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Thermal–mechanical damage is an important problem threatening the safety of deep rock engineering. In this paper, the effects of coupling damage on the deformation and failure characteristics of rock mass were studied via cyclic loading damage, thermal damage and uniaxial compression acoustic emission (AE) tests, and the microscopic fracture process of the damaged rock mass was numerically simulated. Results showed that during heat treatment, the colour of the sample changed significantly, the mass, the P-wave velocity and the number of mineral species decreased. The peak strength and elastic modulus reach their maximum values at 600 °C and 300 °C, respectively, exhibiting a trend of initial increase and subsequent decrease. The rapid growth period of AE activity increased noticeably with increasing temperature, and the effect of energy accumulation became more significant at higher peak strengths. The failure mode was influenced primarily by the cyclic loading amplitude. In addition, an increase in the stress or temperature after crack initiation leads to a sharp increase in the damage within the rock. Temperature had a more significant effect on the generation of damage than stress. Stress-induced microcracks were concentrated in the weakly bonded particles, whilst temperature-induced microcracks were concentrated in the strongly bonded particles. Highlights: Temperature had a significant effect on the mineral composition of rock mass. The rapid growth period of AE activity was prolonged with the temperature increase. The failure mode of rock mass was mainly influenced by cyclic loading amplitude. Temperature-induced microcracks were concentrated in strongly bonded particles. [ABSTRACT FROM AUTHOR]

China is renowned for its extensive underground engineering projects and the complex geological and hydrological conditions it faces. Grouting treatment technology is widely employed in deep-buried mines and tunnels, where grouting parameters such as materials, pressure, volume, and hole arrangement significantly impact the effectiveness of grouting. This review paper comprehensively examines current research on grouting materials, theories, experiments, and numerical simulations. It summarizes the various factors that must be considered during the grouting process of fissures and explores the diffusion mechanisms of grout under their influence. Furthermore, further research is needed on the mechanisms and treatment methods for poor grouting in rock masses, the distribution patterns of fissures, optimization methods for grouting parameters, and grout quality assessment techniques. Future research should focus on developing more efficient experimental methods with higher accuracy levels while advancing grouting technologies. Establishing comprehensive and accurate rock mass models along with improving monitoring capabilities are also crucial aspects to consider. Therefore, studying the diffusion mechanisms of grout in fissured rock masses is of significant importance for the practical operation of underground engineering projects. [ABSTRACT FROM AUTHOR]

Due to the presence of natural joints and weak interlayer interfaces in the rock mass, the rock mass will be damaged or even deformed to high degree under the action of dynamic loads such as strong seismic activity, resulting in significant engineering safety accidents and casualties. In light of the aforementioned dynamic issues with rock discontinuities, complete rock samples and interface rock samples containing cemented material (gypsum) underwent a series of SHPB impact compressive and splitting tensile tests. To investigate the dynamic properties and change laws of rocks, theoretical analysis, high-speed camera systems, and "binary method" fracture extraction technology were employed. It was concluded that the peak strength of impact tension and impact compression of the samples increased with the increase of strain rate in a power function relationship. The cemented material (gypsum) interface causes stress wave and energy dissipation to be attenuated. When compared to an intact rock sample, the interface causes the number, area, and transmission coefficient of the cracks to decrease, preventing further crack development. However, the initial position and development direction of the crack and the overall stress loading of the sample are not affected. When the energy input is too much, the rock crack gradually changes from peritectic to transgranular, showing that the dissipative energy increases and reaches the peak strength. The findings can serve as a guide and a point of reference for major projects involving joined rock mass and broken rock mass in terms of safety design and operation. Highlights: Peak strength of the rock sample increases as a power function of strain rate. Strain rate does not affect the initial position and development direction of the crack. The interface decreases the number, area, and transmission coefficient of the cracks. Cemented interface causes the stress wave and energy dissipation to be attenuated. Cracks will change from peritectic to transgranular as the input energy is too great. [ABSTRACT FROM AUTHOR]

A catastrophic landslide occurred in Xinmo village, Maoxian County, China, on June 24, 2016. The landslide destroyed the whole village and caused 10 deaths, and 73 people were reported to be missing. In this study, the contributions of historical earthquakes to rock damage and progressive deformation are the main focus. A detailed field investigation revealed that historical earthquakes seriously damaged the rock mass in the source area, and there were five macrocracks in this area before the landslide. The landslide was initiated along the cracks and foliation plane of the phyllite under long-term gravity. The rock damage induced by historical earthquakes was tested by uniaxial cyclic loading tests. The results indicate that the damage in the phyllite induced by earthquakes is significantly greater than that to slate and metamorphic sandstone. Moreover, the static loading after earthquakes can trigger the long-term deformation of phyllite, and it is accelerated with the incremental damage in the phyllite. Finally, catastrophic failure was triggered when the phyllite was in a significantly damaged state. As a result, the joint influence of historical earthquakes and long-term gravity loading contributed to the progressive deformation of the Xinmo landslide. [ABSTRACT FROM AUTHOR]

