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The GAW-2000 microcomputer-controlled electro-hydraulic servo rock single-axis testing machine was used to carry out tie classification cyclic loading and unloading test based on the underground deep granite as the test material. The changed of compressive strength, deformation, failure characteristics, residual strain and elastic modulus trend of the specimens were studied under the same increasing load amplitude. The results shown that the peak strength of the rock sample shown a decreasing trend with the increase of the number of cycles per stage under the action of the uniaxial hierarchical cycle plus unloading test, and the cyclic load stress-strain curve was enveloped by a monotonic load stress-strain curve. Rock sample failure characteristics were mainly shear failure, and the failure cracks were more dispersed with the number of cycles increases. During the loading process of the rock sample, the structure inside the rock mass was continuously adjusted so that the structural surface becomes dense and the fissure is more closed. There was a lag phenomenon in the rebound deformation during unloading, and the load start point does not coincide with the unload end point. There were similar features of elastic strain and residual strain on axial and circumferential with the number of cycles changed. Elastic modulus curve of the rock sample changes alternately in a "wavy" pattern. The research results can provide theoretical basis for the gradual rock failure process of underground engineering under action of circulating loads. [ABSTRACT FROM AUTHOR]

Tunnel excavation, mining and other projects involve the phenomenon of periodic compression and tension of rock stress. The mechanical effect produced by construction conditions has a significant impact on rock structure. In order to study the deformation and failure characteristics of rock under different loading and unloading rates, the cyclic load test of sandstone at different loading and unloading rates under uniaxial conditions is designed. Based on tie stress-strain curve, hysteresis loop and residual strain, the evolution law of damage, instability and deterioration of sandstone rock mass under cyclic load is deeply analyzed. The results show that compared with the monotonic loading condition, the cyclic load can strengthen the peak stress of rock samples. The residual strain of banner cyclic load is related to the number of cycles in a logarithmic function. In the graded cyclic load, the change of the upper limit stress to the strain is greater than that of the lower limit stress to the strain. The fitting result of residual strain and cycle times is a linear correlation function with the same slope. The loading mode has a certain influence on the elastic modulus of rock samples, and the same loading and unloading rate has the greatest influence on it. [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]

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The existing fatigue models are not sensitive to load cycles and do not consider the effect of osmotic pressure. To study the strain characteristics and fatigue constitutive model of limestone under osmotic pressure and cyclic disturbance coupling, 300 cycles of loading and unloading tests under different osmotic pressures were carried out and the evolution characteristics of irreversible accumulative deformation curve were analyzed. Then, an improved Nishihara fatigue model was established based on the theoretical results of rheological model by introducing the nonlinear function into Nishihara model. The results show that the irreversible cumulative deformation under different osmotic pressures experiences three evolutionary processes of initial deformation stage, stable deformation stage, and accelerated deformation stage with increasing cycle indexes, showing the typical fatigue deformation characteristics. The axial deformation at initial deformation stage increases significantly, the stable deformation stage shows approximately linear growth and accounts for 40% to 73% of total cycle indexes, and the accelerated deformation stage shows a sudden increase indicating the overall instability of limestone. The nonlinear function is introduced into the Nishihara fatigue model based on rheological theory, and a fatigue deformation model that can reflect the entire process of irreversible accumulative deformation curve is established. The physical meanings of model parameters are discussed, and the relationships between model parameters and cycle indexes are inverted. The model parameters have good convergence and verify the rationality and applicability of the fatigue constitutive model based on experimental data. The results have reference value for revealing the fatigue characteristics of rock under osmotic-cyclic stress coupling. [ABSTRACT FROM AUTHOR]

To ensure the safety of coal mining, lots of coal pillars were reserved in the coal mine. The stability of coal pillar and roof composite structures determines the safety of stope. Under the action of cyclic loading and unloading such as mining face, chamber blasting, and roadway excavation, the coal pillar, and roof composite structure will gradually instability, which will affect the safety production of the coal mine. Therefore, this paper takes the dynamic disaster of coal pillar and roof composite structure as the research background, carries on the cyclic loading and unloading test of the coal-rock composite structure with different coal-rock height ratios, and studies the influence of coal-rock height ratio on the progressive instability characteristics, mechanical properties and damage variables of the coal-rock composite structure. The crack initiation stress, crack propagation time and failure pattern of coal-rock composite structure are analyzed. The improved load-unload response ratio considering the damage variable is established, and the stability of the coal-rock composite structure under cyclic loading and unloading is quantitatively analyzed. It is concluded that the larger the coal-rock height ratio is, the greater the load-unload response ratio of the composite structure is, and the more unstable the composite structure is. This conclusion is consistent with the crack initiation stress and fracture propagation time, and can quantitatively reflect the progressive instability process of coal-rock composite structure, and provide a basis for revealing the dynamic disaster of coal pillar and roof composite structure. [ABSTRACT FROM AUTHOR]

