was not damaged in the first two mining stages of the super thick coal seam gob side entry project. The research results can provide reference or theoretical support for the study of permeability evolution characteristics of slender coal pillar in different mining stages, and the prevention and control of gas water disasters in adjacent goaf under the condition of gob side entry mining in hard roof extra thick coal seams.

Drilling gas drainage boreholes in coal mines is a major measure to prevent gas disasters in coal mines. However, coal mining disruptions can affect the stability and shape of gas drainage boreholes. Therefore, multi-level cyclic loading and unloading tests with a constant lower limit and constant-amplitude multi-level cyclic loading and unloading tests were conducted on hole-containing specimens. The results show that cyclic loading and unloading enlarges the deformation modulus of specimens with holes. The stress change during cyclic loading affects the deformation modulus of the specimens, rendering them more brittle and vulnerable to damage under high stress-induced disturbances. Additionally, the dissipated energy per unit volume of the specimens containing holes increases exponentially during multi-level cyclic loading and unloading. The cumulative dissipated energy per unit volume of the hole-containing specimens exhibits logarithmic growth during constant-amplitude multi-level cyclic loading and unloading. Furthermore, a discontinuity in the curves is observed as the stress level rises, which promotes brittle failure around the hole. The research results expound the deformation and energy evolution characteristics of hole-containing specimens during cyclic loading and unloading, and provide a reference for clarifying the failure modes of boreholes in the gas extraction process.

Weakly cemented rocks have a loose structure, poor mechanical properties, and soften and disintegrate upon contact with water. Mining operations cause damage and ruptures to rocks under cyclic loading and unloading, leading to serious disasters. This study investigated the effects of cyclic loading and unloading on the mechanical properties of weakly cemented sandstone (WCS) with various water contents (0–7.72%). A numerical model based on the particle flow theory simulated the behavior of WCS particles. The stress–strain relationships, damage and rupture patterns, energy evolution, and damage properties of WCS were examined using loading–unloading simulations. Water negatively affected the strength and elastic modulus of WCS. High water contents (>2.31%) increased the rupture probability and affected the rupture modes. Ruptures mainly occurred via the main fissure and caused cleavage damage; however, instances of tensile damage and shear slippage increased with an increasing water content. The elastic, dissipation, and total energies gradually increased with increasing cyclic loading and unloading. The damage factors of WCS with different water contents gradually increased with the growth rate. The mechanical properties of the sandstone were deteriorated by water, which increased the peak value of the damage factor from 0.77 for 0% moisture to 0.81 for 7.72% moisture. [ABSTRACT FROM AUTHOR]

In the mining of nearly horizontal open-pit coal mines, transportation flat plates are usually arranged on the end-slope. Reciprocating transport operations of large heavy-duty trucks cause significant cyclic loading and unloading effects on the overburden rock, leading to initial damage to the coal rock body. At the same time, the strong impact load generated by large-scale blasting during open-pit mining further deteriorates the mechanical properties of the coal rock body, thereby inducing large area instability of the end-slope. Therefore, it is crucial to conduct experimental research on the dynamic mechanical properties of coal rock under cyclic loading and unloading damage conditions for the stability of the end-slopes and safe mining in open-pit coal mines. The dynamic mechanical properties of coal samples under the initial damage condition of cyclic loading and unloading and their damage and fracture mechanism were systematically analyzed by using the split Hopkinson pressure bar test system and scanning electron microscope test system. The results show that: ① With the increase of the loading strain rate, the dynamic compressive strength and elastic modulus of the coal sample increase rapidly, and the strengthening effect of the strain rate on the mechanical properties of the sample is significant. The dynamic compressive strength and elastic modulus decrease linearly and rapidly as the initial damage increases under the condition of fixed strain rate, indicating that the initial damage weakens the mechanical properties of the coal sample; ② There exist small-scale primary cracks and pore defects in the natural state of the coal samples, and the occurrence of damage leads to the emergence of new cracks and pores inside the coal sample. With the increase of the damage intensity, the defect scale and number gradually increase and expand rapidly, which is the fundamental reason for the weakening of its mechanical properties; ③ The failure mode of the coal sample changes from tension failure to shear failure with the increase of the initial damage and strain rate, and at the same time, its degree of damage, dissipation energy during the failure process, and energy dissipation capacity show a rapid increasing trend; ④The dynamic failure of the damaged coal sample is mainly brittle failure, but under the condition of high initial damage, significant sliding separation and toughness fracture morphology can be observed locally, which is also the main reason for the high energy dissipation capacity of the coal sample under the condition of high initial damage.

