Deformation Of Sandstone Research Articles

Characteristics of Deformation and Damage and Acoustic Properties of Sandstone in Circular Tunnel Morphology under Varying Inundation Depths

When water damage occurs in a mine, variations in the immersion levels of tunnels at different burial depths can be observed. There is a significant relationship between the stability of the surrounding rock and the depth of immersion. Therefore, studying the deformation and damage characteristics of sandstone with circular holes at varying immersion depths, along with their acoustic properties, plays a crucial role in maintaining the stability of water-rich roadways. The TAW-2000 press and static strain system were utilized to investigate the mechanical properties, crack evolution, and deformation field distribution of sandstone with circular holes at varying immersion depths. Additionally, this study analyzed the impact of immersion depth on the characteristic parameters of acoustic emission. The results indicate that immersion depth is negatively correlated with the compressive strength and modulus of elasticity of sandstone; as immersion depth increases, the duration of the compression and yield phases of the rock samples also increases, while the duration of the elastic phase remains relatively unaffected. Furthermore, greater immersion depths correspond to a decrease in the total number of cracks, although the proportion of tensile cracks increases, making the formation of secondary cracks less likely. The frequency of acoustic emission events (transient elastic waves generated by the formation, extension, or closure of tiny cracks within the rock) shows a closely correlated dynamic with the stress–time curve of the rock sample. The acoustic emission ringing counts generated by rock samples under submerged water conditions tend to stabilize with a slight increase before signs of rupture appear. Additionally, the cumulative total energy of acoustic emissions from the rock samples decreases as the water level rises. These research findings provide significant reference value for addressing issues related to water immersion and the extent of water saturation in roadways within rock engineering.

Effect of stress unloading rate on fine-scale deformation mechanism of rock under high osmotic pressure

To explore the influence of the working face excavation rate on the rock deformation mechanism and seepage characteristics, deformation and seepage tests of sandstone under different loading and unloading stress paths, such as constant axial pressure unloading confining pressure and loading axial pressure unloading confining pressure, were carried out. Particle Flow Code in 3 Dimensions (PFC3D) and Python were used to realize fluid-solid coupling, and numerical simulation calculations were performed along the test path to analyze the influence of the unloading rate on the fine-scale deformation mechanism and permeability characteristics of sandstone, and the relationship between crack type and permeability was obtained. A sandstone fracture mechanics model is established to analyze the stress concentration degree at the end of the branch crack of the test path. The results show that the rate of confining pressure unloading is inversely proportional to the strain. Additionally, permeability correlates with the principal stress difference in an exponential manner. Interestingly, the sensitivity of permeability to stress shows an inverse trend with the unloading rate of confining pressure. Furthermore, there exists a linear relationship between permeability and the number of cracks. During the unloading process, tensile cracks predominate, and the propagation of shear cracks lags behind that of tensile cracks. The proportion of tensile cracks decreases with the increase of the unloading rate when the axial pressure is unchanged but increases when axial pressure is added, resulting in axial compression deformation and expansion deformation along the unloading direction. These research outcomes offer theoretical insights for the prudent selection of mining rates, and they hold significant implications for mitigating water inrush disasters in deep mining operations.

Experimental study on deformation and fracture evolution of sandstone under the different unloading paths

Unloading changes the stress state of the rock. It will lead to deformation and failure of rock towards unloading direction and the permeability of rock changes accordingly. Taking the sandstone in Yunnan Province as the research object, the seepage stress coupling triaxial unloading confining pressure test was carried out by using four paths. The deformation, permeability evolution, acoustic emission (AE) characteristics and its mechanism are studied. The experimental results show that: ①Compared with the conventional triaxial loading (CTL), the lateral expansion of rock under the three unloading paths is more obvious, and the rock has obvious lateral expansion. Under the CTL, the sandstone shows obvious single shear slip failure. However, under the unloading paths of constant axial pressure unloading confining pressure (CAUC) and adding axial pressure unloading confining pressure (AUC), the sandstone mainly presents the characteristics of tensile shear composite failure. ②Under different unloading paths, the permeability evolution law is almost the same, but the value of permeability is different. ③Under the four unloading paths, the order of AE ringing count from large to small is CTL, unloading axial pressure and unloading confinement pressure (UAUC) and CAUC. And no matter what kind of stress path, the maximum value of permeability always lags behind the maximum value of AE ring count, that is, the maximum value of permeability is always after unloading failure of rock. ④The main reason for the different changes of permeability and fracture evolution characteristics under different unloading paths is that the deviatoric stress is different. The high confining pressure limits the lateral deformation and improves its strength. ⑤From unloading to sandstone failure, the time is shortest under the path of AUC, and the time is longest under the path of UAUC.

