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In this study, a series of impact-induced rockburst experiments were carried out by applying multilevel cyclic stresses on a cubic sandstone with a circular opening. Information on rockburst evolution was monitored using video and AE techniques together. Experimental results show that the opening in the sandstone sample was observed to undergo rock grain ejection, rock spalling and buckling in sequence. AE activity under the lower cyclic stress level followed the Kaiser's effect, while the Felicity's effect of AE was also involved at the higher cyclic stress level. Multi-axial cyclic loading can shorten the fatigue life and fatigue strength of the sandstone sample. However, the multi-axial cyclic loading can obviously reduce the AE hits and its energy rate, in which case the proportion of the AE single with low frequency is less than those under the uni-axial cyclic compression. It is also found that the failure zone in the sandstone is characterized by V-shaped notch. The depth of the failure zone is reduced sequentially under uni-axial, bi-axial, and tri-axial cyclic compression, respectively. In contrast, the extent angle of the failure zone will increase. According to the observed profiles of the failure zone, the changes of friction angle of the sandstone are interpreted by the Mohr–Coulomb strength criterion, as well as movement of the failure line due to the fatigue damage. These findings can provide insights into the study of rock failure mechanisms under various dynamic disturbances. Highlights: Impact-induced rockburst in laboratory was simulated by applying uniaxial, bi-axial, and tri-axial cyclic loadings, respectively to cubic sandstone samples with an opening. The opening in the sandstone sample was observed to undergo rock grain ejection, rock spalling and buckling in sequence. AE activity under the lower cyclic stress level can be explained by the Kaiser's effect, while the Felicity's effect of AE was also involved at the higher cyclic stress level. Failure zone around the opening in the sandstone was characterized by V-shaped notch, whose depth of the failure zone was reduced sequentially under uni-axial, bi-axial, and tri-axial cyclic compression. [ABSTRACT FROM AUTHOR]

This study aimed to explore the effect of four-sided rapid unloading on delayed pillar rockbursts under high pressures. To this end, true triaxial four-sided rapid unloading rockbursts and uniaxial compression experiments were conducted on two sandstones with different strengths. The differences in damage under two different loading paths and characteristics of the rapid unloading point of the pillar rockburst were analyzed. The results indicate that rapid unloading on the four sides under true triaxial high-pressure loading caused unloading damage. The stress and strain at the rapid unloading point would produce an unloading platform, and the acoustic emission (AE) signal and dissipated energy would increase rapidly. The rapid unloading damage in the elastic stage is mainly caused by the tension cracks generated around the specimen because of the rapid expansion of the specimen. As the center part of the specimen was not affected, the stress–strain curve after unloading was still in the elastic stage. Although the initial stress of the high-strength sandstone pillar rockburst during rapid unloading was higher than that of low-strength sandstone, the unloading damage caused was less than that of low-strength sandstone, indicating that rapid unloading damage is closely related to the rock properties. [ABSTRACT FROM AUTHOR]

The fast unloading delayed rockburst experiments on single-side, double-side, three-side, and four-side were carried out using the self-developed true triaxial rockburst experimental apparatus, which can realize the fast unloading of multiple loading surfaces in the horizontal direction under high stress. The rockburst experiment's stress–strain curve, energy evolution, failure characteristics, and acoustic emission (AE) characteristics are investigated. The results show that the rockburst gradually reduces the axial peak stress and strain when increasing the number of unloading surfaces (NUS). The rise in the axial strain at the fast unloading point will progressively grow when the NUS increases, indicating that the sample's expansion in the unloading direction will gradually increase. Fast unloading causes unloading damage, which causes a sudden rise in dissipated energy at the unloading point. The total energy and elastic strain energy input by axial stress at the bursting point gradually declines when the NUS increases, but the dissipated energy increases. The release rate of elastic strain energy at the bursting point increases when the NUS increases, and the rockburst becomes more intense. The abrupt value of the cumulative AE hits at the fast unloading point and the cumulative AE hits at the burst point increase with the NUS increase. The changing trend of the multifractal parameter, which initially decreases and then rises abruptly, can be regarded as the precursor of rockburst. The hysteresis of the precursor of rockburst grows when the NUS increases. Highlights: The fast unloading delayed rockburst tests on single-side, double-side, three-side, and four-side were carried out using the self-developed true triaxial rockburst experimental apparatus. Energy analysis and high-speed recording of the rockburst process found that the severity of the rockburst does not depend on the amount of elastic strain energy stored in the rock sample but rather on the release rate of energy at the rockburst point. The changing trend of the multifractal parameter, which initially decreases and then rises abruptly, can be regarded as the precursor of rockburst. The hysteresis of the precursor of rockburst grows when the NUS increases. [ABSTRACT FROM AUTHOR]