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In order to study the post-peak permeability evolution of limestone under triaxial cyclic loading and unloading conditions, this paper takes limestone from the Suoyuwan South Station of Dalian Metro Line 5 as the research object. A rock multi-field coupled triaxial tester is used to carry out triaxial cyclic loading and unloading tests and post-peak permeability tests under different pre-peak fracture confining pressures. The analysis is carried out from the aspects of stress–strain, the change of mechanical parameters, the influence of confining pressure and seepage pressure on permeability, and the characteristics of the fracture surface. The hydraulic parameters and post-peak permeability evolution characteristics of limestone at different stress stages are summarized, and the test data are fitted to obtain the relationship between the permeability of the fractured rock sample and the change of the pre-peak fracture confining pressure or seepage water pressure. According to the macroscopic fracture morphology, the limestone fracture stress-seepage coupling program is compiled by FLAC3D embedded FISH language. It is used to numerically simulate the permeability of limestone at different fracture angles, which is in good agreement with the experimental values. The test results show that with the increase in the pre-peak fracture confining pressure, the peak strength of limestone increases, and the fracture angle with the axial direction also gradually increases, showing shear failure as a whole. The elastic modulus shows an upward trend in the pre-peak stage, while the evolution law of Poisson's ratio is just the opposite. The permeability increases with the increase in the pre-peak fracture confining pressure and pore water pressure. The results of this study can provide references for the study of the permeability evolution law of surrounding rocks in karst water-rich areas. [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.

Given the roadway deformation and surrounding rock control in the process of thin coal seam mining, the coal-rock combination with inclination angles for 0°, 15°, 30°, 45°, and 60° was designed. The rock mechanic test machine was used to conduct a uniaxial compression test and a graded loading and unloading test on the coal-rock combination; the failure characteristics of the two were compared and analyzed. The stress–strain data obtained by the experiment is plotted by Origin software. On the basis of the obtained curve, energy density is integrated and the energy density data is fitted. The results show that ultimate compressive strength of coal-rock combination under uniaxial loading is higher than that under cyclic loading and unloading. The failure of uniaxial primary loading mainly presents a single shear failure mode, while the coal and rock mass present X-type shear failure mode under graded loading and unloading, which makes the coal body more break. From the energy density curve, it can be observed that with the increase of inclination angle, the ultimate compressive strength of the coal-rock combination decreases, and its input energy and elastic energy also decreases, and the dissipation dissipates faster. In the follow-up research process, the deformation characteristics of coal and rock mass with a large dip angle will continue to research roadway support stability. The adaptability of roadway support provides a reference for roadway support engineering of coal and rock with large dip angle, which has a specific reference value. [ABSTRACT FROM AUTHOR]

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.

This paper mainly explores the deformation characteristics of limestone specimens under constant load cyclic loading. For limestone specimens under uniaxial compression, the stress-strain curve can be divided into three stages: compaction stage, elastic stage and sudden failure stage. Under cyclic loading, the hysteresis loop on the stress-strain curve is long and thin, taking the shape of "toothpicks". The axial strain and radial strain both change with the stress amplitude and cycle number, but in different variation patterns. There is a stress amplitude "threshold" for radial deformation, indicating that the radial deformation is more sensitive to stress amplitude than the axial deformation. It is calculated that the incremental deformation between peaks includes both plastic deformation and the deformation recoverable after unloading, and the recoverable deformation is positively correlated with the load amplitude of the cyclic loading.

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.

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.

In deep underground engineering, surrounding rocks are subjected to cyclic loading and become prone to fractures due to the seepage pressure. To explore the mechanical properties, damage evolution, and permeability response characteristics of sandstone under cyclic loading action, three tests, namely, monotonic loading hydraulic coupling, constant amplitude and variable amplitude cyclic loading and unloading tests, were performed in this work. The mechanical characteristics, damage evolution, and permeability response characteristics of sandstone were then analyzed. Results show that, 1) under the monotonic loading hydraulic coupling action, the elasticity modulus of sandstone increases along with the confining pressure and decreases along with an increasing osmotic pressure, whereas the change in its strength is positively correlated to the elasticity modulus; 2) Given the limited compressible volume of broken sandstone, its permeability significantly decreases along with increasing confining pressure. Given that the permeable channels are changed along with the crack development laws under cyclic loading, the permeability change of sandstone after the yield stage differs from that under the conventional loading action, and the failure mode is classified as tensile-shear mixed failure; 3) A damage variable model of cyclic disturbance under cyclic loading-hydraulic coupling action was established. The conclusions obtained in the study provide references for deep underground engineering construction. [ABSTRACT FROM AUTHOR]