Uniaxial step cyclic loading and unloading tests on siltstone were conducted to investigate the mechanisms and evolution characteristics of rock deformation, including elastic, viscoelastic, and plastic aspects. This study proposes a method for separating dissipated energy into damage energy, which is used for particle slippage and structural fractures, and plastic energy, which remains in cracks that do not open after unloading. Additionally, elastic energy is divided into particle elastic energy, released by particle rebound, and crack elastic energy, released by the reopening of compacted cracks. The results indicate that as the stress amplitude increases, the damage energy consumption, plastic energy consumption, particle elastic energy, and crack elastic energy increase. At peak stress, significant expansion and penetration of cracks within the rock sample occur, leading to a sharp increase in damage energy consumption and a dramatic decrease in the rock sample’s mechanical properties, with the particle elastic energy dropping quickly. Plastic energy dissipation relates solely to cracks that do not reopen during unloading, with minimal change after reaching peak stress. The calculated damage variables, based on damage energy consumption, align with the deformation and energy characteristics of the rock, providing a reasonable description of the damage development process of the rock under cyclic loading and unloading.

A series of freeze–thaw cycling tests, as well as cyclic loading and unloading tests, have been conducted on nodular sandstones to investigate the effect of fatigue loading and freeze–thaw cycling on the damage evolution of fractured sandstones based on damage mechanics theory, the microstructure and sandstone pore fractal theory. The results show that the number of freeze–thaw cycles, the cyclic loading level, the pore distribution and the complex program are important factors affecting the damage evolution of rocks. As the number of freeze–thaw cycles rises, the peak strength, modulus of elasticity, modulus of deformation and damping ratio of the sandstone all declined. Additionally, the modulus of elasticity and deformation increase nonlinearly as the cyclic load level rises. With the rate of increase decreasing, while the dissipation energy due to hysteresis increases gradually and at an increasing rate, and the damping ratio as a whole shows a gradual decrease, with a tendency to increase at a later stage. The NRM (Nuclear Magnetic Resonance) demonstrated that the total porosity and micro-pores of the sandstone increased linearly with the number of freeze–thaw cycles and that the micro-porosity was more sensitive to freeze–thaw, gradually shifting towards meso-pores and macro-pores; simultaneously, the SEM (Scanning Electron Microscope) indicated that the more freeze–thaw cycles there are, the more micro-fractures and holes grow and penetrate each other and the more loose the structure is, with an overall nest-like appearance. To explore the mechanical behavior and mechanism of cracked rock in high-altitude and alpine areas, a damage model under the coupling of freeze–thaw-fatigue loading was established based on the loading and unloading response ratio theory and strain equivalence principle.

In order to investigate freeze–thawed red sandstone failure processes under cyclic loading and unloading conditions, real-time acoustic emission (AE) and scanning electron microscopy (SEM) techniques were used to reveal the fracture process of the saturated red sandstone after cyclic loading and unloading tests using uniaxial compression. The results show that the stress–strain curves of the freeze–thawed sandstones show signs of hysteresis and exhibit a two-stage evolution of “sparse → dense”. In the cyclic loading and unloading process, the modulus of elasticity in the loading process is always larger than that in the unloading process, while the Poisson’s ratio is the opposite, and the radial irreversible strain and cumulative irreversible strain are larger than those in the axial direction. As the number of freeze–thaw cycles increases, the rock specimens need more cycles of loading and unloading to make the crack volume compressive strain Δεcv+ reach the maximum value and tend to stabilize, while the crack volume extensional strain Δεcv− tends to decrease gradually. This study also shows that the growth phase of the cyclic loading and unloading process has more ringing counts and a shorter duration, while the slow degradation phase has more ringing counts with loading and less with unloading. In addition, the F-T cycle gradually changes the internal microcracks of the red sandstone from shear damage, which is dominated by shear cracks, to tensile damage, which is dominated by tensile cracks. This study’s findings contribute to our knowledge of the mechanical characteristics and sandstone’s degradation process following F-T treatment, and also serve as a guide for engineering stability analyses conducted in the presence of multiphysical field coupling.