Energy evolution and damage model of sandstone under seepage-creep-disturbance loading

The geological environment of deep coal seam is complex, and the failure mechanism of coal seam under the action of seepage pressure and disturbance stress is unclear, which restricts the development of deep ground engineering. In this study, triaxial compression test of sandstone under seepage-creep-disturbance loading (SCD) was carried out to analyze the characteristics of disturbance deformation, creep rate and permeability. The focus was on the energy evolution law of sandstone under SCD, and the damage creep model was constructed based on the energy proportion. The results show that the creep increment of sandstone under disturbed load was significantly higher than that under undisturbed load, and the stress level can promote the expansion and development of microcracks, in which the creep increment under high stress level showed a jump increase. The total energy and elastic energy of sandstone showed a step change with time. The damage factor under disturbance load was defined in the form of energy dissipation ratio, and the corresponding coupling damage factor was determined by considering the seepage factor and creep factor, which can reflect the coupled damage deformation of sandstone under SCD. Based on fractional order theory, a nonlinear damage creep model describing sandstone deformation under SCD was established by connecting the acceleration element in series with the Nishihara model and introducing the coupling damage factor. This study can provide research ideas for water inrush of coal seam rock mass.

Thermal-mechanical modelling on cumulative freeze-thaw deformation of porous rock in elastoplastic state based on Drucker-Prager criterion considering dilatancy

The comprehensive understanding of freeze-thaw (F-T) deterioration and deformation properties of rock is a critical basis for frost damage prevention on cold regional rock engineering. In freezing process, plastic strain generates in rock when the stress induced by pore ice pressure (PIP) becomes higher than yield strength, and unrecoverable deformation accumulates in rock under multiple F-T cycles. Therefore, this study presents a thermal-mechanical modelling on cumulative F-T deformation of rock under cyclic F-T action. In the modelling of mechanical action, the microporomechanics theory is applied with a spherical element model in the derivation of equilibrium equations of rock, and the Drucker-Prager criterion is utilized with dilatancy effect of rock considered to describe unrecoverable plastic deformation cumulated in rock under multiple F-T cycles. The thermal process is modelled with unfrozen water content and latent heat accounted. The proposed thermal-mechanical modelling method is validated based on experiments and numerical simulations of F-T deformation of sandstone, and it is able to simulate the temperature and F-T deformation of rock in acceptable precision. The numerical distributions of temperature, PIP, maximum principal strain and displacement are exhibited and show that the plastic region begins to develop in rock matrix with PIP exceeding the elastoplastic critical pressure when temperature drops to about −2 °C and grows rapidly when temperature drops from −2 to −6 °C. Several F-T cycles later, large unrecoverable plastic strain remains in sandstone though PIP dissipates after thawing. The cumulative F-T deformation, porosity increment and maximum of plastic volume ratio are much greater under condition with freezing temperature of −20 °C, compared with condition −10 °C. Besides, as cycle number increases, the plastic volume ratio becomes smaller and its decrease rate attenuates.