To study the acoustic emission evolution characteristics of saturated limestone under different loading and unloading paths, three cyclic loading and unloading tests were conducted with different loading rates and initial cyclic peak stresses, and acoustic emission monitoring was performed simultaneously. The results indicate that, during loading and unloading, the intermediate-frequency signals of saturated rock exhibit a variation trend of sparse–dense–sparse signals, whereas the low-frequency signals are continuously and massively produced. With the increase in the loading rate, the development trends of cumulative hits and energy become closer, and the development form of ringing count changes from N-type to U-type and then to N-type. The slight increase period and attenuation period are extended, whereas the intense growth period and postpeak calm period are shortened. With an increase in the initial cyclic peak stress, the change in cumulative energy is more obvious than that in cumulative hits near the rock failure. The development form of the ringing count changes from U-type to W-type and then to N-type, and each period is first shortened and then extended. With the increase in loading rate, the increase in the slow-change period tends to change from gradually increasing to increasing and then decreasing. By contrast, the increase in the step tends to change to a gradual increase. With the increase in the initial cyclic peak stress, the duration of and increase in the energy in the step and the slow-change period tend to decrease and then increase.

Coal bumps occur frequently during coal mining, and monitoring of stress variations of coal mass is a prerequisite for coal bump prediction. To investigate a new method to reflect coal stress variation based on the pressure stimulated current (PSC), localized cyclic loading–maintaining–unloading experiments on coal samples were carried out and weak currents were measured. The response characteristics of PSCs with stress variations were analyzed, physical models showing the mechanisms of PSCs were established, and mathematical models explaining the relationships between PSCs and mechanical behaviors were derived. The results show that PSCs (tens of nA) flowing from the stressed volume to the un-stressed one can be observed upon load application. The currents are repeated with cyclic loading, load maintenance and unloading, which indicate that PSCs correspond well to stress variations. During loading, the PSC increases to a peak value at approximately 0.6 MPa and then it fluctuates until the stress stops increasing. When load is kept constant, the current decays exponentially to a stable value. During unloading, the PSC decreases at a lower rate when stress is greater than 0.6 MPa, after which it decreases at a higher rate to a minimum when stress is removed. Fitting results show that PSC increases exponentially with loading rate and that the stable current increases linearly with the maintained stress. Positive holes activated by the fracture of chemical bonds in the side chain of coal macro-molecules are believed to be the carriers, and the PSCs are generated due to the movement of the holes along stress gradients. Deduced formulas relating PSCs with coal mechanical behaviors show that PSC is proportional to strain rate; that is, PSC is inversely proportional to elastic modulus when stress rate is constant, as verified by the experimental results. These research results are expected to provide a new idea for monitoring of stress variations of coals during mining, which is significant for the prediction and early-warning of coal bumps. [ABSTRACT FROM AUTHOR]

In underground engineering, such as mining engineering and deep tunnel engineering, the rock is often loaded and unloaded repeatedly. The strength of rock under cyclic load is lower than that under static load. To obtain the fracture response of the rock, the three-point bending tests of notched granite beams under cyclic loading and unloading were carried out with Electro-hydraulic Servo Material Test System. The acoustic emission technology was adopted to monitor the acoustic emission events of sample in the process of fracture. It is revealed that the fracture toughness of granite under cyclic loading and unloading is lower than that under static loading. Based on the acoustic emission energy obtained from monitoring, the damage evolution during cyclic loading and unloading was analyzed. The fracture mode of granite samples is analyzed by the RA value-average frequency correlation method. And the Felicity ratio during the loading and unloading cycle was calculated to evaluate the severity of initial damage of the material. It is revealed that Kaiser effect appears only in the elastic deformation stage of cyclic loading unloading bending. The Holmquist–Johnson–Cook damage constitutive model and Weibull distribution were used to establish the heterogeneous granite model. And the three-point bending of the model under cyclic loading and unloading was simulated to disclose the crack growth mechanism of rock. The study may provide some references for rock instability control in geotechnical engineering construction. [ABSTRACT FROM AUTHOR]

In this paper, the permeability and porosity of self-prepared artificial columnar jointed rock masses (CJRM) with different columnar dip angles under different pore pressures during cyclic loading-unloading of confining pressure were measured. The experimental results indicate that the gas permeability of the artificial CJRM gradually decreases with the rise of pore pressure due to the existence of Klinkenberg effect, and Klinkenberg effect gradually decreases with the rise of columnar dip angle. The existence of it improves significantly the intrinsic permeability. Besides, the intrinsic permeability and porosity of artificial CJRM are more sensitive to the first loading than that of the intact cement mortar specimen. The permanent deformation of pores in the specimens mainly appears in the first cycle, and the subsequent cycles have smaller effects on the intrinsic permeability and porosity. The permanent deformation results in the intrinsic permeability, and porosity in the loading process is always higher than those in the unloading process. The intrinsic permeability gradually increases with the growth of columnar dip angle, exhibiting obvious permeability anisotropy. Under high confining pressure, the permeability anisotropy of artificial CJRM is less than that under low confining pressure. Meanwhile, under the same confining pressure, during the repeated loading-unloading cycles, the permeability anisotropy decreases gradually. The existence of a representative element volume (REV) for CJRM is verified; thus, the equivalent continuum media model can be utilized to analyse the seepage characteristics of CJRM. [ABSTRACT FROM AUTHOR]