Rockburst warnings are a significant problem in the field of rock engineering. In this study, a rockburst experimental system was developed to conduct a true triaxial single-side rapid unloading rockburst experiment. The rockburst process and time were recorded using a high-speed camera. Based on the critical slowing-down theory, the acoustic emission (AE) RA/AF, duration, and rise time in the rockburst process were studied to obtain rockburst precursor information. The main conclusions of the experiment are as follows. (1) In the early stage of rockburst, the AE signal appears to be a critical-slowing phenomenon at the critical point, which is manifested as an increase in the variance and autocorrelation coefficient, and it also appears in the increase in the variance and autocorrelation coefficient during rapid unloading. A recording made using a high-speed camera shows that the rockburst occurrence time does not necessarily correspond to the peak stress point of the stress–strain curve. (2) The variances in the AE RA/AF value, duration, and rise time are the same. The sudden change time of the precursor signal is the same, the waveform fluctuation is similar, and they all appear before the rockburst. At this time, the AE localization event shows that many high-energy events occur in the sandstone, and cracks appear on the sandstone surface. (3) The autocorrelation coefficient fluctuates considerably compared with the variance; therefore, the variance can be considered the main criterion for rockburst precursors. This study provides a theoretical basis for predicting rockbursts. [ABSTRACT FROM AUTHOR]

This study examines the effects of the rock block percentage (RBP) on the cyclic behaviors in soil rock mixture (SRM) by conducting cyclic triaxial tests at a constant stress amplitude and low strain level. The SRM samples were prepared using compaction testing with different RBPs. For the evaluation of the mesoscopic damage and stress dilatancy behaviors, the in-situ X-ray computed tomography (CT) technique was employed. It was revealed that the hysteresis loop presents a sparse-dense-sparse pattern that is caused by the plastic strain accumulation. Adding rock block to SRM influences the shape of the hysteresis loop, and the area of the hysteresis loop decreases with increasing RBP for samples at the same stress amplitude, the energy loss becomes smaller for SRM samples with a high RBP. On the other hand, after extracting cracks from the original CT images, it shows that damage of the SRM with different RBPs is different under the same loading cycle, the geometric parameters of crack decrease as the RBP increases. Localized deformation in the SRM sample results in the differential stress dilatancy behavior, there is interlocking among rock blocks and restriction of localized bands, which is affected by the different RBP in the SRM sample. [ABSTRACT FROM AUTHOR]

To explore the effect of rapid excavation unloading under high pressure on delayed rockburst, a true triaxial double-sided rapid unloading rockburst experiment and a biaxial compression failure experiment without unloading were carried out. The results show that the rapid unloading of excavation before the occurrence of rockburst will cause unloading damage, and the axial strain, acoustic emission (AE) count, and AE energy will suddenly increase at the unloading point. The AE source shows that the rapid unloading produces many tensile cracks, mainly distributed near the free surface. The axial peak stress of the rockburst and the stored strain energy before failure are higher than those of biaxial compression, so the ejection velocity of free surface fragments is also higher at failure. [ABSTRACT FROM AUTHOR]

