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Accurate estimation of potential rockburst pit (PRP) depth contributes to assessing rockburst risks and developing prevention programs, while the existing empirical methods lack of reasonable consideration for the different damage distributions within surrounding rocks during blasting excavation. This study provides the PRP depth estimation formula matching two damage distributions, and subsequently establishes a quantitative estimation procedure. After obtaining the uniaxial compressive strength of the rock, the initial maximum principal stress, the geological strength index GSI , the disturbance factor D s at tunnel profile and depths of high-damage zone (HDZ) and weak-damage zone (WDZ), the PRP depth can be determined by the given estimation formula. Acoustic wave testing and quantification of disturbance factors are utilized to obtain parameters D s and depths of HDZ and WDZ. Taking two deep tunnels as examples, the accuracy of the proposed method is verified. The results show that the estimated PRP depth coincides well with the observed rockburst pit depth, and the estimation accuracy is obviously superior to existing empirical methods. Basis on this, the variation law of PRP depth during blasting excavation is analyzed. It is found that the formation and expansion of the PRP is attributed to the continuous reduction of rock mass strength during blasting excavation, and the PRP depth is positively correlated with D s , HDZ and WDZ depths. [ABSTRACT FROM AUTHOR]

The excavation-induced weakening effect of rock mass parameters is detrimental to the stability of deep tunnels. This study presents a quantification method for the weakening effect of rock mass parameters. In this method, the weakening factor is introduced as a quantitative index of the weakening effect of rock mass parameters, which can be calculated by the acoustic P-wave velocity measured in the surrounding rock and the Hoek–Brown criterion. Taking three deep tunnels as examples, the weakening law of rock mass parameters is investigated. The results indicate that the weakening effect is most severe near the excavation outline and gradually decays with increasing distance. Significant differences in the weakening effect can be observed at different parts of the tunnel cross-section and under different excavation methods. The weakening effect can be further exacerbated by multiple excavation disturbances. Among the strength parameters of rock masses, the weakening effect of uniaxial compressive strength is the most obvious, followed by uniaxial tensile strength, cohesive strength, and equivalent internal friction angle. The FLAC3D software is utilized to reproduce different weakening effects and analyze the energy distribution around tunnels. It is found that as the weakening effect intensifies, the accumulated elastic strain energy within the surrounding rocks gradually diminishes, and the peak elastic strain energy is at the junction of damaged and undamaged rock masses. • A quantification method for the weakening effect of rock mass parameters is proposed. • The weakening effect is most severe near the tunnel contour and decays with increasing distance. • Multiple excavation disturbances could further exacerbate the weakening effect. • The energy distribution of surrounding rock with varied weakening effects is numerically reproduced. [ABSTRACT FROM AUTHOR]

• 1114 rockburst cases from 197 underground rock projects are collected. • The qualitative and quantitative characteristics of rockbursts are systematically analyzed. • "Rock–Environment–Excavation" three-element rockburst mechanism is introduced. • A strength–stress coupling criterion considering absolute relationships between rock strength and geostress is proposed. • The proposed rockburst criterion has high accuracy in predicting rockburst intensity. Rockburst is a kind of engineering geological disaster induced by artificial engineering excavation. Qualitative and quantitative statistical analysis of rockburst cases is the primary work and research basis for rockburst prediction and discrimination. In this study, 1114 rockburst cases are collected and the qualitative and quantitative characteristics are analyzed. It is found that rockburst is prone to occur in rocks that are hard, complete, dry, and unweathered. Strong rockburst occurs more frequently in rocks with high uniaxial compressive strength (σ c), elastic modulus, and elastic energy index, and in rock masses with high initial maximum principal stress (σ 1)and tangential stress. Notably, rockburst may occur only when σ c ≥ 60 MPa and σ 1 ≥ 10 MPa. On this basis, the "Human–Rock–Environment" three-element mechanism of rockburst is presented. Guided by this conception, the rationality and defects of the traditional strength-stress ratio (SSR) criterion are investigated. The SSR criterion does not adequately consider the absolute difference between the rock strength and geostress, as well as their combined effects. Subsequently, a strength-stress coupling (SSC) criterion is proposed from a two-dimensional scale.This criterion is characterized by considering both the absolute boundary conditions of σ c and σ 1 , as well as their coupling interval. Five intervals of weak, weak–moderate, moderate, moderate–strong, and strong rockburst are divided in the two-dimensional chart and their distinguishing conditions are given. The existence of two transition zones for weak–moderate and moderate–strong reflects the coupling effect of rock strength and geostress when rockburst occurs, confirming the rationality and scientificity of the SSC criterion. The verification results show that the SSC criterion features higher accuracy as compared with the SSR criterion. [ABSTRACT FROM AUTHOR]