To study the mesodamage of migmatite under cyclic loading, single cycle loading and unloading tests of migmatite were carried out under different confining pressures (σ3) and stress ratios (p), and acoustic emission (AE) technology was used to monitor the evolution of microcracks. Additionally, the mesodamage of migmatite was analyzed from the porosity and T2 spectrum distribution measured by nuclear magnetic resonance (NMR) spectroscopy. The results show that the peak stress ratios (pmax) of migmatite under different σ3 are different, and as the value of σ3 increases, the migmatite gradually undergoes a change from tension to shear failure. Moreover, with the increase in σ3, high-frequency signals gradually appeared, indicating that the increase of σ3 to a certain extent limited the generation of large size microcracks. The variations in damage variables defined by the number of AE hits and porosity are the same, "steep increment type," and increase suddenly when approaching pmax. Furthermore, σ3 slightly affects the pore throat distribution of migmatite. In addition, σ3 has a certain inhibitory effect on the porosity of migmatite and accumulation of mesodamage. However, when σ3 exceeds a critical value, the inhibitory effect becomes a promoting effect. [ABSTRACT FROM AUTHOR]

Gas permeability in coal characterizes the migration capacity of gas in coal, and it is an important parameter influencing the gas flow in a coal seam. Energy dissipation and energy release are important causes of rock mass failure. By utilizing a THM-2-type thermo-fluid–solid coupling test system for gas-bearing coal developed at Chongqing University, China, experimental research was conducted on raw coal samples from the Jinjia Coal Mine, Guizhou Province, China, under two different cyclic loading paths. The experimental results demonstrated that the overall change trends of permeability were consistent under the different loading and unloading paths and decreased exponentially with increasing stress. As the unloading level increased, the permeability hysteresis loops changed from sparsely to densely distributed and from spindle-shaped to crescent-shaped and ladder-shaped. The change laws of the upper and lower limits of permeability were closely related to the stress paths. The change trends of the lower limit of permeability under the two paths were consistent with that of the overall permeability and decreased exponentially with the increase in energy density. When the lower limit of unloading stress was constant, the upper limit of permeability gradually increased exponentially with increasing energy density, indicating that the gas permeability in the coal samples had strong hysteresis characteristics. The energy densities under the different paths nonlinearly increased with increasing stress; these relationships could be fitted by quadratic functions. The total input energy density showed the fastest rate of increase, followed by the elastic energy density, while the dissipated energy density had the slowest rate of increase, demonstrating that the energy evolution showed nonlinear characteristics. The experimental results were in good agreement with the evolution characteristics of the permeability of the stress loading and unloading zones in front of the working face. Understanding permeability from the perspectives of different stress loading and unloading paths and energy densities provides certain insights into the safe production and extraction of coalbed methane from coal mines. [ABSTRACT FROM AUTHOR]

In the process of deep roadway excavation, the rock mass is subjected to periodic load and high geostress, so the general strength criterion cannot describe the stress-strain curve of rock. However, the cyclic loading and unloading constitutive model of rock under high geostress is the key to predict the long-term stability of deep roadway under periodic load, thus it is urgent to carry out the study of rock constitutive model under high confining pressure. The process of rock subjected to external load to failure presents the propagation of original cracks, growth and propagation of new crack. Previous studies have shown that the number of cracks in rock obeys Weibull distribution and the Griffith criterion assumes that the failure of rock is caused by the crack propagation. Based on these criteria, this paper establishes a statistical damage constitutive theory of cyclic loading and unloading of rock under high confining pressure by extending Weibull statistical damage constitutive. Through the equation transformation of constitutive model, the threshold value of constitutive damage is studied. Furthermore, the constitutive parameters was fitted with the data of even cyclic loading and unloading test, and the evolution rule of the parameters is obtained. By comparing the constitutive theory with the odd cyclic loading and unloading test, the accuracy of the constitutive model is verified, which provides a new insight for the study of the constitutive model of rock under high confining pressure. [ABSTRACT FROM AUTHOR]

