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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]

• 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]

Strain rockburst poses a great threat to the construction safety of deep engineering, especially induced by dynamic impact loads such as blasting from the adjacent tunnel excavation. Aiming to clarify the failure characteristics and mechanism of strain rockburst induced by the dynamic impact load, a series of dynamic impact tests under different biaxial prestress conditions were conducted on the sandstone cube specimens with prefabricated circular holes by using the new biaxial Hopkinson pressure bar equipment. The real-time rockburst process and fracture characteristics on both sidewalls were captured by the high-speed camera. The test results show that the strain rockburst process can be divided into: the calm stage, slab buckling and spalling stage, rock slabs ejection stage, and V-shaped notch formation stage. Ultimately, a symmetrical V-shaped notch failure zone was formed on the sidewall, similar to the site. The strain field evolution characteristics and displacement deformation characteristics around the circular hole during the rockburst process were further explained by using digital image correlation (DIC). The failure mechanism of sidewall strain rockburst was closely related to spalling failure, characterized by dynamic tensile failure. The static stress provides the initial compressive stress field and governing the stress distribution around the tunnel. Dynamic impact load disrupts the original stress equilibrium of the surrounding rock, inducing significant alterations in internal stresses and strains within the surrounding rock, consequently inducing spalling or rockburst failure. The influences of lateral pressure on rockburst were also analyzed in detail, while the lateral pressure influences the severity of strain rockburst, which determines the dynamic load response characteristics, influence range of strain concentration zone, displacement deformation characteristics of the tunnel surrounding rock, and the severity of strain rockburst decreases with the increase of lateral pressure. [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]

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]

Judgement of rock burst proneness of rock materials is one of the basic problems in the field of rock burst research. In this study, a peak-strength strain energy storage index is proposed for estimating and classifying the rock burst proneness of rock materials. The method for determining this index is also introduced in this paper. The peak-strength strain energy storage index is defined as the ratio of the elastic strain energy density to the dissipated strain energy density corresponding to the peak compressive strength of rock specimen. In order to obtain the elastic strain energy density and the dissipated strain energy density at the peak strength, a series of single cyclic loading-unloading uniaxial compression tests on nine rock materials were conducted. The relationships between the total input energy density and the elastic strain energy density at different unloading stress levels were investigated. The results show that, for every rock material, the elastic strain energy density increases linearly with the increase of the total input energy density. Based on this linear storage energy law, the elastic & dissipated strain energy density at the peak strength can be calculated for each specimen, and the peak-strength strain energy storage index can be obtained accordingly. A new criterion for rock burst proneness of rock materials is proposed. The rock burst proneness of nine rock materials estimated with the proposed criterion agreed well with the laboratory test results. [ABSTRACT FROM AUTHOR]

Abstract To investigate the dynamic Mohr-Coulomb and Hoek-Brown strength criteria under the low confining pressures at high strain rates, the dynamic uniaxial and triaxial compression tests of sandstone were conducted with a conventional and modified triaxial SHPB system, respectively. The confining pressures are 5.0 MPa, 7.5 MPa, 10.0 MPa, 12.5 MPa and 15.0 MPa, and the strain rates varied from approximately 40 s−1 to 160 s−1. The results show that the cohesion is a linear positive correlation with the logarithm of strain rate, and the internal friction angle is a linear negative correlation with the logarithm of strain rate. The change of cohesion is more obvious than that of the internal friction angle. The rock parameter m also exhibits a linear positive correlation with the logarithm of strain rate. Furthermore, the relationships between cohesion, internal friction angle and rock parameter m and strain rate were obtained. On this basis, the unified dynamic Mohr-Coulomb and Hoek-Brown strength criteria at high strain rates were established. According to the established dynamic strength criteria, the strength of sandstone under high strain rates and low confining pressures can be estimated theoretically. By comparing the theoretical curve and test data, it can be concluded that established dynamic Mohr-Coulomb and Hoek-Brown strength criteria of sandstone are appropriate for both high strain rates and low confining pressures. [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]