DIC-Based Hydration Absorption Detection and Displacement Field Evolution of Outcrop Porous Sandstone

In order to study the hydration absorption behaviors and characteristics of sandstone in Mogao Grottoes in China, the pressure-less hydration absorption experiment on the outcrop porous sandstone of Mogao Grottoes was carried out by using the self-developed real-time monitoring experimental system. The hydration absorption was measured and the curve of hydration absorption with time was drawn. At the same time, the digital image correlation method (DIC) was used to measure the full-field deformation, and the speckle pattern of the sample was analyzed using Match ID, and the displacement field and strain field of the sandstone sample at different hydration absorption moments were computed. Moreover, the sparse area and dense area of sandstone are used as regions of interest (ROI) for DIC analysis. According to the test results, it is concluded that the hydration absorption of sandstone increases rapidly in the initial stage, and gradually tends to be stable with the change of time. This corresponds well with the deformation characteristics of sandstone analyzed using DIC. In the initial stage, the deformation of sandstone increases rapidly. With the change in time, the deformation of sandstone samples gradually slows down. When the hydration adsorption reaches saturation, the sandstone continues to deform for a period of time before stopping hydration absorption. The results of the mercury injection test and the XRD test show that the porosity of the sparse area is larger than that of the dense area and the particle content of the dense area is lower. When the sandstone is saturated with water, the liquid is immersed in the pores between the solid particles, which makes the sparse area more prone to stress concentration, and the deformation in the sparse area is larger. Therefore, when analyzing the hydration absorption deformation of sandstone, the porosity should be considered.

Energy evolution mechanism of structural surfaces in sandstones with different dips based on the energy principle.

A uniaxial compression test was conducted on sandstone specimens at various inclination angles to determine the energy evolution characteristics during deformation and damage. Based on the principle of minimum energy dissipation, an intrinsic model incorporating the damage threshold was developed to investigate the mechanical properties of sandstone at different inclination angles, and the energy damage evolution during deformation and damage. This study indicated that when the inclination angle of the structural surface remained below 40°, sandstone exhibited varying mechanical properties based on different inclination angles. The peak strain was positively correlated with the inclination angle, whereas the compressive strength and modulus of elasticity showed negative correlations. From an energy perspective, the deformation and damage of sandstone under external loading entail processes of energy input, accumulation, and dissipation. Moreover, higher inclination angles of the structural surface resulted in a smaller absorbed peak strain and a reduced proportion of dissipated energy relative to the energy input, thereby affecting the evolution of energy damage throughout the process. As the inclination angle of the structural surface increased, the absorbed total strain at the peak value decreased, whereas the proportion of the dissipated energy increased. Additionally, the damage threshold and critical value of the rock specimens increased with the inclination angle. The critical value, a composite index comprising the peak strain, compressive strength, and elastic modulus, also increased accordingly. These findings can offer a novel perspective for analyzing geological disasters triggered by fissure zones within underground rock formations.

Investigation of Sandstone-like Material for Damaged Rock Mass Based on Orthogonal Experimental Method

The investigation of the mechanical properties of rock mass can be effectively carried out through rock-like material experiments. In this study, polystyrene foam particles were utilized as a novel material for simulating initial damage within rocks. Our research involved the development of sandstone-like materials with comparable mechanical properties to actual sandstone. These materials were then subjected to orthogonal mechanical tests, allowing us to identify the key factors that have a substantial impact on the mechanical parameters of sandstone-like rocks. The influencing factors considered in the orthogonal mechanical tests were the proportion of aggregate and binder, the proportion of polystyrene foam in the entire model, the proportion of binder and regulator, and the size of polystyrene foam. Five levels were set for each factor, and mechanical parameters such as compressive strength, tensile strength, elastic modulus, axial strain, and Poisson’s ratio were tested for each group of samples. The changes in mechanical parameters with the levels of the above four factors were studied. The study found that modifying the proportion of aggregate to binder can alter the elastic modulus, tensile strength, and compressive strength values of sandstone-like material. The size of polystyrene foam can be modified to alter the axial strain values of sandstone-like materials. Additionally, adjusting the ratio of binder and regulator can modify the value of Poisson’s ratio. The comparison of mechanical parameters between sandstone-like samples and sandstone reveals that sandstone-like materials can better simulate the deformation and failure mechanisms of sandstone. The error in the main mechanical parameters, such as modulus of elasticity, strength, and Poisson’s ratio, is less than 7%, indicating a greater resemblance between sandstone-like materials and sandstone. Therefore, sandstone-like materials can be used to investigate the deformation law, damage evolution law, and failure mechanism of sandstone. This can help alleviate the difficulty of obtaining specimens of deep damaged rock and the high cost of testing.