Copyright of Industrial Minerals & Processing / Huagong Kuangwu yu Jiagong is the property of Industrial Minerals & Processing Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Chinese Journal of High Pressure Physics is the property of Chinese Journal of High Pressure Physics Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this study, the compressive deformation of crushed sandstone was tested using a crushed rock deformation-seepage test system, and the effects of various factors, including crushed rock grade, grade combination, water saturation status, and stress loading method (i.e., continuous loading or cyclic loading and unloading), on the compressive deformation of crushed sandstone was analyzed from four perspectives including stress-strain, bulking coefficient, deformation mechanism and energy dissipation. The results indicate that the stress-strain relations of crushed sandstone are closely associated with all factors considered, and are well represented by exponential functions. The strain observed for a given applied stress increased with increasing crushed rock grade throughout the loading period. Crushed sandstone grades were combined according to a grading index (n), where the proportion of large-grade rocks in the sample increased with increasing n. The bearing capacity of a water-saturated crushed sandstone sample with n = 0.2 was less than that of an equivalent dry sample for a given applied stress. The stress-strain curve of a water-saturated crushed sandstone sample with n = 0.2 under cyclic loading and unloading was similar to that obtained under continuous loading. Observation and discovery, the deformation mechanism of crushed sandstone was mainly divided into four stages, including crushing, rupture, corner detachment and corner wear. And 20% of the work done by testing machine is used for friction between the crushed sandstone with the inner wall of the test chamber, and 80% is used for the closing of the void between the crushed sandstone, friction sliding, crushing damage. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Heilongjiang University of Science & Technology is the property of Journal of Heilongjiang University of Science & Technology Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

During the constructions of motorways and high-speed railway lines in the Yanji Basin, large amounts of excess mudstones due to the enormous tunnel excavations and slope cuts would be deposited as landfills. Assessing the deformation and permeability of Yanji mudstone became important for the design, construction and operation of the landfills. This paper presents an experimental study on the deformation and permeability of Yanji mudstone by carrying out a series of oedometer tests with loading/unloading cycles. The results show that the sample with a lower initial water content exhibited greater swelling deformation after inundation, a lower yield stress, greater deformation and a higher hydraulic conductivity during the loading/unloading cycles. As the number of loading/unloading cycles increased, the yield stress and accumulated plastic deformation increased, while the compression index, rebound index and hydraulic conductivity decreased. The samples became stiffer and their hydromechanical behaviour tended to be stable after three cycles. The compression curves could be divided into pre-yield and post-yield zones. The post-yield zones of compression curves and the rebound curves could be normalized into a unique line, and the pre-yield zones of the compression curves could be described as lines. Basic equations were developed to predict mudstone deformation under cyclic loading and unloading. Additionally, an empirical relationship between the hydraulic conductivity and void ratio was also proposed. The ability of the proposed methods was verified by the overall good agreement between the experimental results and predicted values. [ABSTRACT FROM AUTHOR]

• The cyclic loading and unloading path based on gas extraction borehole is designed. • The strength characteristics and energy dissipation characteristics of samples under different unloading rates are studied. • The crack evolution process of specimen failure is described. • The influence mechanism of stress adjustment lag on strength, crack evolution and damage under different unloading rates is proposed.. In coal mining, the gas pre-drainage boreholes in front of working faces are often repeated loading and unloading. Studying the failure characteristics of boreholes under cyclic loading has a guiding significance for clarifying the failure mechanism of gas pre-drainage boreholes. To simulate the influence of the advancing speed of the working face on the deformation instability of the borehole, a cyclic loading path under the influence of different unloading rates was designed. In this paper, the uniaxial cyclic loading test was conducted by the DYD-10 testing machine, and the variation law of mechanical strength characteristics with the unloading rate was obtained. Combined with digital image technology (XTDIC), the sample deformation characteristics were analyzed. The samples' compressive strength rose with the unloading rate. The sample was damaged in the tensile-shear composite form at different unloading rates. Crack opening displacement (COD) with the increase in the number of cycles shows a "stepwise" increase. Near the moment of destruction, COD suddenly increased. The number of surface fractures that could be seen reduced as the unloading rate increased, and reduced fragmentation when the sample failed. The various types of energy of the sample increase nonlinearly with the increase of the number of cycles, and the plastic damage energy drops as the unloading rate increases. When the external stress state enters the next stress state, the internal stress adjustment of the sample takes a particular time, which is manifested as lagging behind the change of the external stress state. The influence mechanism of stress adjustment lag on strength, crack evolution and damage under different unloading rates was proposed. The results of the study contribute to the research of destabilization control technology for gas pre-drainage boreholes. [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.