Rockburst poses a great threat to the construction safety of deep tunnels under high geostress conditions. Recently, numerous deeply buried tunnels have been constructed in China, especially in the Sichuan–Tibet railway area. In this study, two rockburst experiments (strainbursts and impact-induced rockbursts) were conducted under different initial geostresses using the strainburst experimental apparatus and impact-induced rockburst experimental apparatus equipped with an acoustic emission (AE) monitoring system. A video acquisition system was used to capture the rockburst process. The results demonstrated that the rockburst process is similar under different initial geostresses, including grains ejection, rock plates spalling/buckling, and fragments ejecting violently. Based on the result of the AE characteristics, the large- amplitude AE signals were concentrated in the lower frequency band and the rockburst intensity was increased gradually with increase in the initial geostress. The b -value and β t -value fluctuated slightly at first, subsequently increased steadily, and then decreased abruptly when rockburst occurred. Furthermore, the displacement and divergence fields on the specimen surface were calculated using digital image correlation (DIC) technology. The result shows that the zone of large deformation and high divergence is consistent with the rockburst zone. Moreover, 80%σ peak is a critical stress state where rocks begin experiencing slight damage. • Failure process of strainbursts and impact-induced rockbursts is reproduced in laboratory. • Frequency-amplitude characteristics of AE under different geostress are analyzed. • Evolution of b -value and β t -value during the experiments is investigated. • Deformation and divergence fields are calculated, and microcracks begin to initiate and propagate at 80% of the peak stress. [ABSTRACT FROM AUTHOR]

This study aims to simulate the process of rockburst and spalling failure of roadway surrounding rock under three-dimensional stress in deep rock engineering. Utilizing an independently developed true triaxial rockburst experimental setup, the failure process of a circular tunnel under initial in-situ stress at a depth of 500 m was investigated using red sandstone with prefabricated holes. A miniature camera device and acoustic emission (AE) monitoring system were used to monitor and record the experimental process in real time. Using the collected data, the process of rockburst and spalling failure of the circular tunnel was reproduced. Finally, the difference between rockburst and spalling failure was analyzed and compared based on four aspects of stress characteristics, acoustic emission characteristics, fragment characteristics, and V-shaped notch morphology characteristics. The experimental results show that the failure of surrounding rock was more likely to occur under dynamic disturbance load, resulting in particle ejection. The spalling failure was found to be a slow and gradual static failure process. The mechanism of rockburst was more complex involving tension-shear coupling failure, whereas, the mechanism of spalling failure was simple involving a tensile failure. Compared with spalling failure, rockburst was more intense, producing more debris, and the V-shaped notch was narrow and deep. Highlights: Failure process of rockburst and spalling is reproduced in a laboratory. The stress characteristics of rockburst and spalling failure are calculated based on elastic theory. Frequency-amplitude characteristics and crack types based on acoustic emission are analyzed. The failure intensity of rockburst and spalling is investigated based on the rock fragments and morphology of V-shaped notch. The failure process of rockburst and spalling is discussed along with strain energy. [ABSTRACT FROM AUTHOR]

Rockburst may cause damage to engineering equipment, disrupt construction progress, and endanger human life. To this day, the occurrence of rockburst remains complex and difficult to predict. This study proposes the D-P-Transformer algorithm to address this issue by improving the embedding structure of the Transformer for specific applications to rockburst data. To reduce the computational requirement, sparse self-attention is adopted to replace self-attention. A distilling operation and multiple layer replicas are simultaneously used to enhance the robustness and speed up the algorithm's process. Taking all relevant rockburst factors into consideration, multiple experiments are conducted on seven large-scale rockburst datasets with different training ratios to verify the reliability of the proposed D-P-Transformer rockburst prediction algorithm. As compared to the original algorithm, the proposed algorithm shows average reductions of 24.45%, 46.56%, 17.32%, and 48.11% in the mean absolute error (MAE), mean square error (MSE), root mean square error (RMSE), and mean absolute percentage error (MAPE), respectively. The results indicate that the novel D-P-Transformer rockburst prediction algorithm is superior to the Transformer prediction algorithm, and could be used for coal mine rockburst prediction analysis. [ABSTRACT FROM AUTHOR]

This paper introduces a new high-pressure servo true triaxial rockburst experimental apparatus that can realize multi-face rapid unloading in the horizontal direction. The apparatus is used to simulate different types of strain burst phenomena in deep underground engineering, such as those at roadways, roadway intersections, and pillars with three and four free faces. In the research and development process, apparatus technologies were included to overcome five key challenges: rapid unloading under high pressure, loading gap removal fixture, three-way loading alignment, symmetrical loading, and system stability after unloading. The apparatus is used to carry out multi-face unloading strain burst experiments and can fully satisfy the experimental requirements. The entire strain burst process is recorded using a high-speed camera. The apparatus can provide experimental support for strain burst mechanism research and an experimental basis for different types of strain burst research. [ABSTRACT FROM AUTHOR]

