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The deep excavation unloading stress path and initial high stress difference ( σ 2 0 - σ 3 0 ) are distinctive features that differentiate deep engineering from shallow engineering. To investigate the mechanism of deep hard rock failure induced by initial stress differences under excavation unloading stress paths, a series of typical true triaxial unloading tests are conducted using a true triaxial experimental system. The results show that as the initial stress difference increases, there is a gradual enhancement in both the strength, brittleness and ultimate energy storage capacity of the rock. Additionally, the direction of rock fracture propagation gradually aligns with σ2, leading to an increase in the macroscopic failure angle of the rock. Moreover, the roughness of the rock fracture surface decreases, while transgranular and tensile fractures become more prevalent, often exhibiting sudden, high-energy fracture events. Building upon the non-uniform stress distribution characteristics induced by the initial stress difference, a principle governing the differential development of rock fractures is proposed. This principle underscores the pivotal role of the initial stress difference in triggering spalling of the surrounding rock parallel to the sidewall. Furthermore, a functional relationship between the initial stress difference and the propensity for rockburst events is established. These findings hold significant implications for understanding the failure mechanisms of surrounding rock in deep hard rock engineering excavations and for evaluating the risks associated with spalling and rockburst hazards. Highlights: The failure mechanism of hard rock caused by the initial stress difference under the true triaxial excavation unloading stress path is revealed. The principle of differential development of rock fracture induced by initial stress difference is proposed, which reveals that initial stress difference is the decisive factor for the spalling of surrounding rock parallel to the side wall. The increase of initial stress difference can improve the rockburst tendency of rock, and the relationship between initial stress difference and rockburst tendency index is the exponential function. [ABSTRACT FROM AUTHOR]

The failure of a rock core will result in the inaccuracy of in situ stress tests when the overcoring stress relief method is used in deep rock engineering. To investigate the stress test error caused by rock failure, stress test simulations under different loading conditions and post-peak parameters are designed and implemented. The results show that the stress measurement error increases exponentially as the degree of rock core failure increases. The greater the severity of the rock failure, the greater the estimation error of each stress component. The strain test error is the primary cause of stress test error, not the residual stress at the rock core. Based on this, the applicable conditions of the overcoring stress relief method are analyzed. To keep the stress test error within an acceptable range, the test borehole axis should be kept close to the direction of the maximum principal stress. Additionally, the fracture depth should be less than 0.2 times the radius of the pilot hole. This research will provide the basis for the error analysis of stress test results and optimization of a borehole drilling process for the overcoring stress relief method in deep hard rock engineering. [ABSTRACT FROM AUTHOR]

Based on the frequent occurrence of stress-induced hazards in deep rock tunnels, a physical model experimental study of the development process, failure mechanism, and onset conditions of deep hard rock spalling in horseshoe-shaped tunnels is carried out. The test process mainly includes model preparation, loading, excavation, and monitoring. A high-speed camera, distributed fiber optic sensing, acoustic emission (AE) system, and visual imaging correlation-2D (VIC-2D) techniques are integrated to monitor the mechanical response of the surrounding rocks. The test results indicate that the physical model test successfully simulates the spalling phenomena of the deep hard rock. The integrated monitoring system successfully captures the information on deformation, AE signals, and crack propagation in the instantaneous process of the surrounding rock spalling failure, which provides a sound basis for a more comprehensive understanding of the mechanism. Microcracks and dilation appear first on the sidewall surface; the cracks then continue to propagate and coalesce, forming thin rock flakes and spalling in a lamellar manner from shallow to deep. The quantitative relationships between the onset stresses, the critical strains, and the initial boundary stresses of the spalling failure are established. At the same depth from the excavation boundary, the radial strain of the sidewall is larger than that of the crown, and it decreases away from the excavation boundary to the interior. The increasing loading in vertical direction leads to the stress concentration in the shallow part at the sidewall and a sharp increase of deformation. There is a positive correlation between the spalling failure degree and the initial boundary stresses. [ABSTRACT FROM AUTHOR]