In this study, a method was introduced to validate the presence of a Representative Elementary Volume (REV) within marine clayey sediment containing cracks during cyclic loading and unloading of confinement pressure. Physical testing provided the basis for this verification. Once the existence of the REV for such sediment was confirmed, we established a machine-learning predictive model. This model utilizes a hybrid algorithm combining Particle Swarm Optimization (PSO) with a Support Vector Machine (SVM). The model was trained using a database generated from the aforementioned physical tests. The machine-learning model demonstrates favorable predictive performance based on several statistical metrics, including the coefficient of determination (R2), mean residual error (MSE), mean relative residual error (MRSE), and the correlation coefficient R during the verification process. Utilizing the established machine-learning predictive model, one can effortlessly obtain the permeability tensor of marine clayey sediment containing cracks during cyclic loading and unloading of confinement pressure by inputting the relevant stress condition parameters. The original research cannot estimate the permeability tensor under similar loading and unloading conditions through REV. In this study, the physical model test was used to determine the REV of marine cohesive sediments with cracks by cyclic-constrained pressure loading and unloading. Referring to the results of physical tests, we developed a machine-learning prediction model that can easily estimate the permeability tensor of marine cohesive sediments with cracks under cyclic loading and constrained pressure unloading conditions. This method greatly saves time and computation and provides a direct method for engineering and technical personnel to predict the permeability tensor in this case.

Coal gangue is a byproduct of coal mining and processing, and according to incomplete statistics, China has amassed a substantial coal gangue stockpile exceeding 2600 large mountains, which poses a serious threat to the ecological environment. Utilizing gangue as a coarse aggregate to produce gangue concrete (GC) presents a promising avenue for addressing the disposal of coal gangue; however, gangue concrete presents several challenges that need to be tackled, such as low strength and poor resistance to repeated loads. In this study, polypropylene fibers (PPFs) were incorporated into gangue concrete to enhance its utilization rate. Uniaxial compressive and repeated loading experiments were then conducted to investigate the uniaxial strength and fatigue properties of polypropylene fiber-reinforced gangue concrete (PGC) with varying gangue substitution rates (20%, 40%, and 60%) and different polypropylene fiber admixtures (0, 0.1%, 0.2%, and 0.3%). The findings indicate that incorporating gangue at a substitution rate of 40% could notably enhance the uniaxial compressive strength of PGC, resulting in a maximum increase of 19.4%. In the repeated loading experiments, the ductility of PGC was enhanced with the incorporation of PPFs, resulting in a reduction of 33.76% in the damage factor and 19.42% in residual strain for PGC-40-0.2 compared to PGC-40-0. A PPF content of 0.2% was found to be optimal for enhancing the fatigue performance of PGC. Scanning electron microscope (SEM) testing proved the improvement effect of polypropylene fiber on gangue concrete from a microscopic perspective. This study provides crucial experimental data and a theoretical foundation for the utilization of gangue concrete in complex stress environments.

To investigate the dynamic response of fractured rock under cyclic loading and unloading, a WHY-300/10 microcomputer-controlled electro-hydraulic servo universal testing machine was used to conduct uniaxial cyclic loading and unloading tests. Simultaneously, acoustic emission (AE) and a CCD high-speed camera were employed to monitor the fracturing characteristics of sandstone. The mechanical properties, energy evolution, AE characteristics, and deformation of 45° sandstone were analyzed. The results indicate that as the load cycle level increases, both the elastic modulus and deformation modulus exhibit a “parabolic” increase, with a rapid rise initially and a slower rate of increase later. The damping ratio generally shows a decreasing trend but tends to rise near the peak load. The total energy, elastic energy, dissipated energy, damping energy, and damage energy all follow exponential function increases with the load level. The b-value fluctuates significantly during the stable crack propagation phase, unstable crack propagation phase, and peak phase. When the FR (Felicity ratio > 1), the rock is relatively stable; when the FR (Felicity ratio < 1), the rock gradually extends towards an unstable state. The Felicity ratio can be used as a predictive tool for the precursors of rock failure. Shear fractures dominate during the compaction and peak phases, while tensile fractures dominate during the crack propagation phase, ultimately leading to a failure characterized by tensile fracture. High-speed camera observations revealed that deformation first occurs at the tips of the prefabricated cracks and gradually spreads and deflects toward the ends of the sandstone. This study provides theoretical support for exploring the mechanical behavior and mechanisms of fractured rock under cyclic loading and unloading, and it has significant practical implications.

