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The elastic modulus of rock is an important parameter in rock engineering, but the common methods based on laboratory tests are laborious, especially for obtaining the probability distribution of the elastic modulus that is required in reliability-based design. Many scholars have studied the regression model between the elastic modulus and P-wave velocity; however, most previous reports have ignored the characterization of parameter variability and model uncertainty. To address this problem, a large number of granite samples are collected from the Yingliangbao hydropower station (YLB), compressive wave velocity (P-wave velocity) and uniaxial compression tests are carried out in the laboratory. Then, four different regression models based on the frequentist method and Bayesian method are established to estimate the elastic modulus, the normal priors are adopted by prior analysis and the widely applicable information criterion (WAIC) is used to select the most appropriate Bayesian regression model. Finally, the effects of sample size and sample selection on different methods are studied, the results obtained from different priors are compared. The results show that the Bayesian method provides estimations that are more consistent with the test data and has better robustness in given sets of different sample selections, especially in small sample size. [ABSTRACT FROM AUTHOR]

Investigation into rock damage is of great significance for evaluating and predicting the stability of underground rock applications, such as deep mining or tunneling structures. Considering the energy dissipation properties during rock deformation, this paper proposes a novel theoretical characterization of the damage induced by compressive-shear stress and its evolution in intact rocks. The linear energy dissipation (LED) law is derived from shear stress and deformation data of rocks resulting from the preset angle shear experiment. Based on the LED law, two damage variables are separately constructed from the theoretical and experimental aspects. Several sets of experimental data are subsequently utilized to validate the two constructed damage variables. Results show that both damage variables grow first slowly and then rapidly with shear displacement or shear stress in nonlinear relations. By comparison, however, it is found that the theoretical damage variable outperforms the experimental damage variable, which can accurately reflect the stress and deformation data during progressive rock damage with favorable continuity. This study contributes to a novel theoretical approach to quantifying the pre-peak damage in intact rocks subject to compressive-shear stress. [ABSTRACT FROM AUTHOR]

To investigate the energy evolution and storage characteristics of the rock during the mixed-mode fracture failure, several single loading-unloading tests under preset different unloading levels were conducted on the cracked straight-through Brazilian disc (CSTBD) red-sandstone specimen with crack inclination angles of 10° and 20°. The input energy, elastic energy, and dissipated energy parameters were obtained by calculating the area integral of the load-displacement curve. Test results display that the total input energy, elastic energy, and dissipated energy of rock specimens increase in quadratic function with the increase of unloading level at different unloading levels. It was found that there are significant linear energy storage and dissipation laws of rock during the mixed-mode fracture process. The concepts of fracture energy storage coefficient (FESC) and fracture energy dissipation coefficient (FEDC) were proposed to express the energy variation of rock in the prepeak stage. Under the same loading conditions, FESC and FEDC remain unchanged regardless of crack inclination angles, indicating that the linear energy storage and dissipation laws are an inherent property of the rock. Besides, the peak fracture elastic-dissipation index W ed (the ratio of peak elastic energy to peak dissipation energy) was obtained through quantitative analysis and demonstrated its invariant feature. [ABSTRACT FROM AUTHOR]

In practical engineering, rocks are often subjected to repeated loading and unloading caused by natural or artificial actions. In this study, a novel staged cyclic damage constitutive model for brittle rock is proposed and verified. Firstly, aiming at the pre-peak deformation characteristics, the damage variable of brittle rock defines as the ratio of dissipation strain energy to constitutive strain energy is developed. Based on the linear energy dissipation law, a new damage variable characterization including compressive energy dissipation coefficient and strain energy parameters is deduced. Secondly, a power function correction coefficient of strain is introduced to modify the traditional damage model based on the strain equivalence hypothesis. Finally, the novel staged cyclic loading–unloading damage constitutive model considering the accumulation effect of plastic strain is established by using the iteration relationship. The established model includes basic parameters such as compressive dissipation coefficient, strain energy and plastic strain, and the feasibility of this model are verified through the experimental data of three brittle rocks (including shale, green sandstone, and red sandstone). The comparison results show that the established model can accurately reflect the cyclic loading–unloading behaviours and hysteresis process of brittle rocks, which provide an important reference for studying rock damage and constitutive model for brittle rocks. Highlights: Damage variable characterization of brittle rock containing energy dissipation coefficient is deduced using the linear energy dissipation law. By introducing a power functional correction coefficient of strain, a novel staged cyclic damage constitutive model is established through iterative relationship, which reproduces the loading–unloading process of brittle rock. The applicability of established model is verified by comparing \ the test results of brittle rock subjected to different cycles of loading–unloading. [ABSTRACT FROM AUTHOR]