Considering the linear energy dissipation law under uniaxial compression, a damage characterization of rocks including the compression energy dissipation coefficient is proposed, which can make the existing damage calculation formula more quantitative. Damage variables of brittle rocks can be directly calculated by using the compression energy dissipation coefficient and the input strain energy of rock under uniaxial compression. Furthermore, because the classical strain equivalence hypothesis has a large error in describing the constitutive relation of rock, a correction coefficient considering the compaction effect is introduced to modify the classical model. The modified model is extended to single-cyclic loading–unloading conditions, and the corresponding constitutive equation is established to further verify the applicability of the damage model and correction coefficient. The conventional uniaxial compression and single-cyclic loading–unloading uniaxial compression models are validated with experimental results of four typical brittle rocks. The results show that the theoretical curves are highly coincident with the actual stress–strain curves, indicating that the established damage constitutive model can accurately reflect the conventional uniaxial compression and single-cyclic loading–unloading processes of rocks. The new model in this paper considers the linear energy dissipation and compaction effect, which provides an insight for the study of brittle rock constitutive model from the perspective of energy. [ABSTRACT FROM AUTHOR]

• The fracture toughness strength weakening effect of pre-static load CSTBD sample subjected to dynamic disturbance was investigated. • The pre-static load level dominates the weakening level of fracture toughness, while the dynamic disturbance plays a promoting role and affects the weakening degree. Highly static pre-loaded rock with cracks will be generated during the deep rock engineering excavation, is subjected to low-frequency dynamic disturbance (F d) to induce fracture failure. To investigate the fracture mechanisms of the high static pre-loaded rock with cracks, subjected to low-frequency dynamic disturbances, several tests were conducted on cracked straight through Brazilian disc (CSTBD) granite specimens using the MTS system. Based on the average static peak fracture load (F max) of the CSTBD specimens, three high static pre-load levels, F p (70%, 80%, and 90% of F max), and the corresponding disturbance amplitude, F a (30%, 20%, and 10% of F max), and low-frequency disturbance frequency, f (1, 5, 10, 30, and 50 Hz), were determined. The results revealed that the fracture toughness of the static pre-loaded CSTBD specimens subjected to low-frequency dynamic disturbance was lower than that under static load. The higher the F p , the more significant is the decrease in fracture toughness with f. Comprehensive analysis indicates that the degree of fracture toughness weakening is highly dependent on the F p , F d induces strength weakening, and f affects the weakening degree. [ABSTRACT FROM AUTHOR]

• The u t , u e , and u d increase nonlinearly with the increase of the i. • The u e , and u d have a significant linear relationship with the u t. • The A E increase linearly with the increase of age, while A D is opposite. • A novel method was proposed to calculate the u d and u e at ultimate strength point. To investigate the influence of age on energy storage and dissipation laws, uniaxial compression (UC) and single–cyclic loading–unloading uniaxial compression (SCLUC) tests were conducted on C35 concrete specimens with ages of 3 d, 7 d, 15 d, 28 d. The experimental results showed that the input strain energy (ISE), elastic strain energy (ESE), and dissipated strain energy (DSE) increased nonlinearly with the stress level during the pre-peak loading stage. Analyzing the relationship between the three strain energies, both ESE and DSE increase linearly with the increase of ISE in the pre-peak loading stage at different ages, namely, linear energy storage and dissipation laws exist in every ages. Based on the linear energy storage and dissipation laws, a novel method was proposed to calculate the DSE and ESE at the ultimate strength point of the concrete at different ages. In addition, we found that the DSE was greater than the ESE of concrete in the loading stage, because the energy was mainly dissipated in forming cracks, grain dislocation, and producing plastic displacement during compression. The value of compression energy storage coefficient represents the capacity of the concrete to store ESE, whereas energy dissipation coefficient represents the strain energy dissipation capacity of concrete. In the future, based on the linear energy storage and dissipation laws of concrete, the new method for calculating the damage variable may be proposed. [ABSTRACT FROM AUTHOR]

The investigation of rock damage behaviour is an important requirement for ensuring stability control and safety prediction in rock engineering. In this study, based on the linear energy dissipation law and energy dissipation coefficient, a theoretical method is introduced for characterising the damage of intact rocks under uniaxial compression conditions. The existing uniaxial compression test data of 14 kinds of rock materials (including 6 types of granite, 4 types of sandstone, 2 types of marble, 1 type of slate, and 1 type of limestone) were used to verify the effectiveness of the damage characterization method. The results indicate that the damage evolution process of rocks under uniaxial compression can be fully expressed by the proposed theoretical damage characterisation method, and the damage variable exhibits a quadratic functional relationship with the loading stress level. Moreover, with the linear energy storage law serving as a theoretical basis, the peak dissipated strain energy in a rock damage expression under uniaxial compression can be accurately calculated. The new theoretical damage characterisation method overcomes the shortcomings of conventional damage characterisation methods (e.g. estimating the peak dissipated strain energy through a hypothesis), and provides a new means for analysing rock damage from an energy viewpoint. [ABSTRACT FROM AUTHOR]