• A quantitative evaluation model of rockburst proneness for surrounding rocks is proposed. • The residual elastic energy is estimated by the geological strength index GSI and disturbance factor D. • The empirical theory and wave velocity methods are adopted to quantify GSI and D. • The excavation disturbance could reduce the rockburst proneness of surrounding rocks. • Multiple disturbances could further expand the low and medium rockburst proneness range. Rockburst proneness evaluation is crucial for predicting and preventing rockburst disasters. This study proposes a quantitative evaluation model of rockburst proneness for surrounding rocks. In this model, residual elastic energy is applied as an index to quantitatively evaluate the rockburst proneness, which can be calculated by the geological strength index GSI and disturbance factor D. Furthermore, empirical theory and wave velocity methods are adopted to quantify GSI and D. The results show that the GSI value estimated using acoustic P-wave velocity coincides well with the suggested value. The excavation damage zone (EDZ) around tunnels can be divided into the high-damage zone (HDZ) with D = 1 and weak-damage zone (WDZ) with D linearly decreasing to zero. On this basis, a quantitative evaluation procedure for rockburst proneness is presented and applied to five tunnels. It is found that excavation disturbance could reduce the rockburst proneness of surrounding rocks. The rock mass adjacent to the excavation outline is most severely disturbed, resulting in lower residual elastic energy and more likely to exhibit a low or medium rockburst proneness. With the D linearly decreasing within WDZ, the residual elastic energy first slowly and then rapidly increases, and the rockburst proneness gradually enhances. The residual elastic energy and rockburst proneness is positively correlated with GSI. In addition, significant differences exist in the rockburst proneness distribution at different parts of the excavation section, and multiple disturbances could further expand the low and medium rockburst proneness range. [ABSTRACT FROM AUTHOR]

To fully understand the influence of water on rockburst proneness from energy storage viewpoint of rock and to check the performance of energy-related rockburst proneness indexes, several groups of uniaxial compression tests at different stress levels are performed on three types of red sandstone specimens (i.e., the saturated, natural, and oven-dried specimens). The method of combining strain energy calculations and experimental observations is used to assess the rockburst proneness. The progressive failure of the three types of specimens is recorded by a high-speed (H-S) camera, and the rockburst proneness is comprehensively assessed by considering the failure features of the rock specimens (including the fracture sound, fragment ejection, fragment distribution, and far-field ejection mass ratio). The laboratory results reveal that the rockburst proneness of the natural and oven-dried sandstone specimens is similar. However, compared with the natural and oven-dried specimens, the water-saturated specimens exhibit a visible weaker rockburst proneness. It is also evident that the linear energy storage and dissipation laws are applicable to the saturated, natural, and oven-dried specimens. Most interestingly, the energy storage and energy dissipation coefficients are nearly immune to the presence of water in the red sandstone. Based on this, the definitions of absolute energy storage capacity and relative energy storage capacity are adopted to describe the influence of water on the energy storage performance of rocks. It is found that water dramatically weakens the absolute energy storage capacity, but has little and negligible effect on the relative energy storage capacity of sandstone. Considering the direct and indirect indexes, the accuracy of several representative energy-related rockburst proneness indexes is comprehensively compared in terms of the far-field ejection mass ratio, and their performance is discussed from the relative and absolute energy storage aspects. According to their performance, it is strongly recommended to use the direct index for determination of rockburst proneness when energy-related indexes are used. • The rockburst proneness of oven-dried, natural, water-saturated sandstone specimens is analyzed. • The linear energy storage and dissipation laws are applicable for the sandstone under different contained water conditions. • The concepts of relative energy storage and dissipation capacities are proposed. • The performance of several representative energy-related rockburst proneness indexes is discussed [ABSTRACT FROM AUTHOR]

