Search

The b value is a critical parameter that has been used to describe spatial and temporal size distribution characteristics in data from both natural seismicity and laboratory rock acoustic emission (AE) tests. However, unlike the magnitude used for b value estimation in earthquakes which corrects for hypocentral distance, the apparent amplitude of the AE, is most commonly used for b value estimation in the rock AE test. This ignores the effect of attenuation, which may modify the frequency–amplitude distribution and affect the b value results. Here, we investigate the attenuation effect on the estimate of b value from synthetic data generated by random sampling of a doubly truncated frequency–amplitude distribution and a known underlying b value from a statistical perspective. We generated AE source amplitudes that obey an exponential distribution with different theoretical b values and applied an attenuation operator to obtain apparent frequency–amplitude data for comparison with the true underlying distribution. The results show that the b value is unchanged within a specific interval of amplitude distribution after accounting for attenuation under the conditions examined here, and that data truncation is the primary cause of modification of the frequency–amplitude distribution in the presence of attenuation, rather than attenuation alone. In addition, we employed the same synthetic data simulation scheme to estimate the minimum data volume for accurate estimation of the underlying b value from the apparent amplitude distribution. We found that the determination of the minimum data volume is influenced by the theoretical b value of the underlying distribution: the higher the theoretical b value, the larger the minimum data volume required, varying from 200 events from b  < 1.5 to 300 for b  > 1.5. [ABSTRACT FROM AUTHOR]

To investigate the influence of water on stress-induced failure in D-shaped hard rock tunnels, a series of true triaxial tests were performed on naturally saturated red sandstone cube samples containing a D-shaped hole. A self-developed real-time monitoring device was used to observe and record hole failure in real time, and the process and characteristics of stress-induced failure in D-shaped holes were summarized. The influence of water on failure form, energy storage properties, surrounding rock stability, and axial stress on the stability of the surrounding rock was discussed, and the mechanism of water on stress-induced failure was revealed. The results show that the stress-induced failure in a water-bearing D-shaped tunnel can be divided into buckling deformation, tension cracking, macrocrack propagation and coalescence, and slab flaking. The dominant failure mode of the stress-induced changes from tension around the tunnel to shear of the surrounding rock in depth. A quiet period exists between two adjacent significant AE counts, and the length of the quiet period increases as the two horizontal principal stresses increase. Under the influence of water, the capacity of the surrounding rock to store elastic strain energy decreases, and the dissipated energy required for deformation and failure increases; the form of stress-induced failure changes from spalling accompanied by occasional rockburst to spalling. The increase in lateral stress can enhance the stability of the surrounding rock, and the influence of axial stress on the stability of the surrounding rock is related to the magnitude of lateral stress. [ABSTRACT FROM AUTHOR]

• The failure processes of sidewall were simulated under different bedding angles. • The influence of bedding angle on failure severity and position was studied. • The characteristics of the V-shaped notch boundaries were reported. • The AE characteristics during the sidewall failure were discussed. To investigate the influence of the bedding angle on the failure process and characteristics of circular hole sidewalls, true-triaxial lab tests were conducted on cubic phyllite specimens with different bedding angles. When the tunnel axis direction was parallel to the bedding plane, the initial failure vertical stress of the hole sidewall first decreased and then increased with an increase in the bedding angle β. A V-shaped notch was formed in the surrounding rock, and the characteristics of the V-shaped notch boundaries were such that one side was approximately parallel to the bedding plane, and the other side crossed the bedding plane. Three failure modes of surrounding rock under different bedding planes were observed: large-scale slip failure along the bedding plane (β = 0°, 15°, and 30°), large-scale shear failure across the bedding plane (β = 45° and 60°), and local rock fragment exfoliation failure (β = 75° and 90°). The large-scale slip (or shear) failure of the surrounding rock generated high acoustic emission (AE) counts, whereas the local failure of the surrounding rock generated relatively low AE counts. By comparison, the initial failure vertical stress was much lower than that when the bedding plane was perpendicular to the direction of the hole axis. Therefore, the tunnel layout method perpendicular to the bedding plane is beneficial for improving the stability of the surrounding rock in underground engineering and reduces the risk of rockburst occurrence. [ABSTRACT FROM AUTHOR]

To explore the variation in rock acoustic emission (AE) characteristics with strain rate, uniaxial compression tests at different loading rates and impact loading tests were conducted on granite using a MTS322 rock mechanical test system and split Hopkinson pressure bar (SHPB) system, respectively. The effect of the strain rate on the AE characteristic parameters, rock fracture properties, and destruction evolution were systematically analyzed. The results demonstrated that with increasing strain rate, the cumulative AE count decreases as a power function, and the variation between the cumulative AE count and strain rate can be fitted log-linearly with a slope of 0.48. The peak frequencies of the AE signals are mostly distributed in the zones of 0–100 kHz, 175–250 kHz, and 400–550 kHz. The signal proportion in the 0–100 kHz zone gradually increases with strain rate, while the signal proportions in the 175–250 kHz and 400–550 kHz zone exhibit decreasing trends. A transition of a sudden increase in the RA -value and decrease in the AF -value occurs when the stress reaches a certain level, and the stress level corresponding to this transition will increase with strain rate. Meanwhile, the RA–AF distribution is mostly concentrated on the abscissa in the low strain rate tests, but gradually concentrates on the longitudinal axis as the strain rate increases. This indicates that tensile cracking becomes the dominant fracture mode with increasing strain rate. The b -value decreases with increasing strain rate in the uniaxial compression tests; however, the b -value in the impact loading tests is higher than that in the uniaxial compression tests. Furthermore, to distinguish the signals generated by stress wave propagation from the signals generated by rock fracturing in the impact loading tests, a four-parameter k -means algorithm is used to conduct a clustering analysis. The results indicate that the signals can be classified into four clusters: tensile fracturing signals (cluster A), mixed stress wave and shear fracture signals (cluster B), mixed stress wave and tensile fracture signals (cluster C), and stress wave signals (cluster D). [ABSTRACT FROM AUTHOR]