In many underground engineering projects, rocks are often subjected to cyclic loading and unloading, such as repeated excavation of roadway surrounding rock, which will lead to damage to underground rocks, and the energy of rocks also changes. Therefore, to study the energy evolution and damage characteristics of rocks under cyclic loading and unloading, different cyclic loading and unloading tests of bituminous sandstones under constant amplitude were conducted. Under cyclic loading and unloading, the lower limit stress was 40% of the rock peak intensity, the cyclic amplitude was 20–40% of the peak intensity, and the number of loading–unloading cycles was 10–30. The quantitative characterization of the damage degrees of bituminous sandstone was realized by the ultrasonic wave velocity and elasticity modulus methods. The energy evolution and damage characteristics of bituminous sandstone under different amplitudes and number of loading–unloading cycles were investigated through the energy dissipation method. Results showed that under cyclic loading and unloading, the ultrasonic wave velocity and elasticity modulus of bituminous sandstone decreased gradually; The damage variable shows a trend of rapid and then stable growth and has a power function relationship with the number of cycles; The input energy density and dissipation energy density curves were in L-shaped distribution, whereas the elastic energy density remained stable. The results of this study can provide some theoretical references to underground engineering construction. [ABSTRACT FROM AUTHOR]

With the depletion of shallow resources, deep resource mining has become a trend. However, the high temperature and complex stress environment in deep mines make resource extraction extremely challenging. This paper developed a thermal insulation grouting material made of glazed hollow beads, sodium silicate, and cement and tested the compressive strength, gelation time, and stone rate under various curing days in light of the issue of high temperature heat damage in high ground temperature mines and the impact of mining on roadway grouting bolt support. Fatigue strength, fatigue deformation, load-residual strain, energy evolution and microscopic features were studied and analyzed in relation to the damage law of graded cyclic loading and unloading under the number of varying cycles. The findings demonstrate that cyclic loading and unloading strength is lower than uniaxial compressive strength. The fatigue strength is significantly decreased when the number of cycles reaches its limit. Residual strain is less sensitive to changes in stress than load strain. The fitting correlation coefficients of total output energy and elastic energy are higher than 0.71.

The evolution characteristics of coal seam strain and gas pressure in circular mining were explored by conducting physical simulation tests on the influence of cyclic stress on coal seam parameters under different initial gas pressures using a large true triaxial physical simulation test rig. The evolution characteristics of gas pressure and coal seam strain with the number of cycles and gas pressure were discussed. The test results showed that during cyclic loading and unloading, the coal seam is cracked under stress and new cracks are generated, and the new fractures cause the overall pressure of the coal seam methane to decrease by adsorbing more free gas. In the loading stage, the coal skeleton is squeezed by stress, which causes the space of coal seam pores and cracks to shrink, the free gas in the pores and fractures of the coal seam is extruded, and the strain and gas pressure of the coal seam increase with the increase of stress. In the unloading stage, the reduction of stress leads to the coal skeleton tending to return to its initial state, the free gas in the pores is transported and enriched into the fractures of the coal seam, and the strain and gas pressure of the coal seam are gradually reduced. With the increase of the number of cycles, the damage of the coal seam increases and deformation occurs, the increasing amplitude of gas pressure gradually increases during loading, and the decreasing amplitude of gas pressure gradually decreases when unloading, and the closer the distance from the pressurized boundary, the greater the amplitude change. Under different initial gas pressure conditions, the greater the initial gas pressure, the greater the increasing amplitude of gas pressure and the smaller the decreasing amplitude.