Deformation and hysteresis behaviors of sandstone exposed to distinct sequences of variable-frequency compressive cyclic stresses

Deformation and hysteresis behaviors of sandstone exposed to distinct sequences of variable-frequency compressive cyclic stresses

Experiment Study on the Mechanical Behavior and Acoustic Emission Response of Thick and Hard Sandstone Roof in Xinjiang Mining Area

One of the primary threats to coal mine safety production is the sudden extensive fracturing and collapse of thick and hard roof strata. A range of uniaxial compression experiments with acoustic emission (AE) monitoring was performed for studying the mechanical properties and crack evolution of thick and hard roof sandstone specimens. AE temporal response analysis revealed that the damage process of thick and hard roof sandstone specimens exhibited distinct stages. Additionally, AE event localization indicated that microcracks within the thick and hard roof sandstone specimens propagated from the ends to the middle of the specimens during the loading process, eventually leading to severe failure. The b-value analysis demonstrated that during the early loading stage, the scale of internal microcracks within the thick and hard roof sandstone specimens gradually decreased with the optimization of the load-bearing structure. In the later loading stage, the scale of microcracks increased with the deterioration of the load-bearing structure. Furthermore, RA–AF analysis revealed that the specimens experienced combined tensile–shear failure, primarily dominated by tensile failure throughout. The AE characteristics observed during the deformation of thick and hard sandstone can provide references for the monitoring of roof stability and early warning of potential disasters in thick and hard sandstone conditions.

Effects of CO2 on creep deformation in sandstones at carbon sequestration reservoir conditions: An experimental study

We conducted four hydrostatic creep/flow-through experiments to investigate the evolution of rock hydraulic and mechanical properties due to fluid-rock interactions in the context of underground CO2 utilization and storage. The experiments were performed on the macroporosity-dominated lithofacies of the Morrow B sandstone. The Morrow B is an early Pennsylvanian fluvial/marine deposit, and the target reservoir of an ongoing CO2-enhanced oil recovery project at the Farnsworth Unit oilfield, Ochiltree County, Texas. Core samples were held at 38.5 MPa effective stress and 100 °C while flowing through reservoir brackish solution (control) or CO2-enriched reservoir brackish solution at different flow rates (0.01, 0.02, and 0.1 ml/min). We monitored the evolution of the outlet fluid chemistry, sample axial strain, permeability, and bulk modulus, and measured pre- and post-experiment porosities and ultrasonic wave velocities. Results showed mineral dissolution, but no enhanced creep deformation in the presence of CO2. Samples experienced fluctuations in the bulk modulus of elasticity with up to 18% and 3% increases for flow-through with reservoir brackish solution and CO2-enriched reservoir solution, respectively. Permeability decreased monotonically during flow-through with brackish solution, but fluctuated with CO2-rich brackish solution. All samples experienced a decrease in the post-test ultrasonic wave velocities, where the decrease was proportional to the test duration. Chemical reactions were not rate limited, and ion concentrations varied as the fluid volume through the core for all experiments irrespective of flow rate, duration, or experiment type. We believe that the observed fluctuation in sample bulk modulus was due to dissolution/microcracking leading to rock softening versus rock consolidation/pressure solution leading to rock stiffening, which is supported by petrographic observations. CO2-driven dissolution of disseminated ankerite cements did not cause enhanced compaction because of their low occurrence and the presence of the more abundant unreactive pre-compaction quartz cements forming long contacts with the framework grains. Porous sandstone reservoirs whose framework grains are supported by early diagenetic (pre-compaction) quartz cements make good targets for the long-term safe underground storage of CO2.

