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To explore the energy evolution characteristics of rockbursts and the mechanism of excess energy in rockbursts, a self-developed rockburst experimental system was used. We performed true triaxial rapid unloading rockburst simulations and triaxial experiments based on the complete stress-displacement surface (CSDS) model prediction of the post-peak curve of rock uniaxial compression. By analyzing the characteristics of stress–strain curve and energy evolution of the rockburst experiment and comparing the energy storage characteristics of the two experiments, we arrived at the following conclusions Unlike the static failure of rock due to uniaxial compression, the occurrence of rockburst is mainly due to the generation of excess energy, which depends on the elastic strain energy stored before rock failure, outside energy input after rock failure and residual elastic strain energy. The intermediate principal stress of the rockburst experiment stored considerable residual elastic strain energy after the rockburst, resulting in an insufficient release of the elastic strain energy stored before the rockburst. The intensity of the rockburst is closely related to its excess energy release rate (EERR). With an increase in the EERR value, the cumulative energy of acoustic emission (AE) during rockbursts, the fractal dimension of rockburst fragments, and the rockburst intensity gradually increase. Highlights: The energy evolution and energy storage characteristics of true triaxial single-sided rapid-unloading rockburst experiment were studied using the graphic area method. The excess energy mechanism of the rockburst was discussed by comparing the energy storage characteristics of the true triaxial rockburst experiment and the static failure of the uniaxial compression rock. The correlation between rockburst intensity and EERR value was explored. [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 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]

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]

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]

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]

Shale is a fine-grained, clastic sedimentary rock composed of clay minerals and other minerals. The water-shale interaction plays an important role in the borehole drilling and hydraulic fracturing during the process of shale gas exploitation. In this study, laboratory experiments, including water absorption experiments and uniaxial compression tests, were used to investigate the mechanical behaviours of the water-absorbing shale specimens. From the laboratory experiments, the following statements can be drawn: a) the water-absorbing shale specimens experience longer compaction and crack closure stages than the natural shale specimen; b) the mechanical properties of the water-absorbing shale specimens, such as uniaxial compressive strength, Young's modulus and peak strain, are less than those of the natural shale specimens; c) the apertures of the failure cracks in the water-absorbing shale specimens are greater than those in the natural shale specimens under uniaxial compression loading. The acoustic emission technique was also used to capture the acoustic emission events of the shale specimens during the uniaxial compression tests. By the acoustic emission analysis, it is concluded that the water absorption stimulates the AE events in the shale specimens. [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]