Spalling is a typical failure mode encountered on the sidewalls after the tunnel face excavation of deep D-shaped tunnels or caverns. It significantly affects the safe and efficient construction and long-term stability of tunnels. To study the process and failure characteristics of sidewall spalling in D-shaped tunnels, four different three-dimensional initial stress states were established to perform a series of true-triaxial tests on cube specimens with a D-shaped hole. During the tests, a wireless micro camera was used to monitor and record the failure process of the sidewall in real time. Based on the test results, the process and failure characteristics of sidewall spalling were summarised, and the influence of cross-section shape on tunnel failure characteristics and tunnel stability were discussed. The results indicate that both sidewalls perpendicular to the maximum principal stress are cut into approximately parallel rock slabs, demonstrating typical spalling features. Failure zones on both sidewalls are located primarily between the corner and spandrel, and their outlines are "V" shaped (i.e., V-shaped notch). Spalling slabs are thin in both wings, and thick in the middle; they are cut into multilayer thin slices that are not completely detached from one another. With the increase in failure depth, the shape of the cracks first evolved from a straight shape into an arced shape, and subsequently evolved into a "V" shape. When the maximum principal stress is perpendicular to the tunnel axis, tunnel excavation along the minimum principal stress is conducive to the prevention of spalling. Compared with the circular tunnels, the D-shaped tunnels can effectively reduce the severity of rock failure and transform the rockburst into a static spalling. In a relatively low stress environment, the D-shaped cross section can improve the stability of the tunnel, while it is not conducive to the stability of the tunnel in a high stress environment. The result of this study benefits the design and prevention of spalling and rockburst of hard rock tunnels. [ABSTRACT FROM AUTHOR]

• An internal drilling device to simulate the whole excavation path of deep tunnels was developed. • The influence of water on the rockburst mechanism in deep circular tunnels was investigated. • The weakening mechanism of water on rockburst in deep circular tunnels is clarified. Rockburst is one of the most serious engineering geological hazards encountered in deep rock mass. In this study, we investigate the influence of water on the rockburst mechanism in circular tunnels considering the whole stress path of high in situ stress + internal unloading + stress adjustment, based on external loading-internal unloading (ELIU) tests on intact air-dried and saturated sandstone cubic specimens (100 × 100 × 100 mm3). The proposed ELIU method uses true triaxial equipment to first load each intact cubic specimen with two-dimensional high stresses, followed by an internal drilling device for internal unloading to form circular holes. For comparison, cubic specimens with predrilled holes 25 mm in diameter were subjected to internal drilling – external loading (IDEL) tests, i.e., drilling first and then loading. The results show that water reduces the uniaxial compressive strength, Young's and initial moduli, and elastic energy storage capacity of sandstone materials, and increases the significance of shear failure. Furthermore, saturated tunnel structures induce static spalling failure, while dry tunnels lead to dynamic rockburst failure. The influence of water on the stability of the rock mass surrounding a tunnel is analyzed, including the induced weakening of surrounding rock strength and the amplification of the unloading effect. In summary, water prevents rockburst through the following mechanism: (i) water reduces the brittleness of the surrounding rock and enhances its plasticity; (ii) water reduces the elastic energy storage capacity and increases the energy dissipation capacity of the plastic zone; and (iii) water induces the redistribution of stress throughout the surrounding rock, which reduces the stress concentration near the free surface and transfers the maximum stress to the deeper surrounding rock. [ABSTRACT FROM AUTHOR]

To estimate the energy storage and release performances of rock pillars in high stress and gain insights into the prevention and control of rockburst hazards from an energy aspect, several series of single-cyclic loading-unloading uniaxial compression tests with different stress levels were conducted on five types of rocks sampled in standard cylinders and prisms. The concepts of relative energy storage potential and ultimate energy storage potential were proposed to evaluate the energy storage performance of the two shaped specimens, and the energy release potential was determined using the residual elastic strain energy index. The results showed that, under similar loading conditions, the relative energy storage potential of rocks was insensitive to the specimen shape, whereas the ultimate energy storage potential and energy release potential were sensitive to both the specimen shape and rock type. Based on the lab results, a theoretical approach of assessing the rockburst (or instability) risk of isolated rock pillars subjected to high geostress was illustrated by estimating its current energy state and ultimate energy storage potential. Importantly, the residual elastic energy of an isolated pillar can also be estimated based on the lab results, according to which the number of supporting devices such as the energy-absorbing bolts needed for scientific and sufficient support of pillar rockburst was quantitatively estimated. This study provides a new method for estimating the desired number of energy-absorbing bolts during rockburst support in engineering practice. [ABSTRACT FROM AUTHOR]

