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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]

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 study the effect of excavation unloading on hard rock failure, a series of true-triaxial compression tests, biaxial compression tests, and true-triaxial unloading compression tests (two different unloading rates) at different confining pressures was conducted on red sandstone cube samples. The strength and failure characteristics and their relationship for red sandstone unloading at different unloading rates and confining pressures were analyzed. Based on the test results, the effects of the unloading rate and confining pressure on the strength and failure characteristics of hard rock were explored, and a reasonable explanation for unloading-induced spalling in hard rock tunnels was presented. The results show the stress-strain curve of highly stressed red sandstone exhibits a stress step during unloading, and the higher the unloading rate, the lower the stress level required for a stress step. The rock strength-weakening effect induced by unloading was confirmed. The mechanical properties of red sandstone become more unstable and complicated after unloading. After the red sandstone is unloaded to a two-dimensional stress state, with increasing confining pressure, the strength increases first and then decreases; the failure mode changes from a low-confining pressure tensile-shear failure to a high-confining pressure tensile failure; and the geometries of the slabs change from large thick plates and wedges to medium- and small-sized thin plates. At equal confining pressures, the higher the unloading rate, the lower the strength (i.e., the strength-weakening effect is more pronounced), the thinner the slab, and the lower the confining pressure required for the failure mode to change from tensile-shear failure to tensile failure. The unloading rate and confining pressure affect the strength and failure characteristics by affecting the crack initiation type and propagation direction in hard rock. For deep hard rock tunnels with high unloading rate and axial stress, neglecting the effects of unloading rate and axial stress will lead to a dangerous support design. For deep hard rock ore, if the maximal horizontal principal stress exceeds the critical confining pressure, the mining surface should be perpendicular to the direction of the minimal horizontal principal stress. The results of this study are of great engineering significance for guiding deep hard rock tunnel construction and mining. [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 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]