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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]

In this study, the response mechanism between macro- and microscales of deep hard-rock diorite is investigated under loading and unloading conditions. Moreover, the statistical theory is combined with particle flow code simulations to establish a correlation between unloading rates observed in laboratory experiments and numerical simulations. Subsequent numerical tests under varying confining pressures are conducted to examine the macroscopic mechanical properties and the evolution of particle velocity, displacement, contact force chain failures, and microcracks in both axial and radial directions of the numerical rock samples during the loading and unloading phases. The findings indicate that the confining pressure strength curve displays an instantaneous fluctuation response during unloading, which intensifies with higher initial confining pressures. This suggests that rock sample damage progresses in multiple stages of expansion and penetration. The study also reveals that with increased initial confining pressure, there is a decrease in particle velocity along the unloading direction and an increase in particle displacement and the number of contact force chain failures, indicating more severe radial expansion of the rock sample. Furthermore, microcracks predominantly accumulate near the unloading surface, and their total number escalates with rising confining pressure, suggesting that higher confining pressures promote the development and expansion of internal microcracks. [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]

To study the stress distribution characteristics of surrounding rock and the spalling mechanism of deep hard rock tunnels with different arch heights, the complex variable function and angle-preserving transformation method in elasticity theory were applied to the analytic solution of tangential stress distribution of arch tunnels during stress adjustment. In addition, true triaxial tests were conducted on granite cube specimens (100 mm × 100 mm × 100 mm) containing holes with three arch heights (including the 25 mm semi-circular arch, 16.7 mm three-centered arch, 12.5 mm three-centered arch) to simulate the spalling process under different initial ground stresses. The stress distribution solution and experimental results show that the initial failure stress of arch holes is 0.39–0.48 times the uniaxial compressive strength (UCS) of the rock. The initial failure location occurs at the arch foot, where tangential stress maximizes. When the lateral pressure coefficient is in the range of 0.38–0.50, the tangential stress is 3.2–3.5 times the UCS. The rock debris of the hole wall are in thin flake shapes. Symmetrical V-shaped or curved failure zones occurred on hole sidewalls. The stress distribution resolution of the surrounding rock of tunnels with different arch heights shows that with the increasing burial depth, the bearing performance of the semi-circular arch tunnel is optimal. In addition, the maximum tangential stress increases as the height of the arch decreases or the lateral stress increases, making it easier for the initial failure to occur at the foot of the arch. [ABSTRACT FROM AUTHOR]

In this study, the response mechanism between macro- and microscales of deep hard-rock diorite is investigated under loading and unloading conditions. Moreover, the statistical theory is combined with particle flow code simulations to establish a correlation between unloading rates observed in laboratory experiments and numerical simulations. Subsequent numerical tests under varying confining pressures are conducted to examine the macroscopic mechanical properties and the evolution of particle velocity, displacement, contact force chain failures, and microcracks in both axial and radial directions of the numerical rock samples during the loading and unloading phases. The findings indicate that the confining pressure strength curve displays an instantaneous fluctuation response during unloading, which intensifies with higher initial confining pressures. This suggests that rock sample damage progresses in multiple stages of expansion and penetration. The study also reveals that with increased initial confining pressure, there is a decrease in particle velocity along the unloading direction and an increase in particle displacement and the number of contact force chain failures, indicating more severe radial expansion of the rock sample. Furthermore, microcracks predominantly accumulate near the unloading surface, and their total number escalates with rising confining pressure, suggesting that higher confining pressures promote the development and expansion of internal microcracks.

The study on the failure difference of deep hard rock based on the comparison between conventional and true triaxial tests can help us better understand the fracture processes and failure characteristics of the deep rock mass. Therefore, this article carries out a comparative analysis of the failure of hard rock under conventional and true triaxial stress states. Within the scope of this study, it is found that the brittle–ductile transformation properties can be intuitively reflected in the rock stress–strain curve and failure mode. The brittle–ductile transition point of rock can also be determined by the difference between peak and residual strengths. The rock failure strength increases with the increase of σ2, the peak strain decreases with the increase of σ2, the stress drop of the post-peak curve becomes more obvious with the increase of σ2, and the rock tends toward Class II brittle failure after the peak with the increase of σ2. When σ3 is relatively high, the rock fracture angle increases with the increase of σ2 with obvious regularity. Compared with conventional triaxial stress conditions, the differential stress-induced anisotropy failure is the biggest difference in rock fracture characteristics between true and conventional triaxial stress states. This study can supply useful references to the study of failure properties of hard rock under complex stress states.