In order to investigate rock mechanical behavior under cyclic loading and unloading conditions, the clump parallel-bond model (CPBM) combined with the newly-proposed loading procedure was used in numerical simulations and the simulated results were compared with those of the experiment. Then the intact rock under different confining pressures and pre-cracked rock with different flaw inclination angles are also investigated in detail. The related numerical results have demonstrated that: (1) the CPBM, combined with the use of the newly-proposed loading procedure, can be used to modelling the intact rock and pre-cracked rock mechanical behavior under cycle loading and unloading conditions. The simulated results are in good agreement with the experimental results; (2) the number of microscopic cracks increases with the cycle increases during the loading process, while few microscopic cracks are generated during the unloading process. The higher the confining pressure, the less damage the rock has accumulated during the cyclic loading process; (3) the inclination of the pre-existing flaw has a significant effect on the macroscopic fracture pattern. [ABSTRACT FROM AUTHOR]

During longwall mining, the permeability evaluation in a caving zone is one of the most challenging risks for mine operators because it controls the behavior of gas flow and it significantly influences gob gas extraction and spontaneous combustion. Based on the stress environment of repeated mining in the Huainan coal mine, this study discusses the results of a laboratory experiment on broken coal and rock in a caving zone of a gob. The stress–permeability relationship of broken coal, rock, and combination of coal and rock samples with various particle sizes under cyclic loading and unloading was experimentally studied. The laboratory test results imply that permeability stress sensitivity and permeability loss of the first loading and unloading were significantly greater than the succeeding cyclic loading and unloading. Re-crushing and re-arrangement were the main two factors that lead to a drastic drop in caving zone porosity causing permeability reduction in the first loading and unloading process. The initial permeability and initial pore compressibility increased logarithmically when particle size increased, while the changing rate of pore compressibility decreased exponentially. The stress sensitivity of the broken coal samples' permeability increased when particle size increased. Based on the laboratory experiments, the stress–permeability fitting model considering particle size of broken coal and rock in a caving zone during repeated mining was proposed in this study. [ABSTRACT FROM AUTHOR]

The permeability characteristics of a broken coal mass under repeated loading and unloading conditions exert significance on spontaneous combustion of coal in goaf during the mining of coal seam groups. Considering this, by using the seepage test system for broken coal-rock mass, seepage tests under cyclic loading and unloading conditions, were carried out on broken coal masses. The test results show that the fitting curves between permeability and effective stress, strain and porosity are a logarithmic function, cubic function and power function, respectively. Besides, the permeability of a broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law. With increased cyclic loading and unloading times, the permeability loss, stress sensitivity and the crushing amount of the broken coal sample were gradually reduced, but the particle size gradation of the broken coal sample gradually became better. During one loading and unloading cycle, the stress sensitivity of the permeability of coal samples in the loading stage was far higher than that in the unloading stage. In the loading stage, the re-arrangement, breakage and compressive deformation of coal particles can lead to a reduction in porosity, consequently resulting in a decreased permeability. In the unloading stage, only the permeability reduction of coal samples due to particle deformation can be recovered. [ABSTRACT FROM AUTHOR]

Further research is needed on the deformation behaviour and the macro-micro failure mechanisms of frozen soft rock driven by energy under cyclic loading and unloading conditions. Combined with specific engineering projects of freeze shaft construction, taking frozen weakly cemented sandstone (−10 °C) as the research object, uniaxial cyclic loading and unloading (UCLU) tests at different rates and microscopic observations of the fracture surface by scanning electron microscopy (SEM) were conducted. Then the failure mechanisms of frozen weakly cemented sandstone during UCLU were driven by energy, which was revealed by the proposed method for calculating rock damage based on damage energy. The results showed that the peak strength of sandstone was much smaller under UCLU conditions than the uniaxial compression test, proving that the mechanical properties of the frozen sandstone were affected by UCLU. The dissipation energy and damage energy of sandstone grew with increasing rates of loading and unloading, but the elastic energy and damping energy showed opposite trends. The energy evolution eventually shifted from being dominated by damping energy to being dominated by damage energy as the loading and unloading rates increased. The cumulative damage of sandstone grew logarithmically with increasing loading and unloading rates, and tensile cracking considerably increased at fast loading and unloading rates. Hence, the fast loading and unloading rates caused the rock dissipation energy and damage energy to be larger, which was the main reason why damage was more significant at fast than slow rates. • Macro-microscopic behaviour of frozen sandstone with different loading and unloading rates were obtained. • A frozen soft rock damage characterization method based on damage energy was proposed. • Relationship between energy evolution and damage of frozen sandstone during the test process was established. • Fast loading and unloading rates promoted the transformation of dissipated energy to damaged energy. [ABSTRACT FROM AUTHOR]

To analyse the effect of loading rate on the energy evolution of rocks under cyclic loading and unloading, tests on saturated limestone were conducted at loading rates of 0.15, 0.2, and 0.3 mm/min, and the evolution characteristics of plastic, elastic, dissipation, and input energies were examined under different loading rates. The results indicated that the plastic strain in the entire test was directly proportional to the loading rate. In addition, strength, residual stress, plastic energy, and dissipation energy under residual resistance were inversely proportional to the loading rate. The plastic strain exhibited a decreasing–stabilising–increasing trend, and the smaller loading rate delayed the “increasing” trend. The increasing extent of each energy exhibited the following trend: input > elastic > plastic > dissipation energy. Furthermore, the first three types of energy exhibited a slow–fast–slow–fast increase trend. The dissipation energy exhibited a fast–steady–fast–slow–fast increase trend. Additionally, the elastic energy index exhibited a large increase–steady increase–decrease trend, which was proportional to the loading rate. The damping ratio exhibited a decrease–increase–decrease–increase–decrease trend which was proportional to the loading rate in the compaction stage and inversely proportional to the plastic stage.