In order to study the deformation and failure characteristics of rocks under different cyclic loading and unloading paths, three stress path tests were conducted, and acoustic emission (AE) monitoring was conducted simultaneously. The mechanical characteristics and AE characteristics under different stress paths were analyzed, and the influences of the different stress paths on the energy dissipation and deformation damage were investigated. The law of energy evolution considering viscoelasticity under different stress paths was obtained. The concept of ultimate damage energy and its calculation method was proposed. The results show that the "hardening effect" of sandstone and granite under the constant lower limit (CLLCL) is the most significant in maximizing the mechanical property. The CLLCL imparts a stronger elastic property to rocks than the variable lower limit (VLLCL) does, while the VLLCL causes more damage to rocks than the CLLCL. A significant linear relationship between the proportion of damage energy and the proportion of elastic energy was discovered. Based on this linear relationship, the ultimate damage energy can be calculated for sandstone and granite. The evolution of the damage variable based on damage energy was compatible with the real damage condition, which validates the ultimate damage energy calculation method. The research results lay a theoretical foundation for the design and construction of geotechnical engineering. [ABSTRACT FROM AUTHOR]

During the operational phase of pumped storage hydropower stations, rockfill materials within the dam experience cyclic loading and unloading due to water level fluctuations. This cyclic behavior can result in the accumulation of irreversible deformation, posing a substantial threat to dam safety. However, there is an absence of a constitutive model capable of accurately capturing the low-frequency multi-cycle hysteresis behavior of rockfill materials due to the constraints of conventional laboratory test methods. In this study, we employed the combined finite and discrete element method (FDEM) to investigate the mechanical characteristics of rockfill materials and develop an improved constitutive model capable of effectively capturing their hysteretic behavior. The results demonstrate the FDEM accurately reproduces the mechanical behavior of rockfill material under shear and cyclic loading and unloading conditions. The "hysteresis loop" exhibits a discernible densification trend with increasing cycles. And the variations in elastic modulus and strain primarily occur within the initial five cycles. The plastic strain increment exhibits a strong positive correlation with stress level, while its relationship with confining pressure is comparatively less pronounced. The proposed constitutive model successfully captures the complex low-frequency multi-cycle hysteresis characteristics of rockfill material with few parameters, showing substantial potential for practical applications. [ABSTRACT FROM AUTHOR]

Taking coal as the research subject, the damage mechanical properties of coal were studied by grading cyclic loading and unloading tests. Using the Weibull statistical distribution and the introduction of damage variable D, a damage constitutive model suitable for coal at loading and unloading stages was established, which conforms to Mises yield criterion based on the coal microelement strength. According to the test data, the model parameters of each cycle grade were determined by nonlinear fitting method, and the physical significance of the model parameters was analyzed. The results show that the damage constitutive model based on Mises yield criterion can reflect the stress-strain relationship of coal under grading cyclic loading and unloading accurately. With the increase of loading and unloading levels, the residual strain accumulates, and the elastic modulus increases first and then decreases gradually. The hysteresis loop shape shows the characteristics of “fat”-“thin”-“fat”. The constitutive model parameters have clear physical significance.

The paper represents a test investigation of the mechanical properties and acoustic emission (AE) characteristics of low-strength coal specimens subjected to cyclic loading and unloading. From the lab tests, the following conclusions can be obtained: (1) The axial strain is very well linear with the loading–unloading cycle number, and the circumferential and volumetric strains are approximately quadratic functions with the loading–unloading cycle number; (2) Under the same loading stress interval, the elastic modulus firstly increases and then remains stable with the loading–unloading cycle number. In addition, the higher the maximum stress of a loading–unloading cycle, the more significant the plastic strengthening effect produced by this cycle; (3) The damage calculated by the cumulative AE hit count can better reflect the fact that the damage has been increasing in the loading phase and keeps basically unchanged in the unloading phase. So, the AE hit count, as a damage variable, can better describe the damage development of coal specimens. (4) The significant fluctuation of the AE b value can be used as the precursor of coal specimen failure. Additionally, the AE b value decreases rapidly at coal specimen failure. (5) The closer to the loading–unloading cycle of coal specimen failure, the more accurate the predicted "maximum magnitude" at coal specimen failure. [ABSTRACT FROM AUTHOR]