Summary: A FORTRAN program, consistent with the commercially available finite element (FE) code ABAQUS, is developed based on a three‐dimensional (3D) linear elastic brittle damage constitutive model with two damage criteria. To consider the heterogeneity of rock, the developed FORTRAN program is used to set the stiffness and strength properties of each element of the FE model following a Weibull distribution function. The reliability of the program is assessed against available experimental results for granite cylindrical specimens with a throughgoing, flat and inclined fissure. The calibration procedure of the material parameters is explained in detail, and it is shown that the compressive to tensile strength ratio can have a substantial influence on the failure response of the specimens. Numerical simulations are conducted for models with different levels of heterogeneity. The results show a smaller load bearing capacity for models with less homogeneity, representing gradual coalescence of fully damaged elements forming throughout the models during loading. The maximum load bearing capacity is studied for various combinations of inclination angles of two centrally aligned, throughgoing and flat fissures of equal length embedded in cylindrical models under uniaxial and multiaxial loading conditions. The key role of the compressive to tensile strength ratio is highlighted by repeating certain simulations with a lower compressive to tensile strength ratio. It is proven that the peak loads of the rock models with sufficiently small compressive to tensile strength ratios containing two throughgoing fissures of equal length are similar, provided that the minimum inclination angles of the models are the same. The results are presented and discussed with respect to the existing experimental findings in the literature, suggesting that the numerical model applied in this study can provide useful insight into the failure behaviour of rock‐like materials. [ABSTRACT FROM AUTHOR]

To investigate the permeability changes and the mechanisms of fractured rock under dynamic and static stresses produced by earthquakes, permeability experiments on fractured rock with rough surfaces under axial dynamic and static stresses were conducted on the MTS815 Rock Mechanics Testing System. Surface asperity was investigated by scanning the specimen surfaces before and after testing. The results show that the roughness of fracture surface has a great influence on the permeability when the axial displacement is not enough to cause the fracture rock to slip. Moreover, the rougher fracture surface leads to severer surface damage as indicated by the more gouge productions. The accumulation of gouge materials on larger roughness fracture surfaces causes a slow drop in permeability. The fracture surfaces experience larger degradations, but it has small weights of gouge materials on fracture surface after testing under axial dynamic stress. The reason is that the gouge material transport and mobilization tend to occur in process of dynamic loading. Therefore, the permeability drops of axial dynamic stress are larger than those of axial static stress. [ABSTRACT FROM AUTHOR]

The original discontinuous deformation and displacement (DDD) method is greatly refined using the statistical damage theory and contact mechanics. Next, a novel coupled method is proposed to model the continuous to discontinuous failure process of rocks. By hybridizing the finite element method (FEM) and discontinuous deformation analysis (DDA) method, the proposed method inherits the advantages of both and is able to provide a complete and unified description of rock deformation, crack initiation and propagation, and rock body translation, rotation and interaction. Moreover, to improve the deformation results and refine the stress distribution within the model blocks, finite elements are introduced into the blocks. The ability of an intact block to fracture is included as well, i.e., the deformable blocks that contain several finite elements may split into smaller blocks if the strength criteria are satisfied continuously. The boundaries of damaged elements represent newly formed joints, and sliding and opening may occur along these joints, i.e., mechanical interaction is allowed between adjacent blocks. The correctness and validity of the proposed method are verified through a series of benchmark tests. The simulated results are consistent with the analytical solutions, previous studies and experimental observations. Overall, the coupled method is an effective and reliable approach for modelling the entire rock failure process with satisfactory accuracy and shows considerable potential in geotechnical engineering. [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]