Experimental Study of the Mechanical and Acoustic Emission Characteristics of Sandstone by Using High-Temperature Water-Cooling Cycles

In order to study the physical and mechanical properties of sandstone under high-temperature water-cooling cycling conditions, an RMT-150B electrohydraulic servo rock testing system and a DS-5 acoustic emission detection and analysis system were used to conduct uniaxial compression acoustic emission tests on sandstone after high-temperature water-cooling cycles. The deformation, strength, and acoustic emission characteristics of sandstone were analyzed under different temperatures and cycle times. The results show that the high-temperature water-cooling effect caused changes in the physical properties of sandstone. The volumetric expansion rate of the rock samples first decreased, then increased in temperature, and the strength first increased, then decreased, whereas the number of cycles had less of an impact on the physical properties. At 200 °C, with increased cycle number, the elastic modulus increased by 20.1%, and the compressive strength increased from 63.9 MPa to 71.46 MPa. At 300–600 °C, the elastic modulus and compressive strength of sandstone gradually decreased with increases in the temperature and cycle number, with reductions of 6.04%, 7.24%, 28.7%, 35.57%, 17.6%, 18.2%, 20.4%, and 60.5%, respectively. With increased temperature and cycle times, the acoustic emission ringing counts increased, ringing counts and cumulative energy appeared earlier, the rock samples entered elastic deformation earlier, the yield stage length increased, and the samples showed a tendency to transition from brittle to ductile damage.

Creep behavior and permeability evolution of red sandstone in three Gorges Reservoir area subjected to cyclic seepage pressure

The creep behavior of the rock mass in an anchoring section affects the control effect of the stabilizing piles and the long-term stability of the landslide-stabilizing pile system. Additionally, periodic reservoir water level fluctuations cause cyclic water-seepage pressure changes in the rock mass of bank slopes, which may promote rock creep deformation, further compromising the stability of the slope and increasing the risk of landslides. In this experimental study, the creep properties and permeability of red sandstone from the bedrock of the Majiagou landslide in the Three Gorges Reservoir area were investigated under cyclic seepage pressure conditions. Four seepage pressures (0, 1, 2, and 1-2 MPa) were selected for comparison. The results demonstrated that creep strain curves, especially volumetric strain, exhibited fluctuations under cyclic seepage pressure conditions. The creep deformation of red sandstone was enhanced by cyclic seepage pressure. Creep strain increased with increasing deviatoric stress, except at 60 MPa deviatoric stress. Furthermore, the creep time to failure at 108 MPa deviatoric stress was inversely proportional to the seepage pressure. The steady-state creep rate exponentially increased with deviatoric stress, and the steady-state creep rate under cyclic seepage pressure was higher than that under constant seepage pressures of 1 and 2 MPa. Finally, the permeability of the red sandstone fluctuated in accordance with the fluctuating seepage pressure. The permeability evolution of the red sandstone samples under different deviatoric stresses was attributed to the competing processes of preexisting microcrack closure and the propagation of new microcracks. Accurate evaluation of the long-term performance of stabilizing piles and the stability of stabilized landslides necessitates consideration of the influence of cyclic seepage pressure on rock mass creep behavior.

Dynamic response and energy evolution of sandstone under combined dynamic and static loading

Impact disturbance and bedding angle have a substantial effect on the mechanical response of sandstone under prestressed loading. Therefore, in this paper, a combined 1D static and dynamic loading test system is employed for examining the compression of sandstone at various bedding angles under a pre-applied axial pressure of 25 MPa. Under these conditions, the strength, stress-strain curves, energy usage ratio, particle size and deformation of sandstone are obtained. The results are as follows. (1) Strain rebound easily occurs in high-strength sandstone under low impact pressure. (2) The peak strength of sandstone is positively correlated with the impact pressure. The peak strength and combined dynamic‒static strength of sandstone change in a "v" shape. (3) The rock's failure particle size is controlled by the bedding plane at low impact pressure. (4) We propose a new energy calculation method in which the energy utilization rate presents an approximately inverted "v" shape. (5) The sandstone with different bedding angles has different bursting liabilities at an identical impact pressure. (6) The impact pressure is proportional to the crack growth rate of sandstone.

Technique for studying in high resolution poromechanical deformation of a rocklike medium.