The occurrence of rockburst is closely related to initial high stress state and excavation unloading effects. To investigate the effects of three-dimensional (3D) stress state and unloading rate on the rockburst mechanism, triaxial unloading compression tests were performed on cubic fine-grained granite specimens (50 mm × 50 mm × 50 mm). Two unloading rates (0.08 and 40 MPa/s) were employed in the tests with seven initial 3D confining pressures (5, 10, 20, 30, 40, 50, and 60 MPa). For comparison, triaxial and biaxial compression tests were conducted at the same initial confining pressures. The results show that, under the same confining pressures, the variations in peak strength of granite specimens are in the following order: triaxial compression test > biaxial compression test > triaxial unloading compression test under low unloading rate > triaxial unloading compression test under high unloading rate, which indicating that unloading induces an obvious strength-weakening effect on fine-grained granite. In the triaxial unloading compression test with the same unloading rate, a higher confining pressure results in a greater rock bearing strength and stress drop. More energy will be stored in the rock, and the rockburst intensity will be more serious when the rock is destroyed. When the confining pressure is constant, the lower unloading rate is beneficial for improving the rock bearing strength, and more elastic energy will be accumulated and stored in the rock. Once rockburst occurs, the energy released by rock is larger and the degree of rockburst is stronger. The failure mode of fine-grained granite specimens in triaxial unloading compression tests transforms from shear-tension failure to tension failure with increasing confining pressure, which is the main reason for the weakening of rock strength. Establishing a strength criterion for 3D rock considering the unloading effect is a key for understanding the mechanism of rockburst. [ABSTRACT FROM AUTHOR]

Spalling (or slabbing) in the sidewalls of deep rectangular tunnels is a type of rock failure phenomenon under the three-dimensional stress environment, which often occurs on the surrounding rocks after the tunnel face excavation is completed and has a significant impact on the safety of construction and support. In order to investigate the spalling process in deep rectangular excavated tunnels under three-dimensional stress, a granite material was firstly processed into cubic specimens (100 mm × 100 mm × 100 mm) with prefabricated rectangular holes (40 mm × 40 mm). Simulation experiments were performed using a rock true-triaxial electro-hydraulic servo mutagenesis testing system under four different initial stress states. Meanwhile, a wireless micro-camera was used to record and monitor the damage process on the sidewalls of the rectangular tunnel in real time. The results showed that under the four stress conditions, an evident spalling failure occurred on the entire sidewalls of the rectangular hole. It was observed that the spalling-damaged zone gradually developed toward the deep part of the hole in a horizontal direction. Finally, it formed a penetrating symmetric arc-shaped groove along the axial direction, whereas the roof and bottom slab remained stable. The spalling failure process of the sidewalls had four stages: the calm period, particle ejection at the upper and lower shoulders, sidewall crack propagation, and crack penetration spalling failure. Compared with the dynamic failure (rockburst) of a circular tunnel under the same three-dimensional high stress conditions, the spalling failure of the rectangular tunnel sidewalls was of the static failure mode. Under the three-dimensional stress condition, the radial stress had a greater effect on the spalling failure than the axial stress, by increasing the radial stress can be significantly reduced the degree of spalling failure and effectively improved the stability of the surroundings. [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]

In this study, we investigated the failure process and evolution mechanisms in circular tunnels under a complex stress path of deep in-situ stress + excavation unloading + stress adjustment using a simulation test based on loading first and then drilling (LFTD) mode. This test was performed on cubic sandstone specimens using a true-triaxial testing system combined with a self-developed drilling unloading test device. For comparison, another set of sandstone specimens, similar to the original specimens but with Φ 25 mm circular through-holes, were processed to perform another set of simulation tests based on drilling first and then loading (DFTL) mode. Under constant lateral stress, compared with the DFTL test, the initial failure vertical stress was lower, and the surrounding rock was more prone to failure in the LFTD test, indicating that drilling unloading causes damage to the surrounding rock, thereby inducing a strength-weakening effect. Under constant lateral and vertical stresses, sidewall failure in the LFTD test was more severe. The sidewall's initial failure vertical stress difference between the LFTD and DFTL tests increases with increasing lateral stress or initial hydrostatic pressure. The effect of drilling unloading becomes more apparent under a higher initial stress state. Comparing the same types of simulation tests under different stress states, the initial failure vertical stress increases with the lateral stress or initial hydrostatic pressure, inducing a strength-strengthening effect in the surrounding rock. Comparing the strength-strengthening effect caused by increasing the lateral pressure or initial hydrostatic pressure and the strength-weakening effect caused by drilling unloading shows that the strength-strengthening effect is more evident. [ABSTRACT FROM AUTHOR]