The influence of rock mineral composition and mineral grain size on basic rock strength performance and AE characteristics have been studied, 13 different rocks microstructures are analyzed in an optical microscope thin section using petrographic image analysis, making it possible to determine the mineral composition and mineral texture characteristics of rocks. Then, the basic strength parameters of rock and AE signals generated during fracture propagation were obtained by UCT (uniaxial compression test) and BIT (Brazilian intension test). Finally, the relationship between basic strength parameters and AE characteristics of rock with mineral composition and grain size was analyzed. The results showed that different mineral constituents have significant effects on rock strength. The positive influence of plagioclase content on igneous strength was obtained. Sedimentary rocks strength increases initially and then decreases with the increase of plagioclase content. Besides, with the increase in quartz and K-feldspar content, the strength of the rock was weakened obviously. It is also found that the greater the dimensional deviation of mineral grain, the greater the strength of the rock. The strength of igneous rocks was inversely proportional to the mineral grain size, but there is no correlation between the sedimentary rocks strength and the mineral grain size. Furthermore, the tension–shear crack propagation of rock can effectively distinguish by judging that the data set of the AF–RA density graph was nearby the AF axis or RA axis and the peak frequency data sets of below 100 kHz or more than. Alterations in the rock nature are the main key reasons for the differences between AE hit rate, AE count rate, AE energy, and cumulative energy. The plagioclase content and grain size play a decisive role in AE signal characteristics and failure mode. Highlights: The relationship between basic strength parameters (i.e., σc, σt and E) and AE characteristics of rock with mineral composition and grain size was analyzed with 13 different rocks. The positive influence of plagioclase content on igneous strength was obtained. Sedimentary rocks strength increases initially and then decreases with the increasing plagioclase content. The strength of igneous rocks was inversely proportional to the mineral grain size, but there is no correlation between the sedimentary rocks strength and the mineral grain size. The AF–RA nuclear density map and AE peak frequency can effectively distinguish the shear and tensile failure of rocks The plagioclase content and grain size play a decisive role in AE signal characteristics and failure mode. [ABSTRACT FROM AUTHOR]

In deep mining and excavation of tunnels with high geothermal, the surrounding rock is not only subjected to high ground stress but also subjected to high temperature. Temperature will change mechanical characteristics and energy storage capacity of rocks, as well as increase the destructiveness and randomness of rockburst. To reveal the mechanism of high-temperature strain burst in deep rock, the rockburst tests from uniaxial compression to three-dimensional compression were reviewed, and the research results of the minimum principal stress rapid unloading, true-triaxial loading with one free face, and dynamic disturbance triggered pre-heated granite rockburst simulation tests were focused on. According to the occurrence state of country rock for deep high-temperature and stress state in the whole process during excavation, six development directions for high-temperature strain rockburst simulation tests were proposed: (1) constructing the damage constitutive models of high-temperature rocks according to linear energy dissipation law; (2) developing the true triaxial rockburst simulation testing system accomplishing the function of "real-time high temperature + unloading + dynamic disturbance"; (3) considering the true triaxial rockburst simulation test after microwave irradiation; (4) developing the real-time high-temperature rockburst simulation testing device for large-size specimens and internal unloading; (5) focusing on the energy actuating mechanism for deep high-temperature rock failure via rockburst simulation tests; and (6) implementing the three-dimensional rockburst simulation test on the basis of deep in situ coring. [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]

The quantitative determination of crack damage stress is of significant importance for investigating rock deformation and failure. In this study, a new quantitative method for determining the crack damage stress of rock materials was proposed based on acoustic emission (AE) signal detection. Ten rock materials were subjected to uniaxial compression testing, during which real-time AE signals and axial, lateral, and volumetric strain values were recorded. The AE cumulative count curves of the different rock materials were calculated based on the AE counts. These curves were divided into three stages as the axial stress increased: the slowly increasing stage, the steady stage, and the sharply increasing stage. The point where the sharply increasing stage began was defined as the PR point and could be accurately determined based on a fitting optimization method. Then, the damage stress was calculated by using the crack volumetric strain model. Finally, the relationship between the R stress (axial stress corresponding to the PR point) and the crack damage stress was investigated for the ten rock materials. The results showed that there was a good correspondence between the R stress and the crack damage stress. Hence, the R stress can be used to determine the crack damage stress and the starting point of the unstable crack growth stage in rock materials. [ABSTRACT FROM AUTHOR]

This special issue compiles seventy-six original papers covering the following topics: mechanical properties and fracture behavior of deep rock, rock failure behavior under multifield coupling and multiscale systems, dynamic failure characteristics and constitutive properties of deep rock, mechanical behaviours of rock fractures during shearing, energy evolution characteristics during rock fracturing, experimental or numerical simulation of rockbursts, failure mechanism of spalling in deep tunnels, rock and structure response under blast and impulsive loading, and prevention and control of disasters in deep rock engineering. G. Deng et al. numerically simulate the evolution of mining-induced fracture network in a coal seam and its overburden under the top coal caving method. Q. Ma et al. give the dynamic behavior and energy evolution characteristic of deep roadway sandstone containing weakly filled joint at various angles. [Extracted from the article]