In this study, a new strain burst test system was used to conduct two types of strain burst experiments by rapidly unloading single and double faces (in one direction), to simulate the strain burst occurring in surrounding rock of tunnel or double tunnel faces during the opposite tunneling. A high-speed image recording system and an acoustic emission system were used to monitor the failure process of strain burst of red sandstone. The commonalities of these two types of strain burst are as follows: failure stress and percentage of dissipation energy due to crack propagation were approximately equal; the evolution of mean velocity of free face was similar, which suddenly increased first and then decreased, then the gradient increased to the extremum value, and finally decreased to zero. Subregions of the fracture were consistent, and both showed shear, buckling, splitting, and ejection zones. However, the strain burst intensity for unloading double faces was larger than that for unloading single face, as demonstrated by a larger initial ejection velocity, larger volume of burst pit, and lower peak frequency. Furthermore, when double faces were unloaded, the flaky and massive features of fragments obtained due to strain burst were more obvious than the fragments obtained from the unloading of single face. Moreover, from yield to burst, when double faces were unloaded, the sandstone strain burst contained more shear microcracks, whereas the strain burst when unloading single face contained more tensile microcracks. [ABSTRACT FROM AUTHOR]

To investigate the specimen shape effect on rockburst proneness of rock materials, a string of conventional and single-cycle loading-unloading uniaxial compression tests was performed with cylindrical and cuboid red sandstone specimens. Despite similar development paths on stress-strain curves for the specimens with two shapes, the cuboid specimens generally show a higher uniaxial compressive strength than the cylindrical specimens. The energy evolution laws inside the two shaped specimens were explored. The results show that the input energy density (IED), elastic energy density (EED), and dissipated energy density (DED) of the two shaped specimens increased in a quadratic relationship with the increment of unloading level. Moreover, the linear relationships between the EED, DED, and IED were further confirmed for two shaped specimens, which were defined as the linear energy storage and dissipation laws, respectively. The energy storage coefficients and energy dissipation coefficients (the slopes of the linear relationships between the EED, DED, and IED, respectively) are almost independent of the specimen shape. According to the linear energy storage and dissipation laws, the peak EED and peak DED of every specimen can be calculated accurately. Finally, combining the failure process of rock specimens recorded by a high-speed camera, the elastic energy index ( W ET ), the residual elastic energy index (AEF), and the far-field ejection mass ratio (MF) of each specimen were adopted to assess the rockburst proneness of the red sandstone sampled in cylindrical and cuboid. The results show that cuboid specimens exhibited stronger rockburst proneness than cylindrical ones, which favorably agreed with the actual failure phenomena. [ABSTRACT FROM AUTHOR]

The work aims to experimentally investigate the anisotropic fatigue characteristics of marble with different bedding orientations using uniaxial fatigue experiments and posttest computed tomography (CT) scanning. The impacts of interbed orientation on the strength, deformation, fatigue life, and stimulated fracture pattern were systematically analysed. Results show that the axial, lateral, and volumetric strain curves present a three‐stage deformation and are influenced by the interbed orientation. The volumetric strain is maximum and minimum for a marble sample with 30 and 90° orientation, respectively, which is closely related to the failure mode. The crack pattern in the CT images is related to the interbed orientation; the crack density and scale are the largest for a 90° orientation sample and are the smallest for a 30° orientation sample. Good correlation has been established between the fatigue lifetime and the interbed orientation. It is suggested that anisotropic cyclic behaviours of marble is structurally dependent. [ABSTRACT FROM AUTHOR]