High geostresses and stress gradients are the predominant stress conditions in deep excavation-disturbed rock masses. The aim of this study is to determine the triaxial compressive strength properties of hollow cylindrical granite specimens under a radially non-uniform confining stress field with different radial stress gradients determined by coupled external and internal confining stresses. Triaxial compression testing of hollow cylindrical rock specimens was performed to investigate the influence of the radial stress gradient, external confining stress and specimen length-to-diameter (L/D) ratio on the triaxial compressive strength. The experimental results and regressed failure criteria indicate that the triaxial compressive strengths of the hollow cylindrical granite specimens increase with the external confining stresses, but decrease with an increase in the radial stress gradients. The calculated goodness of fit (R2) and root-mean-squared error suggest that the nonlinear failure criterion based on the Hoek-Brown model is more accurate than the linear failure criterion based on the Mohr-Coulomb model for determining the influences of the external confining stress and radial stress gradient on the triaxial compressive strength. In addition, the triaxial compressive strength increases with a decreasing L/D ratio due to the strengthening end effect of the hollow cylindrical granite specimens and the change in the failure pattern of these specimens from shear to slabbing. [ABSTRACT FROM AUTHOR]

• A new multi-crack initiation criterion of anisotropic rock is established. • Effects of crack relative positions and sedimentary angle on maximum SIF ratio is studied. • Two collinear cracks are always initiated at inner-tips in Mode I (α 1 ≥ 28°) or in Mode II (α 1 < 28°). • Mode II fracture occurs only when the two cracks both have small inclination angles. • The new criterion is verified by uniaxial compression tests of double-cracked shale specimens. Multi-crack initiation prediction of anisotropic rock is very important for designing fracture-networks and improving efficiency of shale-gas exploitation. Current literature is mainly focused on multi-crack interacting stress intense factors (SIFs) but less on multi-crack initiation criterion for anisotropic rock. In this study, a new multi-crack initiation criterion of anisotropic rock is established based on the maximum ratio of Mode I and Mode II SIF to its fracture toughness on the critical crack plane (θ IC or θ IIC), K I M θ IC / K IC M θ IC and K II M θ IIC / K IIC M θ IIC , in order to predict the multi-crack initiation parameters (crack initiation tip, angle and mode). Effects of crack relative position (α 1 , γ , D h and D s) and sedimentary angle (β) on K I M θ IC / K IC M θ IC and K II M θ IIC / K IIC M θ IIC are analyzed for anisotropic rock. Research results show that under uniaxial compression stress, two collinear cracks are always initiated at two inner-tips simultaneously in Mode I (with large crack incline angle, e.g., α 1 ≥ 28°) or in Mode II (with small crack incline angle, e.g., α 1 < 28°). Two vertical cracks (α 1 = 45°, α 2 = 135°, with extension line of one crack passing through the center of another crack) are initiated at only one crack-tip for any sedimentary plane angle (β = 0°∼180°) in Mode I. Two arbitrary cracks are initiated at one crack-tip in Mode II only when the absolute value of two-crack incline angles with respect to horizontal direction are both small (e.g., |α 1 |=10°<28°, |α 2 |=20°<28°). Theoretical prediction results are in good agreement with uniaxial-compression test results of shale and sandstone plates with two arbitrary cracks. The new multi-crack initiation criterion can be further developed to predict the whole multi-crack propagation process of anisotropic rock. It provides a theoretical basis for safety assessment of rock engineering and cracking-network formation of shale-gas hydraulic exploitation. [ABSTRACT FROM AUTHOR]

A three-dimensional (3D) numerical model for polymer grouting in rock mass fractures based on reaction kinetics theory is established in this article. The model integrates polymer reaction kinetics, compressible Newtonian fluid control, the energy balance accounting for foaming agent evaporation, and slurry density and viscosity models. A 3D rock fracture model simulates grouting in complex fractures and is validated by conventional viscosity simulations. In this study, the impact of fracture medium parameters on polymer diffusion is assessed, and slurry flow, pressure, viscosity, and density distributions during grouting are predicted. The research results indicated that (1) wider fractures reduce the overall slurry viscosity, rough fractures yield an uneven viscosity distribution, and smooth fractures (Joint Roughness Coefficient, JRC = 0) exhibit symmetrical viscosity. An increased fracture inclination boosts the slurry viscosity and reduces the reaction time. (2) Compared with viscosity, slurry density inversely trends with distance. (3) Larger fractures exhibit lower overall slurry diffusion pressures, which decrease with distance. Rough fractures experience higher pressure and fluctuations. A greater fracture inclination increases the overall diffusion pressure, with a monotonic increase at 60° and a parabolic distribution at 0°, peaking at the grouting port. [ABSTRACT FROM AUTHOR]

