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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]

To investigate the influence mechanism of geostress on rockburst characteristics, three groups of gneiss rockburst experiments were conducted under different initial geostress conditions. A high-speed photography system and acoustic emission (AE) monitoring system were used to monitor the entire rockburst process in real time. The experimental results show that when the initial burial depth increases from 928 m to 1320 m, the proportion of large fracture scale in rockburst increases by 154.54%, and the AE energy increases by 565.63%, reflecting that the degree and severity of rockburst increase with the increase of burial depth. And then, two mechanisms are proposed to explain this effect, including (i) the increase of initial geostress improves the energy storage capacity of gneiss, and then, the excess energy which can be converted into kinetic energy of debris ejection increases, consequently, a more pronounced violent ejection phenomenon is observed at rockburst; (ii) the increase of initial geostress causes more sufficient plate cracks of gneiss after unloading of σh, which provides a basis for more severe ejection of rockburst. What's more, a precursor with clear physical meaning for rockburst is proposed under the framework of dynamic response process of crack evolution. Finally, potential value in long term rockburst warning of the precursor obtained in this study is shown via the comparison of conventional precursor. [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]

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]

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 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 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]

Zonal disintegration widely exists in the excavation roadways and caverns in deep rock masses. Ultra-low friction can be generated in the deep fractured rock masses, which reduces or disappears under dynamic loading. In this paper, granite blocks were processed into cube specimens with a cylindrical hole in center. Static loads and sinusoidal disturbances were applied to study the sliding phenomenon between blocks. During the experiments, real-time images, and acoustic emissions were recorded. Results indicate blocks above the cylindrical hole slide during the second half of the sinusoidal disturbance, and block sliding is caused by a reduction of friction between the blocks. The existence of ultra-low friction can be inferred from this experiment. [ABSTRACT FROM AUTHOR]

This study used a self-developed rockburst-inducing system to apply dynamic load at different frequencies to true triaxially loaded sandstone specimens pierced by a central hole. After failure, the sandstone fragments generated were collected, as were those generated by a similar compression experiment where only static load was applied. The quantity of fragments falling within coarse, medium, fine and micro-scale size ranges and the mass distribution were analyzed. Image processing software was then used to process images of the medium grains, and the measurements obtained were used to calculate the fractal dimensions of the granularity–quantity, perimeter length–quantity and area–quantity relationships. The experimental results show that fine and micro-grains make up a higher proportion of the fragmented mass when the dynamic loading frequency is higher. Fractal analysis of medium grains shows that the fractal dimension increases with an increase in dynamic loading frequency. The degree of crushing in impact rockburst experiments is found to be higher than in the compression experiment. [ABSTRACT FROM AUTHOR]

In the roof cutting pressure releasing gob-side entry retaining technology, roof cutting affects roof's mechanical properties greatly. In order to further improve the theory and method of roof cutting design, this paper puts forward a new design idea about roof cutting angle based on the mechanics theory analysis and model tests. Firstly, the surrounding rock stress state evolution process is summarized as four stages in this paper: supporting stress state, roof cutting stress state, pre-mining stress state and post-mining stress state. And what is pointed out is the stress state before mining can also affect the stability of the cutting roof. Then, gypsum specimens are taken as examples to do uniaxial compression tests and whose structural plane is constructed under the regular roof cutting angle [0°, 60°]. Tests get the cθ and φθ of structural surface are 3.54 Mpa and 10.46° respectively. The results prove the single plane theory and are conductive to the cutting angle design under pre-mining stress state. Under the tests conditions, in order to ensure the roof entry retaining has high strength and elastic modulus, and can also take advantage of the function of the advanced stress concentration to make the roof slit more easily developed, the cutting angle is approximately designed as 23°. [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]

Failure evolution of cemented paste backfill (CPB) is crucial to the stope stability in the mining industry. While many effects have been done on the strength requirement and macroscopic deformation behaviors of CPB, the mesoscopic failure mechanism is not yet well understood. In this work, a uniaxial compressive test on the CPB sample from Lilou iron mining was conducted under topographic monitoring using a 450-kV industrial X-ray computed tomography (CT). A specific self-developed loading device was used to match the CT machine to realize the real-time CT scanning. A series of 2D CT images were obtained by carrying out CT scanning at seven stages throughout sample deformation and from different positions in the sample. Clear CT images, CT value analysis, and void identification and extraction reveal that the sample experiences compression, damage, crack initiation, crack propagation, crack coalescence, and collapse stages. Besides, a damage constitutive equation is proposed first using the CT data, which can be used to predict the stress strain response of CPB. What is more, volumetric dilatancy characteristics caused by the damage and cracking behavior are investigated from the stress strain curve and the CT images, the result reveals the influence of localization deformation on the failure evolution of CPB. Through a series of meso-structural changes analysis, the meso-mechanisms of failure evolution in CPB have been first documented. [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]