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Strainburst often occurs in the loading process of tangential stress concentration from surrounding rocks after excavation and unloading of deep rock mass. The concentrated tangential stress is relatively larger on the tunnel walls, which decreases towards the interior of surrounding rocks with a certain gradient. To explore the fracture evolution characteristics of surrounding rocks under different tangential gradient stresses and understand various phenomena in the strainburst process, a large‐scale true‐triaxial gradient and hydraulic‐pneumatic composite loading rockburst test device was adopted to carry out the strainburst tests under four different gradient stresses. Based on the macroscopic failure phenomena, distribution of rockburst debris, macro‐micro morphology of failure cross section, and monitored data of acoustic emission (AE), the fracture evolution of rockburst specimens was analyzed under different stress gradient loadings. The results indicate that the stress gradient has an obvious influence on the fracture characteristics, resulting in different rockburst phenomena. With the increase of stress gradient, the failure stress of rockburst decreases, while the dynamic failure characteristics of rockburst trends to be significant. The increase of stress gradient contributes to higher fragmentation degree of debris, more uniform distribution and better continuity. The total number of cracks in the model decreases with the rise of the stress gradient, and the proportion of shear failure increases, which is the main reason for the enhancement of the rockburst intensity. [ABSTRACT FROM AUTHOR]

To study the effects of different stress gradient on rockburst hazards, a novel experimental technique was proposed and a series of tests under different stress gradient paths were conducted on specimens (400 mm × 600 mm × 1000 mm) by using a true-triaxial gradient and hydraulic-pneumatic combined test device. The damage phenomena, force characteristics and acoustic emission (AE) signals of the specimen during rockbursts were systematically analyzed. The experimental results indicated that different stress gradient distributions not only affect the failure mechanism of the specimen, but also affect the energy conversion characteristics. With the stress gradient increasing from 0 to 6, the index r decreasing from 0.93 to 0.37, and the power law growth characteristics of the energy acceleration tends to be obvious. The energy released after rockburst accounts for 31.1%, 41%, 77.7%, and 87.5% of the total energy, respectively, which is an important reason for the severe damage tendency of rockburst. Rockburst intensity can be judged according to the stress/strength ratio corresponding to the inflection point of the AE energy fitting curve under different stress gradients, which can be used as an important precursor of rockburst. [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]