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The tangential stress of surrounding rocks is large on tunnel walls. While from tunnel walls to the interior of surrounding rocks, the tangential stress declines and approaches the in situ stress in a gradient manner. To study the influences of stress gradient on the failure mechanism of rockbursts, a mesoscopic model was established based on the discrete element software particle flow code (PFC). The model was used to simulate rockburst disasters in the loading process of gradient stresses and analyze the failure modes and energy evolution process under different gradient stresses. Using the PFC platform, an acoustic emission (AE)–based simulation method at the mesoscopic scale was proposed according to the moment tensor theory to explore features of AE events, including the spatio-temporal distribution and fracture strength of the model during rockbursts. By analyzing the failure process, the increase in the applied stress gradient is found to accelerate the deterioration process of materials and promotes samples to fracture rapidly along dominant main cracks. The number of derivative cracks and the total number of cracks are significantly reduced, and the model shows a change from tensile failure to shear failure. As the applied stress gradient grows, the proportion of elastic energy storage in the model increases before a rockburst, and the rate of release of energy rises accordingly during the rockburst. The AE count at fracture points on the unloading face of the model is normally distributed with changes in the strength M, and the overall AE intensity is enhanced as the gradient increases. [ABSTRACT FROM AUTHOR]

To investigate the influence of stress gradients in the surrounding rock of tunnels on the frequency features of acoustic emission (AE) waveforms, a true-triaxial gradient, and hydraulic-pneumatic loading apparatus were used to conduct rockburst experiments under four different stress gradients. The AE waveforms were recorded in real time throughout the entire loading process using an AE instrument. A fast Fourier transform was applied to extract feature points from the frequency spectrum of the AE waveforms. The main contribution of this study is the qualitative analysis of the feature points using the proposed variance/mean ratio (VMR) under different stress gradients. It was found that: (1) the cumulative amplitude of the dominant frequency of the feature points decreases with increasing stress gradients, indicating a reduction in crack size in the specimens. (2) The feature points exhibit varying evolutionary patterns with loading under different stress gradients, with low-frequency, high-amplitude feature points tending to concentrate at the moment of rockburst as stress gradients increase. (3) Fluctuations in the VMR curve can be utilized to identify stages of specimen deformation, and regular changes in these stages were observed with increasing stress gradients. (4) As the stress gradient increases, the fluctuation of the VMR curve gradually decreases, indicating that the increase in stress gradient accelerates the alternating development of macroscopic scale cracks and small-scale cracks. The results of the experiments are expected to provide insights into the mechanism of rockburst in underground constructions. [ABSTRACT FROM AUTHOR]

Rockburst is a common geological disaster in the process of deep rock excavation, and stress gradient is an important factor affecting rockburst. In this paper, the concept of “stress concentration” was proposed. The relevant data of a large number of engineering rockburst cases were statistically analyzed, and a new rockburst criterion of stress–strength ratio considering stress gradient of surrounding rock was established. The research results revealed that the rockburst criterion considering the stress gradient distribution of surrounding rock could effectively overcome the problem of non-uniform division of rockburst criteria in the traditional strength-to-stress ratio. It can fully reflect the influence of tunnel size effect on rockburst. There was a significant correlation between rockburst strength and stress gradient, and different rockburst strengths showed obvious zoning concentration phenomenon on the two-dimensional plane. The established rockburst criterion considering the stress intensity ratio of surrounding rock had strong applicability, and the prediction accuracy could reach more than 90%. The proposed rockburst criterion in this paper has certain theoretical and engineering guiding value for the early warning of underground engineering disasters. [ABSTRACT FROM AUTHOR]

Strainburst is a kind of safety concern caused by the release of elastic strain energy from surrounding rocks under a tangential stress gradient. In this paper, laboratory rockburst model tests under four different stress gradients were conducted. Based on the acoustic emission (AE) data, the influence of tangential stress gradient on the energy evolution of strainburst was studied. The results indicate that (1) by controlling the tangential stress gradient loading at the top of the specimen, the change process of tangential and radial stress gradient from surrounding rocks caused by excavation disturbance, to a large extent, can be simulated. (2) The failure phenomena and failure stress of the specimen are both related to its tangential stress gradient distribution. As the tangential stress gradient increases, the failure stress is reduced, but dynamic failure phenomena become more evident. (3) With the increase of tangential stress gradient, the accumulation period of elastic strain energy lengthens, whereas the dissipation and release periods shorten during the loading process. As the tangential stress gradient rises, when the specimen rockburst occurs, the more dramatic decrease and faster decreasing rate of AE b-value reveal the increase in the specimen's proportion of shear failure. Also, the critical index (r) of probability density distribution shows a downward trend, which indicates that the AE energy in the low disturbance area decreases. In contrast, the AE energy span in the high disturbance area widens and the energy level becomes higher. [ABSTRACT FROM AUTHOR]