Spalling is a typical failure mode encountered on the sidewalls after the tunnel face excavation of deep D-shaped tunnels or caverns. It significantly affects the safe and efficient construction and long-term stability of tunnels. To study the process and failure characteristics of sidewall spalling in D-shaped tunnels, four different three-dimensional initial stress states were established to perform a series of true-triaxial tests on cube specimens with a D-shaped hole. During the tests, a wireless micro camera was used to monitor and record the failure process of the sidewall in real time. Based on the test results, the process and failure characteristics of sidewall spalling were summarised, and the influence of cross-section shape on tunnel failure characteristics and tunnel stability were discussed. The results indicate that both sidewalls perpendicular to the maximum principal stress are cut into approximately parallel rock slabs, demonstrating typical spalling features. Failure zones on both sidewalls are located primarily between the corner and spandrel, and their outlines are "V" shaped (i.e., V-shaped notch). Spalling slabs are thin in both wings, and thick in the middle; they are cut into multilayer thin slices that are not completely detached from one another. With the increase in failure depth, the shape of the cracks first evolved from a straight shape into an arced shape, and subsequently evolved into a "V" shape. When the maximum principal stress is perpendicular to the tunnel axis, tunnel excavation along the minimum principal stress is conducive to the prevention of spalling. Compared with the circular tunnels, the D-shaped tunnels can effectively reduce the severity of rock failure and transform the rockburst into a static spalling. In a relatively low stress environment, the D-shaped cross section can improve the stability of the tunnel, while it is not conducive to the stability of the tunnel in a high stress environment. The result of this study benefits the design and prevention of spalling and rockburst of hard rock tunnels. [ABSTRACT FROM AUTHOR]

Rockburst disaster is one of the prominent problems faced by deep underground engineering. In this study, rockburst in four elliptical holes with different axial ratios in sandstone under biaxial loading is studied as an analogue for underground roadways. Video and acoustic emission (AE) equipment is used to monitor the biaxial loading tests. Experimental results indicate that each of the elliptical holes goes through four stages: quiet period, small particle ejection, spalling, and rockburst. The duration of quiet and spalling periods increased with increasing axial ratio of ellipse. The duration of the ejection and rockburst periods remains unchanged. All the four elliptical holes have V-shaped pits after rockburst occurs. The fragments produced during rockburst are divided into coarse, medium, fine, and micro grains. The quantity of coarse and medium grains increases with increasing axial ratio. The mass ratio of coarse and medium grains increases and that of fine and micro grains decreases. The depth, angle, and area of the V-shaped pits decrease with increasing axis ratio. Tensile cracks play an important role in rockburst failure. Tensile cracks are the dominant crack types formed during rockburst and account for over 70% of all cracks in the samples. The number of tensile cracks increased and the number of shear cracks decreased. This paper has some reference value for practical engineering design and prevention of rockburst. [ABSTRACT FROM AUTHOR]

The ability of fracture propagation (fracability) is a key parameter of evaluating the fracture network generation during hydraulic fracturing. In order to consider the in situ conditions of the shale formation, four factors should be taken into consideration, which are brittleness, fracability, in situ stresses and natural fractures. The current shale brittleness and fracability evaluation methods rarely involve all the above factors. In this paper, coupling brittleness, stress-strain data and fracture morphology, a new shale fracability evaluation method is proposed. The brittleness is calculated by the mineral components and elastic parameters; the stress-strain data obtained by triaxial compression experiments are used to describe the shale breaking process; and the fracture morphology is evaluated by the fractal dimension method. This method is used to calculate the fracability index of the Longmaxi shale in Sichuan, China. The results show that: (1) the shale fracability under 5 MPa confining pressure is higher than that of under 20 MPa confining pressure; (2) the fracability in 7 coring angles shows a significant difference under the same confining pressure, the highest fracability is at 15∘ and the lowest is from 60∘ to 75∘. Compared with Jin and Rickman’s models, our model matches to the fracture morphologies and the fragments distribution closely. [ABSTRACT FROM AUTHOR]

