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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]

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]

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]

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 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]

• 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]