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The investigation of the shear fracture mechanism under cyclic loading holds significant implications for controlling instability in the surrounding rock of roadways. Both uniaxial constant amplitude and stepwise linear loading and unloading tests were carried out to reveal the fatigue damage and shear fracture mechanisms of the coal–rock mass under cyclic stress disturbance. The rock samples for the tests were taken from the lane side of the coal mine's haulage trough, and the evolution of microcrack extension and shear damage were analyzed from the perspective of deformation modulus and energy evolution using the CT scanning technique and excavating the hidden stress–strain information. The results indicate that the microcracks undergo intermittent expansion under constant amplitude cyclic loading and unloading, while they exhibit progressive expansion under stepwise linear cyclic loading and unloading. In both cases, the cumulative damage caused to the rock samples is irreversible. The test discovered a rarely reported phenomenon of shear fracture oscillation, showing a process of "hysteresis–reciprocation–hysteresis–reciprocation," and the corresponding fracture zone formed by shear fracture exhibited a pronounced domino structure. The results also show that the shear oscillation phenomenon is a unique stress–strain state of domino structural evolution. During the failure process of the rock sample, the trends of deformation modulus, energy, stress–strain rate, and rebound strain rate show a high spatial and temporal correlation and spatial synergy. The process of shear fracture oscillation can be subdivided into six stages: initial crack micro-extension, initial crack extension, main shear fracture extension, metastable fracture extension, rapid fracture extension, and accelerated failure. The rotation–gyration motion of the domino structure can slowly release and dissipate excessive energy, providing a potential coping strategy for optimizing the supporting system and controlling the stability of surrounding rock in coal mines by fully utilizing the structure. Highlights: A shear oscillation phenomenon has been discovered in which the typical stress–strain curve evolves in a twice hysteresis–reciprocal shape during the failure stage in stepwise linear cyclic loading and unloading test. This shear oscillation phenomenon is closely related to the motion of rotational–gyration of the unique domino structure in the shear fracture zone. The rotational–gyration motion of the domino structure slowly releases the overload energy, temporarily preserving the integrity of the rock sample and significantly prolonging the duration of the damage. The unloading condition is the main stress environment that generates and triggers the process of shear fracture oscillation, which is closely related to actual underground engineering. [ABSTRACT FROM AUTHOR]

The complex mechanical behaviours of steeply inclined and layered surrounding rock in strong and active fault zones result in control measures that cannot adapt to asymmetric squeezing tunnel and are still unsolved. Hence, the Yuntunbao Tunnel was taken as an example to study this issue based on geological survey and indoor and outdoor tests. The results showed that strong geological structures and abundant groundwater undoubtedly deteriorate the mechanical properties of rocks containing many water-sensitive minerals, approximately 45%. The stepwise growth of deformation characteristics before reaching the rock peak strength and the gradient to abrupt failure characteristics after reaching the rock peak strength are determined via triaxial cyclic and static load tests. According to field test results, the unilateral squeezing deformation is severe and greater than 1.5 m, the average extent of the excavation loosening zone is approximately 10 m, and the highest deformation rate reaches 12 cm/d. The gradual and sudden changes in tunnel deformation are demonstrated to be consistent with the postpeak deformation characteristics of layered rock in indoor tests. Moreover, the steel arch exhibits composite failure characteristics of bending and torsion. Finally, reliable and practical controlling measures are suggested, including the optimised three-bench excavation method with reserved core soil, advanced parallel pilot tunnel, long and short rock bolts, and large lock-foot anchor pipe. Compared with tunnel deformation before taking measures, the maximum convergence deformation is reduced from 2.7 to 0.9 m, and the bearing force of the primary support is also reasonable and stable. [ABSTRACT FROM AUTHOR]