Pressurized fluid injection into underground rocks occurs in applications like carbon sequestration, hydraulic fracturing, and wastewater disposal and may lead to human-induced earthquakes and surface uplift. The fluid injection raises the pore pressure within the porous rocks, while deforming them, yet this coupling is rarely captured by experiments. Moreover, experimental studies of rocks are usually limited to postmortem inspection and cannot capture the complete deformation process in time and space. In this Letter we will present a unique experimental system that can capture the spatial distribution of poromechanical effects in real time by using an artificial rocklike transparent medium mimicking the deformation of sandstone. We will demonstrate the system abilities through a fluid injection experiment, showing the nonuniform poroelastic expansion of the medium and the corresponding poroelastic model that captures completely the results without any fitting parameters. Moreover, our results demonstrate and validate the underlying assumptions of the poroelastic theory for fluid injection in rocklike materials, which are relevant for understanding human-induced earthquakes and injection induced surface uplift.

Experimental study of real-time temperature-dependent nonlinear deformation of sandstone

In contrast to thermal heating, the real-time heated rock exhibits significant nonlinearity during the elastic deformation stage. Hence, an experiment assembled by a high and low-temperature test chamber and a microcomputer-controlled rock direct shear instrument is built to evaluate the effect of real-time temperature on the nonlinear deformation behavior of rock. A set of nonlinear stress–strain relationships of forty sandstone samples under moderate real-time temperature conditions are obtained from experiments and agree well with the theoretical predictions from the two-part Hooke’s model (TPHM). The soft and hard parts respond to the real-time temperature are quite different. The ratio of the soft part (γt) increases and the elastic modulus of the hard part (Ee) decreases with the temperature below 125℃. On the contrary, γt decreases and Ee increases with the temperature above 125℃ up to 200℃. Thermally-induced and expandable microcracks (thermal weakening) incur structural damage of the hard part below 125℃, while thermal expansion (thermal strengthening) can lead to tighter compaction between grains above 125℃. Therefore, there is a threshold temperature 125℃ under real-time heating condition due to the thermal cracks and expansion. The elastic modulus of the soft part (Et) decreases continuously with increasing temperature, indicating inconsistent thermal expansion effects on the soft part with mixture of impurity particles and the hard part whose mineral particles are closely contacted at higher temperature. Based on the experimental data and TPHM, empirical relationships between the real-time temperature and nonlinear deformation are proposed. This study will provide support for the safe production and disaster prevention of geological engineering under thermo-mechanical coupling working condition.

Effect of Physical Properties on Mechanical Behaviors of Sandstone under Uniaxial and Triaxial Compressions.

Mechanical properties of sandstone, such as compressive strength and young's modulus, are commonly used in the design of geotechnical structures and numerical simulation of underground reservoirs using models such as the digital groundwater, equivalent porous medium, and Discrete Fracture Network (DFN) models. A better understanding of the mechanical behaviors of sandstone under different loading conditions is imperative when assessing the stability of geotechnical structures. This paper highlights the effect of the physical properties (i.e., porosity, mean grain size) and environmental conditions (i.e., water content and confining stress) on uniaxial compressive strength, triaxial compressive strength, and young's modulus of sandstone. A series of uniaxial and triaxial compression experiments are conducted on sandstone formations from Wyoming. In addition, experimental data on sandstones from the literature are compiled and integrated into this study. Prediction equations for the compressive strengths and young's modulus of sandstone are established based on commonly available physical properties and known environmental conditions. The results show that the mean Uniaxial Compressive Strength (UCS) decreases as the porosity, water content, and mean grain size increase. Furthermore, a predictive empirical relationship for the triaxial compressive strength is established under different confinements and porosity. The relationship suggests that the mean peak compressive strength increases at a higher confinement and decreases at a higher porosity. The results and recommendations provide a useful framework for evaluating the strength and deformation of most sandstone.