With the depletion of shallow mineral resources, the sustainable development and utilization of deep mineral resources will become a normal activity. As a type of clean energy to promote sustainable development, gas in deep coal seams has attracted wide attention. A better understanding of the permeability evolution induced by mining disturbance and the geological environment is of great importance for underground coal exploitation and gas extraction. In order to analyze the evolution of the mechanical properties and permeability of deep coal that are induced by high ground temperature, coal of the Pingdingshan Coal Mine has been investigated, and the seepage tests were carried out by keeping the confining pressure constant and loading and unloading axial stress under different temperature conditions. The effect of temperature on the peak strength and the initial elastic modulus of coal samples is analyzed. The evolution of permeability, which is estimated with the transient pulse method, based on fractional derivative and fracture connectivity, are discussed by establishing the relationship between fracture connectivity and fractional derivative. Meanwhile, the damage variable that is caused by stress and temperature is introduced and the contribution of thermal damage on coal damage accumulation is discussed. A theoretical model is proposed regarding permeability evolution with temperature and stress based on the Cui–Bustin model, which is verified by experimental data. It has been found that the strength and elastic modulus of deep coal decrease nonlinearly with increasing temperature, which demonstrates that temperature has a weakening effect on the mechanical properties of coal. The fracture connectivity and permeability evolution trends with axial strain are consistent under different temperatures, which decrease slowly in the compaction and linear elastic stages, reach the minimum at the volumetric dilation point, gradually increase in the yield stage, then have a sharp increasing trend in the post-peak stage and, finally, become steady in the residual stage. The damage induced by temperature increases with rising temperatures under different external load conditions. When the external load increases gradually, the thermal damage still accumulates, but the thermal damage variable ratio decreases. The proposed permeability model considering temperature and stress can describe the trend of the experimental data. With axial stress increasing, the influence of temperature on permeability decreases, and its leading effect is mainly reflected in the compaction stage and the linear elastic stage of coal. [ABSTRACT FROM AUTHOR]

The deformation and failure features of rock formation in deep coal mines are basic mechanical problems in the complex geology environment and complicated excavation process. Under the effect of cyclic loading and unloading, the bearing capacity weakens and damage degree exacerbates significantly, which seriously threatens the safety and stability of the working face. To study the damage characteristics of sandstone, especially the precursor characteristics of acoustic emission (AE), this paper conduct the AE response experiments on sandstone under cyclic loading and unloading. The results show that with the increasing number of cycles, the loading modulus, unloading modulus, total strain energy, elastic energy, and dissipation energy of sandstone in the cycle stage all increase continuously. In the initial loading stage, the sandstone has fewer cycles and lower stress levels, fewer AE ringing counts and energy, and less rock damage. With the increasing cyclic times and loading stress, the damage degree of sandstone increases rapidly in a very short time. The damage variable represented by ringing count is more sensitive than by energy. Just before rock failure, the ringing count and the energy value increase significantly, and the damage curve rises sharply. In addition, AE waveform signals have obvious aggregation characteristics and four main bands. Just before sandstone failure, the main frequency band becomes wider, the low frequency bands f1 and f2 become connected, and the main signal frequency appears abnormally low and high. The waveform signals before sandstone instability and failure show a phenomenon where the low-frequency amplitude is generally at a high level, the high-frequency signal decreases, the amplitude becomes low, and the multipeak phenomenon weakens. The above characteristics of the AE time domain and waveform analysis can be used as the precursor characteristics of sandstone failure and instability. This study can reveal the process of the sandstone deterioration and AE response under the cyclic loading and unloading condition, and has certain guiding significance for roof and floor control in deep roadway, instability warming monitor of working faces, and guarantees for safety production. [ABSTRACT FROM AUTHOR]