• The mechanical properties of red sandstone increase with the increase of pre-existing single crack angle. • The linear energy storage and dissipation laws of pre-existing single crack rock are obtained. • The peak strain energy densities are determined using the linear energy storage law. • The linear energy dissipation law is applied to express the damage of rock with single crack. To investigate the effects of the pre-existing single crack angle on the mechanical behaviors and energy storage characteristics of red sandstone, a series of uniaxial compression and single-cyclic loading–unloading uniaxial compression tests were conducted on red sandstone specimens with five crack angles. The relationships among the crack angle and the mechanical properties, failure modes, and energy storage characteristics of red sandstone were analyzed using the acoustic emission (AE), stress–strain curve and curvilinear integral. The results indicate that the peak compressive strength, peak strain, and elastic modulus of specimens increase with the crack angle increasing, and the specimens with five different angles all present "anti-N-type" shear failure. Throughout the loading process, the total input energy density, elastic strain energy density, and dissipated energy density of the red sandstone at five different crack angles exhibit significant quadratic growth in response to the actual unloading stress. Importantly, the elastic strain energy density and dissipated energy density of red sandstone with different crack angles all show highly linear relationships with the total input energy density, and the linear energy storage and dissipation laws of pre-cracked rocks are obtained. Based on the linear energy storage law, the energy densities at peak strength of red sandstone with different crack angles are determined quantitatively. In addition, the linear energy dissipation law is applied to the expression of rock damage, and the damage constitutive equations of uniaxial compression specimens with different crack angles are obtained. Finally, the effects of crack geometric parameters on the energy storage and dissipation characteristics were discussed. [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]

• The linear energy storage and dissipation laws during rock flexure are identified. • The elastic and dissipation energies at rock failure are determined using the linear energy storage and dissipation laws. • The relationship between the energy properties and mechanical properties is obtained. • The influence of pre-existing flaws on the linear energy storage and dissipation laws is discussed. To investigate the energy storage and dissipation characteristics during rock flexure fracturing, a series of single cyclic loading–unloading flexural fracture tests was conducted on rectangular rock beams using the three-point loading technique. The results show that under different experimental unloading levels, the elastic and dissipation energies increase linearly as the input energy increases. On this basis, the linear energy storage and dissipation laws were obtained, which were immune to the experimental unloading level. The flexural energy storage coefficient and flexural energy dissipation coefficient were subsequently introduced to characterize the potentials of the rock for storing and dissipating energy, respectively. A positive or negative relationship between the two coefficients and the rock mechanics properties (the brittleness index and average flexural strength) was observed. The influence of pre-existing rock flaws on the linear energy storage and dissipation laws was discussed. In addition, highly quadratic increases in the elastic energy, dissipation energy, and input energy were observed with an increasing experimental unloading level. For the energy characteristics at rock failure (the failure point of the rock specimen), the peak input energy, peak elastic energy, and peak dissipation energy varied considerably under different experimental unloading levels, while the associated peak elastic dissipation index (the ratio of peak elastic energy to peak dissipation energy) remained constant. Similar to the flexural energy storage coefficient, the mean value of peak elastic dissipation index is also positively related to the rock mechanics properties. The linear evolution law of energy storage and dissipation not only permits a quantitative determination of the energy parameters at rock failure (the peak elastic energy and peak dissipation energy), but also relates the energy properties with the rock mechanics properties. [ABSTRACT FROM AUTHOR]