Mechanical and Energy Evolution Characteristics of Sandstone under True Triaxial Cyclic Loading

To study the mechanical and energy evolution characteristics of sandstone under true triaxial cyclic loading, a sandstone mechanical test with different intermediate principal stress under true triaxial loading was conducted using the rock true triaxial disturbance unloading test system. The influence of axial load on the deformation, energy evolution, and macroscopic failure characteristics of sandstone under different intermediate principal stress in a true triaxial test was systematically analyzed, and the damage evolution law of sandstone under true triaxial cyclic load was revealed. Results showed that the failure mode of sandstone under true triaxial compression changed from tension–shear composite failure to tension failure. Grading cyclic load σ1 greatly influenced maximum principal strain ε1 and minimum principal strain ε3 but had little influence on intermediate principal strain ε2. Under the same σ2 condition, the input energy and elastic energy in σ1, σ2, and σ3 directions increased nonlinearly. Under different σ2 conditions, the dissipated energy in σ1, σ2, and σ3 directions decreased with the increase in σ2. With the increase in σ2, graded cycles σ1, ε2, and ε3 decreased considerably, and the failure mode changed from tensile failure to shear failure. When the cyclic loading rate increased, the σ1, ε1, ε2, ε3, and volume strain εv of sandstone failure decreased, but the expansion point increased. Under true triaxial grading cyclic loading and unloading, the total dissipated energy of sandstone increased exponentially. The larger σ2 was, the smaller the damage variable was.

Experimental and numerical analyses of double flawed sandstone with a circular cavity under static-dynamic loads

To study the damage deformation and crack evolution of sandstone with combined fractures under static-dynamic loads, in this paper, the rock roadway of a mine of Henan Pingdingshan Tian'an Coal Industry Co., Ltd. was selected. Cyan sandstones specimens from the field were processed into a double flawed cube with a circular cavity (100 mm × 100 mm × 100 mm) specimens. The static-dynamic loads mechanical test system independently developed by Chongqing University was used to conduct the physical tests. The discrete element method (PFC2D) was used for the numerical simulations of static-dynamic loads with intermediate strain rates. The results indicated that with the increase of dip angle, the average strain rate and damage degree of the specimens first increased, then decreased, and finally slightly increased under the same dynamic frequency. They reached the maximum at β = 45 °. Additionally, the degree of damage deformation was reduced, as well as the degree of crack development under the same dip angle, with the increase of dynamic frequency. The specimens finally produced tensile failure and shear failure, mainly tensile failure, and the wing crack was the initial macro-cracks. After we compared the experimental and simulated results between the initiation stress, the peak strain and the crack patterns, it can be stated that the physical tests and the numerical simulations were in good agreement which also proved the effectiveness of the simulation method. Moreover, the damage evolution, the crack initiation angle, the failure forms and other flaw forms (length, width, position) of the specimens were discussed.

Experiment on deformation and failure characteristics of sandstone at different unloading rates

The unloading effect is an important factor for the failure of surrounding rock in deep underground engineering projects, especially under high-stress conditions. To investigate the deformation and failure characteristics of sandstone at different unloading rates, the evolution law of the surrounding rock stress caused by excavation was clarified. Then, the single-side unloading test of surrounding rock at different unloading rates was conducted using a true triaxial rock mechanics test system. During the test, the acoustic emission (AE) signals were monitored using an AE monitoring system. The test results show that excavation disturbance leads to complete single-side unloading at the boundary of the surrounding rock, and partial single-side unloading occurs as the depth of the surrounding rock increases. The ultimate strength of the sandstone specimen decreases as a power function with the increasing unloading rate. The unloading rock mass is mainly subject to shear failure. However, the increasing unloading rate raises the proportion of tension cracks. The sudden high strain rate on the unloading side can be used as precursor information of rock fracture, which can effectively prevent accidents caused by loss of rock bearing capacity. The AE signal is active and releases less energy in the unloading stage. At this time, the internal fractures are in the development stage, and the energy is still mainly accumulated. The unloading and failure are not synchronized but suffer from the hysteresis effect. Additionally, the AE and the strain rate change on the unloading side are consistent. Due to the hysteresis effect of damage on unloading, anchor bolts (cables) should be installed to support the surrounding rock immediately to compensate for the stress loss caused by excavation in engineering practice.