With the depletion of shallow coal mine resources, the mining depth of coal mines is deepening year by year, therefore, gas explosion and other disasters occur from time to time. Gas drainage is the main measure to prevent gas disasters, and the permeability of coal is one of the main factors affecting gas drainage. In order to explore the seepage characteristics of coal under different confining pressure loading and unloading conditions and different gas pressure, the seepage test of raw coal samples was carried out by using the coal rock triaxial loading seepage test device. The results show that the permeability of coal samples decreases with the increase of confining pressure in the loading stage, and increases with the unloading of confining pressure in the unloading stage; Through calculation, it is found that the permeability loss of coal body in the loading stage decreases with the increase of loading times, and during unloading, this value also decreases with the increase of unloading times, and reaches the maximum value at the first loading and unloading. With the increase of loading and unloading cycles, the permeability loss of coal samples decreases, and the first loading and unloading is the main stage of permeability loss of coal samples. At the same time, it is found that when the confining pressure increases, the permeability loss of coal samples decreases, and the initial permeability of coal samples maintains a good linear growth relationship with the increase of gas pressure. [ABSTRACT FROM AUTHOR]

In many underground engineering projects, rocks are often subjected to cyclic loading and unloading, such as repeated excavation of roadway surrounding rock, which will lead to damage to underground rocks, and the energy of rocks also changes. Therefore, to study the energy evolution and damage characteristics of rocks under cyclic loading and unloading, different cyclic loading and unloading tests of bituminous sandstones under constant amplitude were conducted. Under cyclic loading and unloading, the lower limit stress was 40% of the rock peak intensity, the cyclic amplitude was 20–40% of the peak intensity, and the number of loading–unloading cycles was 10–30. The quantitative characterization of the damage degrees of bituminous sandstone was realized by the ultrasonic wave velocity and elasticity modulus methods. The energy evolution and damage characteristics of bituminous sandstone under different amplitudes and number of loading–unloading cycles were investigated through the energy dissipation method. Results showed that under cyclic loading and unloading, the ultrasonic wave velocity and elasticity modulus of bituminous sandstone decreased gradually; The damage variable shows a trend of rapid and then stable growth and has a power function relationship with the number of cycles; The input energy density and dissipation energy density curves were in L-shaped distribution, whereas the elastic energy density remained stable. The results of this study can provide some theoretical references to underground engineering construction.

• Active wave velocity and passive AE are combined to monitor rock failure. • Precursors of rock failure are extracted from energy, AE, and wave velocity. • There is a clear linear relationship between wave velocity drop and dissipated energy. • A two-part wave velocity theory of rock is proposed. • A multi-indicator joint early warning method for rock failure is proposed. The precursor characteristics of rock failure under cyclic loading and unloading are crucial to the prevention of underground engineering disasters. Taking sandstone as an example, this work conducted rock failure experiments under uniaxial cyclic loading and unloading combined with active wave velocity and passive acoustic emission (AE). The damage evolution process of rock was analyzed from the perspectives of energy, AE, and wave velocity. The results show that as the cyclic load stress level increases, the rigidity of the rock continuously increases, the proportion of elastic energy may reach a peak, and there will be an extreme time difference between the wave velocity and stress. The occurrence of the peak point of elastic energy proportion is a sufficient and unnecessary condition for the precursor characteristics of rock failure. The appearance of numerous AE events with high amplitude and high count is an effective precursor characteristic of rock failure. The stress continues to increase while wave velocity decreases is a key precursor characteristic of rock failure. There is a clear linear relationship between wave velocity drop and dissipated energy, it implies that both the which can quantitatively evaluate the rock damage under cyclic loading and unloading. The variation of wave velocity exhibits obvious anisotropic characteristics. This paper is not only a beneficial supplement to the theoretical system of rock failure precursors, but also provides new ideas and methods for rock stability monitoring and failure disaster early warning in underground engineering. [ABSTRACT FROM AUTHOR]