Long-term experiments were performed on red sandstones after different thermal treatments (25, 300, 700 and 1000 °C) under multi-step loading and unloading cycles and a confining pressure of 25 MPa. Furthermore, to quantitatively analyse the temperature influence on the deformation behaviours of the specimens, the concept of the temperature-strain rate was proposed to describe the relationship between strain and temperature, and the experimental results were corrected to identical temperatures (i.e., 20 °C), to overcome the influence of periodic fluctuations in ambient temperature. The results show that the axial mean temperature-strain rate first increased as temperature increased from 25 to 300 °C and then decreased with increasing temperature, whereas the lateral mean temperature-strain rate decreased with increasing temperature. The total strain was divided into the instantaneous elastic strain, the instantaneous plastic strain, the visco-elastic strain and the visco-plastic strain. The total axial strain increased with increasing deviatoric stress, and the irrecoverable strain increased with increasing loading and unloading history. Furthermore, the total axial strain increased with increasing temperature; specifically, at 1000 °C, it was approximately two times that at 700 °C and three times those at 25 and 300 °C. The instantaneous elastic strain and the instantaneous plastic strain increased approximately linearly with increasing deviatoric stress, whereas the creep strain varied with deviatoric stress in complicated ways at different temperatures. However, under identical deviatoric stress, the instantaneous elastic strain and the instantaneous plastic strain increased slightly as temperature increased from 25 to 700 °C and then increased substantially as temperature reached 1000 °C, whereas the variations in the creep strain, the visco-elastic strain and the visco-plastic strain were dependent on temperature and stress level. Finally, the permeability first decreased slightly as temperature increased from 25 to 300 °C and then increased with increasing temperature. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Engineering research has shown that the surrounding rock of deep roadways experienced many times of post-peak cyclic loading and unloading. So studying on rock mechanical characteristics of post-peak loading and unloading is helpful to control the deep surrounding rocks. The test using RMT-150B rock mechanics testing system, to experiment on the mechanical properties of sandstone of post-peak cyclic loading and unloading. The results show that: the stress-strain curves of post-peak cyclic loading and unloading have significant plastic hysteretic loops. The area of plastic hysteretic loops gloss back. The enveloping outer enclosure of cycle loading curve is the similar as the stress-strain curves of strain softening stage when the samples failure, which shows that post-peak failure of rocks have significant memory. With the increase of cycles, the cumulative deterioration parameters are gradually increased, and the ultimate bearing capacity, elastic of loading section of samples and cumulative deterioration parameters conform with the exponential attenuation function with constant term. With the increase of surrounding pressure, the total energy, dissipated energy and elastic energy of samples are gradually increased. With the increase of cycles, the total energy, dissipated energy and elastic energy of samples all are gradually decreasing, the rate of reduction decreases gradually. The samples exist in vertical splitting and transverse shear combination failure under the post-peak uniaxial cycles, or exist in shear failure under the post-peak triaxial cycles. The shear plane exists slip traces. [ABSTRACT FROM AUTHOR]

In order to study the mechanic responding mechanism of rock materials under cyclic loading and unloading,uniaxial cyclic loading and unloading tests were carried out to investigate the non-uniform deformation evolution processes of granite specimens.The deformation images of specimens during the tests were collected by CCD camera and based on the digital speckle correlation method,the non-uniform deformation,the evolution laws of the characteristic parameters of the localization during cyclic loading and unloading were analyzed.The research results show that:With the increase of cycle numbers,the non-uniform deformation is more and more obvious.The statistical index Sw of non-uniform deformation increases with the increase of loading stress,and increases with the increase of cycle numbers.After the localization of a specimen appears,the extrusion displacement and the influence width of the localization band all increase with the increase of stress and cycle numbers in the subsequent cycles. [ABSTRACT FROM AUTHOR]