Impact-induced rockburst hazards pose a severe threat to the construction safety of deep-buried, high-stress tunnels. Negative Poisson's ratio (NPR) cable has the characteristics of strong energy absorption and impact resistance. In order to explore the control effect of NPR cable on impact-induced rockburst, this paper designed and developed a model NPR cable based on the similarity principle. With the help of the self-developed new true triaxial rockburst experimental apparatus, indoor simulation experiments of rockburst under dynamic disturbance at varying strain rates were conducted. The rockburst characteristics under the conditions of NPR cable support were compared to those without support. On rockburst, the influence mechanism of strain rate and NPR support was discussed. The results indicate that the rock strength and rockburst intensity increase as the strain rate of dynamic disturbance increases. The support of NPR cables enhances energy dissipation and rock fragmentation, as well as increases the strength of the rock. Importantly, with NPR support, the energy release rate during rock failure slows, which significantly minimizes the rockburst intensity (kinetic energy release). The research results provide reference and guidance for the engineering practice of rockburst control. Highlights: A cable model with negative Poisson's ratio (NPR) structure effect was developed and its mechanical properties were verified. The experiments of rockburst caused by dynamic disturbance with different strain rates were carried out, and the situation with or without NPR support was compared. With the increase of strain rate of dynamic disturbance, rock strength and rockburst intensity increase. NPR cable improves rock-bearing capacity and effectively reduces rockburst intensity by enhancing fracture and reducing energy release. [ABSTRACT FROM AUTHOR]

• The rockburst infrared precursors of sandstone with different bedding dip angles were investigated; • The low-frequency fluctuations of the horizontal Fourier spectrum can be used as the precursor of rockburst in inclined bedding sandstones; • The NARX model, less dependent on the data length, was used to predict rockburst stress by using AISD. In order to determine the infrared precursors of rockburst in sandstone with different bedding dip angles (β = 0°, 30°, 60°, and 90°), four groups of true triaxial rockburst tests were carried out. An infrared thermal imager was used to obtain the infrared temperature field variation process on the free face of the sample. Additionally, the rockburst characteristics were analyzed from the stress–strain curve, failure mode of rock, Average Infrared Temperature Difference (AITD), Average Infrared Standard Deviation (AISD), and infrared thermal images, and the corresponding spectrums. Finally, based on the Nonlinear Autoregressive Model (NARX), the rockburst stress was predicted by using the AISD. The results show that with the increase in β , the failure mode of the sandstone changes from shear failure (β = 0°) to splitting failure (β = 30°, 60°, and 90°). The AITD and AISD curves show a steep increase before rockburst, and the corresponding time are before rockburst 33 ms, 762 ms, 567 ms, and 316 ms, respectively for the β are 0°, 30°, 60°, and 90°. Furthermore, the horizontal Fourier spectrums of the sandstone with inclined bedding planes (β = 30° and 60°) show evident low-frequency fluctuations in a short period before the amplitude of the spectrum increases. Overall, the NARX network predicts peak stress with relatively high accuracy (greater than 85 %); however, the prediction errors of the sandstone with horizontal and vertical bedding planes (β = 0° and 90°) are larger than that for the inclined bedding planes. [ABSTRACT FROM AUTHOR]

The stability of rock blocks near the underground opening may be influenced by the impact loadings caused by either natural hazards or man-made disasters. This paper addressed the dynamic response of the rock blocks under impact loadings. The experimental tests were carried out to record the movement as a function of vibrations using the impact-induced rock burst system. Granite blocks were placed in the steel frame and loaded to the designed quasi-static condition at first. Permanent jumps of displacements toward the opening were observed after the pulse loading was applied. Analytical investigations were carried out to understand the macroscopic friction during the vibration progress. The results showed that the friction force was reduced by 4% due to the pulse loading in the orthogonal direction. The friction angle at the block boundary was also determined and further adopted in the DDA models which were built to understand the micro-friction. Three zones were determined in the model based on the uneven distributions of stresses in the potential sliding block: the concentration zone, the transition zone and the active zone, from the opening to the inner place. Consequently, the variations of stress and displacement as the function of the vibrations in the three distinguished zones were presented to show the dynamic response of the stressed rock block. The results showed that the direction of the major principal stress was turning to be vertical in the concentration zone which was favorable for self-stabilization during the impact loading. [ABSTRACT FROM AUTHOR]