The crack closure, propagation, recovery, and damage characteristics of a granite undergoing uniaxial and triaxial compressive cyclic loading were investigated. The ultrasound speed was measured to quantify the damage and predict the failure of selected granite specimens. The results showed that with the increase of the stress amplitude, new cracks formed in the specimens. The average ratio of the crack recovery stress to the peak stress was greater than that of the crack closure stress, indicating that crack recovery was easier to achieve than crack closure. New cracks can form during unloading, causing the crack recovery strain to increase. In experiments, it was observed that with the increase in the number of cycles, the P-wave velocity differences increased at first, they then reached a steady value, and finally decreased. The ratio of the peak P-wave velocity to the peak S-wave velocity can be used as a precursor to failure, and an abrupt change in that ratio precedes a rock failure. Damage parameters based on crack recovery strain and wave velocity were identified. It was found that the growth rates of the damage parameters first increased and then remained unchanged with the increase in the number of cycles. In the cases of uniaxial cyclic loading and unloading, the results of the two damage parameters were approximately equal. Furthermore, under triaxial cyclic loading, the damage parameters based on wave speed were determined to be greater than those of the other damage parameters. [ABSTRACT FROM AUTHOR]

To reveal the shear-seepage coupling characteristics of fractured specimens under cyclic loading and unloading, the specific test device and test method were designed in this study. The cyclic loading and unloading shear-seepage coupling test on the fractured rock mass under different confining pressures and seepage pressures was carried out by processing "double L-shaped" specimens, and the change laws of the shear characteristics and seepage characteristics of fractured specimens with different roughness were experimentally investigated. The results indicated that the peak shear stress, residual shear stress, and shear stiffness of rough fractures all increase with increasing confining pressure, while the change in normal dilatation displacement is the opposite. Under a constant normal stress, the permeability of rough fracture decreases, increase, and then stabilizes with increasing shear displacement. The peak shear stress of the smooth fracture is 3.7 times lower than that of the rough fracture with the same shear displacement, and the smooth sandstone specimens are all in a shear shrinkage state, with the normal shrinkage displacement of less than 1.0 mm. In addition, during unloading, permeability increases to some extent but cannot recover to the original value. The confining pressure causes permanent damage to the permeability of fractured rock mass. The permeability of sandstone specimens changes primarily in the early loading stage and late unloading stage. Based on the test results, the relationship between permeability and confining pressure follows a negative exponential function under cyclic loading and unloading conditions. Highlights: The shear-seepage coupling test device and test method were designed. The effect of seepage pressure on the hydro-mechanical coupling of rough fractures has a threshold value. The permeability of rough fractured specimen changes in the staged manner of "decrease-increase-stability" with increasing shear displacement. The initial stage of loading and the later stage of unloading are the main stages of permeability change of fractured sandstone specimens. [ABSTRACT FROM AUTHOR]

Cyclic disturbance has a significant effect on rock stability. It is very important to study the nonlinear behavior and failure precursor law of rock to evaluate stability and predict rock mass behavior. In this paper, uniaxial and cyclic loading and unloading experiments of three stress paths were carried out on sandstone and granite, alongside acoustic emission (AE) monitoring. Based on the maximum likelihood estimation method and least square method, the self-organized critical characteristics and AE b-value characteristics of rocks under different stress paths were studied and the nonlinear characteristics and failure precursor laws of rocks were explored according to the AE information before and after the critical point in the process of rock fracture evolution. The sudden change point of AE marks the precursor of self-organized critical behavior of rock from steady state to unsteady state, and the self-organization evolution of rock micro-cracks is affected by high energy AE events. The self-organized collapse phenomenon exists in the whole process of rock failure, from low energy collapse at the beginning to high energy collapse, and the scale gradually increases. The critical point is the region where b-value drops rapidly to the minimum value before failure and the region where b-value starts to fall before failure is the precursor characteristic region. As an important parameter of rock failure and collapse effect, b-value shows nonlinear dynamic behavior of spontaneous evolution, which is closely related to the self-organized critical behavior of rock failure. [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 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]

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]

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]

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 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]

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]

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]

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]

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]