• The incorporation of BF increased the compressive strength by 9.3%-14.2%. • The effect of BF on the fracture pattern of mortar rubble was discussed. • The Felicity effect was observed in the BF mortar rubble. • Based on damage mechanics theory, a intrinsic damage model of BF mortar rubble was developed. To investigate the effect of basalt fibre (BF) content on the mechanical behaviour and damage characteristics of mortar rubble under cyclic loading, uniaxial compression tests and cyclic loading and unloading tests were carried out on specimens fabricated with different BF volume fractions. Based on damage mechanics theory, a damage ontology model of BF mortar rubble was developed. The damage of BF mortar rubble under cyclic loading was identified and analysed using acoustic emission (AE) techniques, and parameters such as the Felicity ratio (FR), ratio of the RA (rise time/amplitude) and the average frequency (RA/AF), response ratio, b-value and peak frequency were used to characterise the failure mechanism of BF mortar rubble. The results of this study revealed that the incorporation of BF increased the compressive strength by 9.3%-14.2%. The response ratios of the initial cycles were 91.2, 53.4 and 24.4 when the BF content was 0.2%, 0.3% and 0.4% respectively. The incorporation of BF helps to delay the onset of critical damage values. The peak frequency signals are concentrated in the region of 50 kHz and 160 kHz. the 50 kHz frequency represents the appearance of fractures in the matrix and the 160 kHz frequency signal represents the occurrence of brittle fractures in the aggregate. The FR trend was a good predictor of damage to the mortar rubble. The damage mode of the mortar rubble changed from tensile fracturing to composite fracturing due to the addition of BF. The performance optimisation concept of mortar rubble proposed in this study provides a reference for the optimal design of BF mortar rubble and thus has a strong application value. [ABSTRACT FROM AUTHOR]

The research of gas-water two-phase flow is significant for gas extraction and CBM in deep coal rock, so a triaxial seepage test system for gas-water two-phase in coal rock is independently developed. The device is equipped has various stress loading modes such as constant pressure, gradient, waveform loading, and manual mode, which can realize the study of the gas phase, water phase, and gas-water two-phase seepage law in coal rock under cyclic loading and unloading. The results of seepage experiments showed that the two-phase seepage test required injection of "gas-driven water" to realize the mixed gas-water seepage, and the appearance of significant seepage in the water phase was about 6100 s earlier than that of single-phase seepage. Compared with single-phase seepage, the damage of the specimens after two-phase seepage tests was more remarkable, the number of cracks and crack area of the specimen were the highest, and the strain hysteresis loops were more obvious. Under the condition of incomplete pressure relief, the permeability of gas in coal rock showed a trend of increasing and then decreasing during two-phase seepage due to the influence of the water phase, and the value was lower than 0.0025 mD. • Triaxial seepage test system for gas-water two-phase in coal rock is developed. • Axial pressure and confining pressure are loaded and unloaded at the same time. • The stress path is designed in a combination of large and small cycles. • Realizing gas-water two-phase mixed seepage in the way of "gas drive water". [ABSTRACT FROM AUTHOR]

The Kaiser effect reflects the memory of the loaded rock to the irreversible damage and deformation. The stress level, loading rate and lithology are the main factors affecting the Kaiser effect of the rock. To identify the accurate stress point of the Kaiser effect, the MTS 816 rock mechanics testing system and the DS5-A acoustic emission testing and analysis system were adopted. The uniaxial cyclic loading−unloading and acoustic emission characteristic test of 90 rock specimens from three types of rocks under different stress level and loading rate was carried out. The evolution of acoustic emission under uniaxial compression of the rock corresponds to the compaction stage, elastic stage, yield stage and post-peak stress drop stage of the rock deformation and failure process and is divided into the quiet period, transition period, active period and decay period of the acoustic emission. The larger the hardness of rock is, the earlier the stress point of the Kaiser effect appears. The loading stress level (σA) has appreciable influence on the Kaiser effect of the rock. When σA ≥ 0.7σc, the Kaiser effect disappears. Usually, the dilatancy stress (crack initiation stress) does not exceed 70% of the uniaxial compressive strength (σc) of the rock, and the stress point can be the threshold to determine whether the Kaiser effect occurs. The influence of loading rate (lr) on Felicity rate (FR) is relatively large when lr < 0.01 mm/s, and FR rapidly grows with increase of the loading rate. When lr ≥ 0.01 mm/s, the influence of the loading rate on FR is relatively small. The findings facilitate the future application of the Kaiser effect and improvement of the accuracy of the acoustic emission data interpretation.

The permeability characteristics of a broken coal mass under repeated loading and unloading conditions exert significance on spontaneous combustion of coal in goaf during the mining of coal seam groups. Considering this, by using the seepage test system for broken coal-rock mass, seepage tests under cyclic loading and unloading conditions, were carried out on broken coal masses. The test results show that the fitting curves between permeability and effective stress, strain and porosity are a logarithmic function, cubic function and power function, respectively. Besides, the permeability of a broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law. With increased cyclic loading and unloading times, the permeability loss, stress sensitivity and the crushing amount of the broken coal sample were gradually reduced, but the particle size gradation of the broken coal sample gradually became better. During one loading and unloading cycle, the stress sensitivity of the permeability of coal samples in the loading stage was far higher than that in the unloading stage. In the loading stage, the re-arrangement, breakage and compressive deformation of coal particles can lead to a reduction in porosity, consequently resulting in a decreased permeability. In the unloading stage, only the permeability reduction of coal samples due to particle deformation can be recovered.