To study the stress distribution characteristics of surrounding rock and the spalling mechanism of deep hard rock tunnels with different arch heights, the complex variable function and angle-preserving transformation method in elasticity theory were applied to the analytic solution of tangential stress distribution of arch tunnels during stress adjustment. In addition, true triaxial tests were conducted on granite cube specimens (100 mm × 100 mm × 100 mm) containing holes with three arch heights (including the 25 mm semi-circular arch, 16.7 mm three-centered arch, 12.5 mm three-centered arch) to simulate the spalling process under different initial ground stresses. The stress distribution solution and experimental results show that the initial failure stress of arch holes is 0.39–0.48 times the uniaxial compressive strength (UCS) of the rock. The initial failure location occurs at the arch foot, where tangential stress maximizes. When the lateral pressure coefficient is in the range of 0.38–0.50, the tangential stress is 3.2–3.5 times the UCS. The rock debris of the hole wall are in thin flake shapes. Symmetrical V-shaped or curved failure zones occurred on hole sidewalls. The stress distribution resolution of the surrounding rock of tunnels with different arch heights shows that with the increasing burial depth, the bearing performance of the semi-circular arch tunnel is optimal. In addition, the maximum tangential stress increases as the height of the arch decreases or the lateral stress increases, making it easier for the initial failure to occur at the foot of the arch.

• The effect of loading angle ω on the time-dependent deformation of natural jointed granite subjected to true triaxial stresses was investigated. • The unfavorable combination relationship between joint orientations and true triaxial stresses was investigated. • The stress-structure-controlled time-dependent failure enters the stage dominated by shear cracks earlier than stress-controlled. • Joint orientations most likely to induce the time-dependent disaster in deep tunnel were predicted according to the true triaxial tests of jointed granite. Joints are an important part of rock masses and the spatial relationship between joint orientation and in situ stress (or tunnel axis) has a significant impact on the deformation, strength, and failure behavior of the surrounding rock. To study the influence of joint orientation on creep characteristics of deep hard rock, the true triaxial creep tests were performed in the laboratory on natural jointed granite under different loading angles ω (defined as the angle between the strike of the joint plane and σ 2 -direction) conditions. The test results show that ω has a significant effect on the time-dependent deformation and failure characteristics of the jointed rock. As ω increases, there is a decrease in creep deformation, steady creep rate in the σ 3 -direction, and value of the DI index (used to evaluate the difference in the time-dependent deformation in the σ 2 - and σ 3 -directions). Stress-structure-controlled failure (sliding along the joint plane) occurs when ω ≤ 30°. However, when ω > 30°, the failure of the jointed granite is not affected by joint and stress-controlled failure occurs. The stress-structure-controlled time-dependent failure evolution enters a stage that is dominated by shear cracks earlier than stress-controlled failure according to AE results. In addition, the time-dependent failure went through stages of microcracks initiation and interaction, stable crack growth and unstable crack propagation. And when the time-dependent failure evolution enters the unstable crack propagation stage, the nature of the cracks changed from tensile-dominated to shear-dominated. [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]

Zonal disintegration refers to the phenomenon whereby fractured zones and intact zones appear alternately in surrounding hard rocks under high stress conditions. In this study, zonal disintegration within hard surrounding rocks of deep tunnels at the China Jinping Underground Laboratory Phase II (CJPL-II) was observed by using a digital borehole televiewer in pre-drilled boreholes. The results revealed that fractured zones within intact high-strength marbles were small and exhibited a higher degree of fracturing, whereas fractured zones within low-strength, low-integrity, marbles showed fewer, yet larger fractures. An evaluation method based on rock mass integrity index using digital borehole televiewer data (RMIBT) has been proposed for the analysis of zonal disintegration process in the deep hard rocks surrounding the tunnels. By using the method we were able to quantify the initiation and evolution of fractures during zonal disintegration. The formation mechanism of the second and third fractured zones induced by the existing fractures was further investigated using numerical analysis. The data provide an insight into the evolution of failures in, and a basis for support design and stability control of, deep engineering works in hard rock. [ABSTRACT FROM AUTHOR]

To investigate the jet breaking mechanism of hard rocks under three-dimensional stress, jet breaking experiments under different ground stresses and horizontal stress differences were performed. The results showed that the rock-breaking holes were in the shape of an "inverted Ω". The ground stress had an obvious effect on the rock-breaking performance of the jet. As the ground stress increased, the rock strength increased, and the rock-breaking performance decreased by 28.56%. At ground stresses greater than 50 MPa, the rock-breaking performance increased by 20%. The horizontal stress difference made the pore wall stress concentrated and loosened the cementation between mineral particles. As the stress difference increased, the roughness increased, and the rock-breaking performance increased by 58.85% while the energy consumption decreased by 25.56%. The results of this study provide data and technical support for the jet breaking of hard rock in deep mine construction. [Display omitted] • High-speed particle jet rock-breaking test was performed under deep in-situ stress. • The effect of stress difference on rock-breaking performance was examined. • The microscopic morphological characteristics of the crushing holes were studied. • The effect of three-dimensional stress on rock-breaking by a jet was determined. [ABSTRACT FROM AUTHOR]