Following the excavation of the rectangular tunnel faces with deep harder rock, spalling often occurs on both sidewalls of the tunnel, which affects the tunnel support and stability. To investigate the spalling phenomenon of the harder rock under the deep initial three-dimensional (3D) stress conditions, true triaxial loading tests of different depths (250, 500, 750, and 1000 m) and different stress combinations (four different horizontal stresses at the same depth 1000 m) were designed on plastic fine grain marble. The overall failure process of the surrounding rock was monitored and recorded by a wireless micro-camera. The results show that, under the shallow stress condition (250 and 500 m), the V-shaped bands will appear on the both sidewall due to the overall shear failure of the specimen. Under the deep stress condition (750 and 1000 m), the obvious spalling was observed on the hole sidewalls of all marble specimens. Moreover, at the time of hole sidewall spalling, the vertical stress is the maximum principal stress. The spalling process of the hole sidewall can be divided into four stages: the microcrack initiation, the appearance of thin rock slabs, the formation of arc-shaped notches, and the expansion to V-shaped notches. If the excavation depth remains the same (1000 m) when the maximum principal stress is perpendicular to the tunnel axis, excavation along the minimum principal stress direction is beneficial to reducing the spalling damage degree. Compared to the brittle granite specimens with rectangular holes under the same 3D stress conditions, the influence of brittleness and plasticity of rock materials on spalling failure was analysed. (1) Spalling failure mode of hole sidewalls: the plastic-marble is static failure without fine particle ejection, while the brittle granite will break dynamically with fine particle ejection. (2) Spalling failure process and zones of sidewall: the cracks on the plastic marble sidewalls develop parallel to the axial direction, and run through the whole hole sidewall along the axis, while the cracks on the brittle granite hole sidewalls propagate perpendicular to the axial direction, and the sidewalls first break locally and then break through the whole hole. Failure zones on both sidewalls of the two kinds of rock specimens will transit from arc-shaped notch to V-shape notch. Those failure notches are symmetrical and finally penetrate the sidewalls. (3) Microscopic failure mode: scanning electron microscope (SEM) shows that the plastic and brittle rock materials are tensile failure during the spalling under 3D stress conditions. The plastic marble is characterized by fracture tension failure, while the brittle granite is characterized by layered tension failure. Some suggestions are given finally, under the condition that the vertical stress is the maximum principal stress, when the tunnel is excavated along the horizontal direction, the resistant ability of the tunnel sidewall in the plastic and brittle surrounding rock should be strengthened by supporting the sidewall, which can effectively reduce the degree of spalling damage. [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]

Scientists have attempted to investigate deep underground rock failure. A key challenge to study underground rock failure is the difficulty to survey its process. Therefore, a large number of important and insightful laboratory investigations of underground rock failure experiments are conducted. In this paper, an experimental method is proposed to explore dynamic failure process of pre-stressed rock specimen with a circular hole. The failure process of a rock specimen under different initial static stress coupled with dynamic loading is clearly illustrated by a high speed camera. The experimental results indicated that high static pre-stress coupled with dynamic loading induces rock debris to be ejected at the surrounding circular hole; however, lower static pre-stress coupled dynamic loading cannot induce rock failure. The dynamic stress concentration surrounding the circular hole by transient wave incidence was further demonstrated at the condition of half-sine wave loading. In this condition, the results demonstrated that combined action of static and dynamic stress concentrations induces the primary fractures of rock specimen. [ABSTRACT FROM AUTHOR]

• The density of structural plane highly correlates with the P-wave velocity. • Deformation prediction method considering wave velocity is established. • Support design combined with wave velocity is an accurate method. With the rapid development of the tunnel project, the problem of soft rock tunnel support has become increasingly prominent. The prestressed anchor support method has made great progress in theory and practice, but the research on its quantitative design is less. The optimization design methods need improvement to meet specifications. This paper uses surrounding rock wave velocity and related geological conditions to further enhance the support optimization model. The joint density corresponding to different wave velocities is analyzed, and the DFN (discrete fracture network) is inserted into the numerical model for simulation. To ensure the accuracy of the calculation, similar material simulations and engineering measurements are compared and analyzed. Deformation of the surrounding rock with different support parameters is predicted and analyzed using a neural network and a database formed by numerical simulation calculation. The results show a linear relationship between the P-wave velocity of rock mass and the structural plane density of the surrounding rock. Then, under the influence of the structural plane and without prestressed anchor support, the surrounding rock produces large asymmetric deformation, and the damaged area is extensive. The surrounding rock also has asymmetric deformation with prestressed anchor support, but the deformation value and plastic area are small. The density of the structural plane, the burial depth of the tunnel, and the length of the anchor cable are the main factors controlling the surrounding rock. With an increase in tunnel burial depth, the deformation of the surrounding rock linearly increases. The reduction of the P-wave velocity of surrounding rock will also significantly impact the deformation of surrounding rock. An increase in anchor cable length has a significant control effect in the initial stage, but the control effect slows down with the continuous increase of anchor cable length. [ABSTRACT FROM AUTHOR]