The permeability and porosity of a porous sandstone under cyclic loading-unloading conditions are measured using an integrated permeability and porosity measurement system. Mineral content analysis of the rock is performed. The rock contains open microcracks and pores, which can be closed after cyclic loading-unloading; as a result, the permeability and porosity decrease. Based on the test results, the empirical relations of confining stress-dependent sandstone permeability and porosity are derived. A power law is suggested to describe the relationship between the stress-dependent porosity and permeability of the sandstone under both loading and unloading conditions within the stress range used in the experiment. The permeability of the sandstone is more sensitive to stress than to porosity. It is inferred from the test results that the evolution of sandstone porosity and permeability is related to the relative movement of particles and the closure of microcracks. [ABSTRACT FROM AUTHOR]

During coal excavation, coal pillars can undergo shear failure due to repeated loading and unloading of mining-induced stress, and this failure may be followed by plastic flow. Change in the permeability of the coal pillar under plastic flow is closely related to the loading path. In this study, the development of the permeability of a coal seam under plastic flow was investigated by conducting permeability testing on a shear-yielded coal sample as it was subjected to cyclic loading and unloading. The results show that the permeability is inversely related to the volumetric strain; that is, the permeability decreases as the volume strain increases and vice versa. However, the permeability at a given strain value is greater during loading than that during unloading. The curve of strain and permeability will form an approximately elliptical hysteresis, and the permeability is a multi-valued function of volumetric strain. As the confining pressure is increased, crack opening decreases, and crack penetration moves from the ends toward the middle of the sample. [ABSTRACT FROM AUTHOR]

Columnar jointed rock masses (CJRM) as a type of jointed rock masses with the distinct structure are widely distributed in nature and have received considerable attention during the last few decades. In this paper, an approach, which is a combination of experiment and graphing method, is proposed to verify the existence of a representative elementary volume (REV) of CJRM during cyclic loading and unloading of confining pressure based on physical model permeability experiments. According to the experiment results, mathematical models for calculating permeability tensor of CJRM were established, and the major principal permeability component (PPC), the minor PPC, and the principal permeability angle (PPA) of CJRM during cyclic loading and unloading of confining pressure are calculated. It was concluded that the existence of a REV of CJRM during cyclic loading and unloading of confining pressure can be verified easily by using the proposed method. During cyclic loading and unloading of confining pressure, a REV does not always exist, especially at high confining pressure. The impacts of confining pressure on the major PPC, the minor PPC, and the PPA are large in the first cyclic loading phase. [ABSTRACT FROM AUTHOR]

Porous reservoir rocks are often anisotropic in permeability. It is necessary, therefore, to study the effect of anisotropy on the permeability of porous coal seams that are subjected to stress during repeated mining of grouped coal seams. In the present investigation, specialized permeability evaluation equipment, known as gas injection to flush methane testing apparatus (GIFMTA), was used to study the directional permeability characteristics of the coal under various stress states according to the repeated mining of group coal seams. The stress states include three cyclic loading and unloading (0.5 MPa–16 MPa–0.5 MPa in a single cycle), and different axial stresses (0.5 MPa to 10 MPa) with the same confining stress, and different confining stresses (2 MPa to 10 MPa) with the same axial stress. The test results showed that the axial and radial permeability of the coal samples decreased with the increase in effective stress. The radial permeability of coal samples (parallel to the bedding structure) and their stress sensitivity were significantly larger than the axial permeability of the samples at the same effective stress level. It was observed that axial permeability was more sensitive to the confining stress than radial permeability, while the sensitivity of radial permeability to axial stress was much larger than that of axial permeability. The axial and radial permeability and their decrease during first loading were much greater than the case during later loading processes. With an increase in cycle time, the losses in axial and radial permeability, and their stress sensitivities, were observed to decrease gradually. Based on the laboratory test data and on a "matchstick model," the axial and radial permeability models of the coal samples during a three-cycle loading/unloading procedure were obtained in this paper. Additionally, an anisotropic model was established to investigate the mechanisms that led to the test results using a digital evaluation model (DEM). [ABSTRACT FROM AUTHOR]