• Rockburst on the red sandstone containing an elliptical hole is reproduced. • The impact disturbance is considered in the experiment. • The average ejection velocity field of rockburst is investigated. • The influence of axial ratio on rockburst in elliptical tunnel is explored. The effect of the ratio of long and short axes on the rockburst of an elliptical tunnel structure was investigated by performing a series of true triaxial tests on red sandstone cuboid samples containing seven types of elliptical holes with different long–short axial ratios, using a self-developed true triaxial compression equipment. The entire process of spalling was monitored and recorded using a miniature camera. According to the test results, the failure stress, geometrical characteristics of the rockburst notch, fragment characteristics, and ejection velocity field characteristics of the rockburst are summarized, and the influence of the axial ratios of the elliptical hole on the failure characteristics and stability of the tunnel are discussed in detail. The test results indicate that the rockburst process of elliptical tunnels with different axial ratios experiences four processes; namely, calm period, small particle ejection period, spalling, and rockburst. The shape of the failure area after the rockburst presents a V-shaped feature. With the increase of the axial ratio, the area, angle, and depth of the V-shaped area show a decreasing trend, the initial failure stress of the cavern and the rockburst failure stress exhibit a trend of first decreasing and then increasing; the tangential stress of the cavern shows an increasing trend; the mass percentage of micro, fine, and medium grain decreases; and the average ejection velocity first decreases and then increases. Under the same stress conditions, rockbursts are more likely to occur on circular sections. Under the same section conditions, the ellipse can bear larger loads when its major axis is parallel to the disturbance stress. These conclusions are helpful to the efficient design of the tunnel structure. [ABSTRACT FROM AUTHOR]

Abstract The time-dependent behavior of rock joints, specifically creep and stress relaxation, govern the long-term stability of underground structures, but have not been fully understood. The effect of asperity degradation on the time-dependent behavior of rock joints is investigated in this test. Triangular synthetic rock joints with four asperity angles (θ = 0°, 10°, 30° and 45°) are tested for creep and stress relaxation. It is revealed that the failure mode of a rock joint during creep and stress relaxation shifts from surface sliding (θ = 0°) to a combination of surface sliding and asperity crushing (θ > 10°). Asperity crushing becomes more severe as θ increases. Consequently, a rougher rock joint with higher θ exhibits higher creep and stress relaxation rates due to more asperity crushing. For each θ , the creep and stress relaxation rates both increase exponentially with increasing shear stress before the stable long-term strength is reached, suggesting a higher degree of asperity degradation as shear resistance mobilizes. Semi-empirical correlations are proposed to quantify the effects of asperity angles on creep and stress relaxation of rock joints at various shear stresses. Highlights • Quantitative long-term mechanical behavior of rock joints considering asperity degradation. • Multi-stage creep and stress relaxation tests on triangular synthetic rock joints with different asperity angles. • Rock joint failure mode shifts from surface sliding to a combined mode of surface sliding and asperity crushing when θ exceeds 10°. • Rock joint with higher roughness exhibits higher creep and stress relaxation rates. • Propose semi-empirical equations to quantify the effects of asperity angle. [ABSTRACT FROM AUTHOR]

[Display omitted] • The initial crack affects the location and intensity of the rockburst. • Three mechanisms dominate the effects of the pre-existing crack on rockburst. • Four different rockburst evolution modes are proposed for the tunnel. The structural planes adjacent to the tunnel have a significant effect on rockburst, which has become an important engineering issue. To investigate the influence of such a rigid structural plane on rockburst, four different types of block specimens containing a through circular hole are designed with a prefabricated unfilled crack. A series of biaxial compression tests are then performed to simulate the rockburst process, followed by a detailed analysis of the strength characteristics, the strain field, the rockburst evolution process, and the microscopic characteristics of fracture surfaces. A prefabricated unfilled crack causes a decrease of the initial damage stress and the overall strength of block specimen, the earlier occurrence of rockburst but a decrease of initial rockburst intensity. Compared with the exposed crack, the non-exposed crack facilitates the occurrence of the rockburst more. Three mechanisms are proposed to explain these effects, including (i) the local stress concentration that alters the location of initial damage and the following crack propagation pattern, further affecting the location and scope of rockburst; (ii) the weakening effect that promotes rockburst evolution and inhibits rockburst intensity; and (iii) the sliding-induced secondary rockburst that contributes with significant energy release once the pre-existing crack is compacted. In light of the experimental observations, four different rockburst evolution modes are summarized to help to understand the rockburst in practical tunnels, and some engineering measures are recommended for the controlling and prevention of rockburst. [ABSTRACT FROM AUTHOR]