Abstract: Groundwater inflow into tunnels can constitute a potential hazard and also is an important factor influencing the speed of tunnel excavation. In this paper the results of numerical modelling are presented to investigate the groundwater flow and the distribution of the pore pressure around tunnels. Two types of tunnels, double-arch tunnel and twin-tube tunnel, were studied. Potential leakage places are identified for the two types of tunnels. The most permeable place in the double-arch tunnel is at the contact interface between the middle wall and the overlying rock. The results of numerical modelling are compared with field observations in the case studies. Based on the results of numerical modelling and the field investigations, an innovative water-gathering system for reducing water leakage was proposed and applied in some tunnels on ChangJi Expressway in China. The water-gathering system can be quickly glued to the rock surface and easily installed for tunnelling. It can be applied in tunnels where water-bearing fractures are well-developed in the rock mass. [Copyright &y& Elsevier]

To explore the variation in rock acoustic emission (AE) characteristics with strain rate, uniaxial compression tests at different loading rates and impact loading tests were conducted on granite using a MTS322 rock mechanical test system and split Hopkinson pressure bar (SHPB) system, respectively. The effect of the strain rate on the AE characteristic parameters, rock fracture properties, and destruction evolution were systematically analyzed. The results demonstrated that with increasing strain rate, the cumulative AE count decreases as a power function, and the variation between the cumulative AE count and strain rate can be fitted log-linearly with a slope of 0.48. The peak frequencies of the AE signals are mostly distributed in the zones of 0–100 kHz, 175–250 kHz, and 400–550 kHz. The signal proportion in the 0–100 kHz zone gradually increases with strain rate, while the signal proportions in the 175–250 kHz and 400–550 kHz zone exhibit decreasing trends. A transition of a sudden increase in the RA -value and decrease in the AF -value occurs when the stress reaches a certain level, and the stress level corresponding to this transition will increase with strain rate. Meanwhile, the RA–AF distribution is mostly concentrated on the abscissa in the low strain rate tests, but gradually concentrates on the longitudinal axis as the strain rate increases. This indicates that tensile cracking becomes the dominant fracture mode with increasing strain rate. The b -value decreases with increasing strain rate in the uniaxial compression tests; however, the b -value in the impact loading tests is higher than that in the uniaxial compression tests. Furthermore, to distinguish the signals generated by stress wave propagation from the signals generated by rock fracturing in the impact loading tests, a four-parameter k -means algorithm is used to conduct a clustering analysis. The results indicate that the signals can be classified into four clusters: tensile fracturing signals (cluster A), mixed stress wave and shear fracture signals (cluster B), mixed stress wave and tensile fracture signals (cluster C), and stress wave signals (cluster D). [ABSTRACT FROM AUTHOR]

Stress conditions around deep underground mine openings can significantly influence rock fragmentation and stability, and thus the cuttability of the targeted rock. In this study, rock breakage experiments and associated regression analyses indicate nonlinear rock cuttabilities (decreasing followed by increasing) with increases in the differences between biaxial confining stresses and the values of uniaxial confining stresses. Rock breakages were found to be efficient and safe under low and no-stress conditions that require low indentation force and depth, cutting work and specific energy to completely split the rock wtih no rockburst risk. Stress concentration initially impeded rock breakage, although high uniaxial stress improved rock cuttability. Inducing high stress to fracture the rock and produce an excavation damage zone (EDZ) via stress release effectively transformed the stress condition into low confining stress or even the stress-free condition, improving rock cuttability significantly and preventing rockburst. Mining of rock in the EDZ around the pillar could be efficient, cost-effective and safe when using roadheaders, which showed high cutting efficiencies, low pick wear failures, high machine stabilities and no rockbursts. In addition, a binary linear regression model was proposed to determine the thickness variation of the EDZ correlated with the excavation span and a coupled index of rock properties and buried depth of opening. The results indicated that the thickness of the EDZ increases with increases in the buried depth of the opening, which can improve the applicability of non-explosive mechanized mining in deep mines. [ABSTRACT FROM AUTHOR]