The processes of deformation and failure in rocks are unavoidably accompanied by the absorption, storage, dissipation, and release of energy. To explore energy allocation during rock shear fracturing, two series of single loading and unloading preset angle shear tests at inclined angles of 60° and 50° were performed on red sandstone and granite by varying the experimental unloading level. The area integral approach was employed to interpret the load–displacement responses of the rock specimens via calculation of the energy parameters (referring to the external input energy, internal elastic energy and internal dissipation energy). The interpretations of the results revealed that the increase in the experimental unloading level nonlinearly increases the internal elastic energy, internal dissipation energy and external input energy; these relationships can be described by quadratic functions. It was also realized that under different experimental unloading levels, not only the internal elastic energy but also the internal dissipation energy is closely proportional to the external input energy. The proportional energy relationship can be used to quantify the internal elastic energy and internal dissipation energy at any expected experimental unloading levels, and a real-time calculation model for the internal elastic energy and internal dissipation energy in the pre-peak duration (including the peak point) was introduced. Meanwhile, an invariable feature for the ultimate internal elastic index Wed (the ratio of ultimate internal elastic energy to peak internal dissipation energy) was captured via quantitative analysis. Additionally, the energy allocation manner and transfer mechanisms of rocks bearing varied loading forms (including uniaxial compression, Brazilian splitting, point load, semicircular bending, and preset angle shear) were also comprehensively compared considering three basic rock fracture modes: the tensile, shear, and hybrid failure (mixed tensile-shear) modes. Thus, the proportional distribution patterns of internal elastic energy and internal dissipation energy or the linear correlations among the three energy parameters can be universally observed during the failure of homogeneous rocks, despite distinct loading forms under one-dimensional stress conditions. [ABSTRACT FROM AUTHOR]

Keywords: Preset angle shear; Direct shear; Triaxial compression; Cohesion; Internal friction angle; Shear strength parameters EN Preset angle shear Direct shear Triaxial compression Cohesion Internal friction angle Shear strength parameters 2505 2519 15 05/08/20 20200501 NES 200501 Introduction Estimation of shear strength parameters, which are generally known as the cohesion and internal friction angle ( I c i and I i ) in the Mohr-Coulomb (M-C) strength criterion, plays an essential role in rock mechanics and rock engineering (Zhao [34]; Kahraman et al. [14]; Alejano and Carranzatorres [1]; Gong et al. [7]). 6 Shear strength parameters of a marble and b red sandstone obtained by three different testing methods Comparison of Testing Methods for Rock Shear Strength Parameters Hereinbefore, data deviations were observed for marble and red sandstone under the three testing conditions. [Extracted from the article]

Identifying Crack Compaction and Crack Damage Stress Thresholds of Rock Using Load-Unload Response Ratio (LURR)Theory To investigate the LURR value and stress thresholds of different rocks in USLUC conditions, ten rocks (the names and origins of ten rocks are shown in Table 1) were collected and used in this study. 4 Stress-strain curves in the uniaxial step load-unload compression test: a, b axial stress-strain curves of ten rocks; c-e axial stress-lateral strain curves, axial stress-volumetric strain curves, and volumetric strain-axial strain curves of nine rocks 6 Axial stress-strain curves, axial stress-volumetric strain curves, LURR curves, and LURR curves for ten rocks. [Extracted from the article]

To investigate the energy evolution characteristics of rock materials under uniaxial compression, the single-cyclic loading–unloading uniaxial compression tests of four rock materials (Qingshan granite, Yellow sandstone, Longdong limestone and Black sandstone) were conducted under five unloading stress levels. The stress–strain curves and failure characteristics of rock specimens under the single-cyclic loading–unloading uniaxial compression tests basically corresponded with those of under uniaxial compression, which indicates that single-cyclic loading–unloading has minimal effects on the variations in the loading–deformation response of rocks. The input energy density, elastic energy density and dissipated energy density of four rocks under five unloading stress levels were calculated using the graphical integration method, and variation characteristics of those three energy density parameters with different unloading stress levels were explored. The results show that all three energy density parameters above increased nonlinearly with increasing unloading stress level as quadratic polynomial functions. Meanwhile, both the elastic and dissipated energy density increased linearly when the input energy density increased, and the linear energy storage and dissipation laws for rock materials were observed. Furthermore, a linear relationship between the dissipated and elastic energy density was also proposed. Using the linear energy storage or dissipation law, the elastic and dissipated energy density at any stress levels can be calculated, and the internal elastic (or dissipated) energy density at peak compressive strength (the peak elastic and dissipated energy density for short) can be obtained. The ratio of the elastic energy density to dissipated energy density with increasing input energy density was investigated using a new method, and the results show that this ratio tends to be constant at the peak compressive strength of rock specimens. [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]

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]

A series of experimental tests were conducted to investigate the effects of loading rate on the tensile strength of sandstone by using four test methods, including a direct tensile method and three typical Brazilian disc methods (plate loading, circular arc loading, and strip loading). The loading rates used in these tests varied from 10−2 MPa/s to 100 MPa/s. The results show that the rate effects are clear for these test methods, and the tensile strength of sandstone will increase linearly with the logarithm of the loading rate. At the same loading rate, it is found that the tensile strengths of the sandstone specimens under plate loading and arc loading are relatively similar and are much greater than the direct tensile strength, while the tensile strength under strip loading is less than the direct strength. A comprehensive comparison suggested that the strip loading method can be adopted for the Brazilian disc test, while the obtained strength should be modified with a coefficient of 1.37 to obtain the direct tensile strength. [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]

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]

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]