This study is aimed at revealing the effect of water on the damage and instability of sandstone under cyclic loading and unloading. To achieve this aim, first, an electromagnetic radiation (EMR) experiment was performed on five groups of sandstone samples with gradient water content under uniaxial cyclic loading and unloading to investigate the time sequence characteristics of EMR signals released by these samples. Based on the EMR signals, the damage evolution process of sandstone was studied. Furthermore, the damage prediction formula of water-bearing sandstone under cyclic loading and unloading was established and verified. Finally, the precursor characteristics of sandstone instability were analyzed with reference to the critical slowing down theory. The experiment results are as follows. The existence of water significantly weakens EMR, but the weakening rate decreases with the increase of water content. Besides, the damage variable of samples with different moisture contents based on EMR energy changes steadily first and then increases with the rise of the peak stress in the loading and unloading stage. Moreover, with the rise of water content, the number of cycles corresponding to the sudden surge of the damage variable gradually decreases. The verification suggests that the damage prediction formula of water-bearing sandstone based on EMR signals is highly accurate. The variance of EMR signals rockets suddenly prior to sandstone instability, so it can be used as a precursor for the instability and failure of sandstone samples. On top of that, the higher the water content is, the earlier the precursor occurs. [ABSTRACT FROM AUTHOR]

The rock mass in the hydro-fluctuation belt of the reservoir bank slope is subjected to long-term water-rock interaction and frequent moderate-low intensity reservoir earthquake. The deterioration of the mechanical properties of the rock mass caused by this directly affects the dynamic response and seismic capacity of the reservoir bank slope. Based on this, the settings of the fluctuating zone of the bank slope of the reservoir are simulated, and the water-rock interaction test to simulate the periodic rise and fall of the reservoir water level is designed and carried out. In the course of the test, the cyclic loading and unloading method is used to simulate the influence of seismic action, and the sequential action of water-rock interaction and cyclic loading and unloading is mainly considered. According to the test results, the following conclusions are obtained. (1) The dynamic parameters of the rock sample generally change exponentially from steep to gentle under the water-rock interaction. After considering the sequential action of water-rock interaction and cyclic loading and unloading, the deterioration rate and trend of dynamic parameters of rock samples increase obviously, indicating that frequent moderate-low intensity reservoir earthquakes can obviously promote the damage development of bank slope rock mass in the environment of water-rock interaction for a long time. (2) Under the action of water-rock interaction and cyclic loading and unloading, the microstructure of rock samples gradually loosens from the compacted state, and the corresponding integrity degradation coefficient and the secondary porosity also show a trend of steepness and then slowness. Among them, the microstructure changes of rock samples under the sequential action of periodic water-rock interaction and cyclic loading and unloading are the most significant, followed by the rock samples with initial cyclic loading and unloading damage, and the change of rock samples under the water-rock interaction alone is the smallest. The changes and differences of the microscopic structure of the rock sample under different schemes also govern the degradation law of its dynamic characteristics. (3) Under the long-term water-rock interaction and frequent moderate-low intensity reservoir earthquakes, the internal damage of the reservoir bank slope will gradually accumulate and develop, which will directly affect the dynamic response characteristics and seismic capacity of the reservoir bank slope. Therefore, the degradation law of the dynamic characteristics of the bank slope rock mass should be systematically considered in the analysis and evaluation of long-term seismic performance of reservoir bank slope.

When the coal-seams are multi-layered, the stress and permeability of the caving zone are disturbed again during the following coal seam mining. Repeated unloading and compaction cause caved coal and rock to break further in the caving zone, which in turn affects the migration of gas in goaf. This paper briefly summarizes prior research on caving zone internal stress, porosity, permeability and other parameters of the process of longwall face advancing. The forecast methods for calculating the stress and permeability of the caved zone during its compaction are listed. On the basis, this paper describes a laboratory experiment of the broken coal and rock in the caving zone of the gob in Huainan coal mine. The laboratory test results imply that the re-crushing, re-arrangement, and compressional deformation of particles in the loading process lead to a drastic drop in the caving zone porosity, which causes the permeability reduction. The permeability sensitivity to stress of broken rock (broken coal) samples was determined by their strength. The greater the proportion of coal samples in the combined coal and rock samples, the higher the permeability stress sensitivity and the smaller the permeability.With the increase of the cyclic times, the stress sensitivity gradually weakened. Based on laboratory experiments, the stress-permeability fitting model of the broken coal and rock in the caving zone during the group coal seam mining is proposed in this paper.