• True Mode II fracture toughness (K IIC) of anisotropic rock is measured by shear-box test. • Shear-box test is proved to be an effective method for measuring K IIC of anisotropic rock at any anisotropy orientation angle. • A new physical factors Y I0 and Y II0 are proposed to calculate K I0 and K II0 for shear-box specimen. • When θ = 0–10°, K Iθ is negative, which promotes occurrence of Mode II fracture. • The K IIC of anisotropic shale is increased monotonously as β (β = 0–90°) increased. Fracture toughness of anisotropic shale is an important parameter in shale-gas exploitation technology. Currently available literature is mainly focused on Mode I fracture toughness (K IC) of anisotropic rock under tensile loading. Although there are the cracked straight through Brazilian disk and crack ring disk used to determine Mode II fracture toughness (K IIC) of anisotropic rock under pure shear stress, their fracture trajectories are deviated from the original crack plane and cannot be regarded as the true Mode II fracture. In this paper, shear-box test was firstly adopted to measure K IIC of anisotropic shale. New physical factors Y I0 and Y II0 were proposed to describe effects of both the geometry and the material parameters on stress intensity factors (SIFs) of the original crack plane (K I0 and K II0) and to derive the calculation formulae of SIFs on arbitrary (K Iθ and K IIθ). Calculated results show that K IIθ reaches its maximum absolute value at θ = 0–10° where K Iθ is negative, which promotes occurrence of Mode II fracture. The predicted planes of Mode II fracture agree well with the tested fracture trajectories (amostly along the original crack plane). The K IIC is increased with increase of β (β = 0–90°). K IIC is 3–4 times as large as K IC and can be regarded as the true Mode II fracture toughness of anisotropic shale. The shear-box test is an effective method for measuring K IIC of anisotropic rock. [ABSTRACT FROM AUTHOR]

• The variable angle shear tests (VASTs) and direct shear tests (DSTs) were conducted. • Shear strength parameters and acoustic emission (AE) characteristics were analyzed. • Micro-crack component was characterized in the compression-shear failure. • Micro-cracking types were classified in shear-dominated and compression-dominated failures. The surrounding rocks of underground engineering are usually subjected to the coupling of compressive and shear stresses. The variable angle shear tests (VASTs) and direct shear tests (DSTs) are two types of lab tests that can easily achieve different compression-shear stress states. The VASTs and DSTs on cubic rock specimens were conducted to analyze the shear strength parameters and acoustic emission (AE) characteristics during rock failure process. The test results show that it is more reasonable to use the test data in VASTs with shear angles greater than 45° for calculating the shear strength parameters of rocks. The essence of changing the shear angles in VASTs or the normal loads in DSTs is to change the micro-crack component produced in the compression-shear failure mode, in which the shear cracking mainly trigger the AE signals with low peak frequency around 100 kHz, and the tensile cracking mainly induce the AE signals with relatively high peak frequency around 300 kHz. Although the main type of micro-crack produced by shear failure and compression failure are both shear micro-crack, the micro-cracks generated by shear failure have higher efficiency in generating AE energy compared with compression failure, which means that the large-scale micro-crack expansion activities are more intense in shear failure than that in compression failure. [ABSTRACT FROM AUTHOR]

This study further develops the discontinuous deformation and displacement (DDD) method to model the nonlinear deformation and failure behaviors of jointed rock masses. The improved DDD (IDDD) method is then proposed to provide a unified formulation for solving the transformation of rock materials from continuum to discontinuum. By embedding finite elements into model blocks, the deformation ability and stress distribution within blocks are refined, and the fracture of an intact block is permitted. Namely, the rock blocks containing a number of finite elements are deformable and can be broken into several smaller parts during calculation. The edges of damaged elements are new joints and mechanical interaction between neighboring blocks along these new interfaces is therefore allowed. A series of numerical tests are conducted to validate the correctness and effectiveness of the IDDD model in terms of bending deformation, frictional contact, strength characteristics, crack propagation, etc. Meanwhile, the failure mode of a block system and flexural toppling of a jointed rock slope are further analysed. The results are in good agreement with the theoretical analyses, previous studies and experimental data. Overall, the proposed IDDD method is effective and reliable to model the nonlinear mechanical behaviors of jointed rock masses and it has shown particular advantages over the conventional numerical methods. [ABSTRACT FROM AUTHOR]

Rock mass characterization is a key issue to derive mechanical properties of jointed rock mass. In this research, the strength, modulus and post-peak behaviour of rock masses with varied joint orientations and confining stresses are characterized by integrating an empirical geological strength index (GSI)/Hoek-Brown system and a synthetic rock mass (SRM) numerical method. The SRM model is a 3D distinct element model supporting explicit simulation of discontinuous joints and crack propagation in the rock mass. The rock mass structure as denoted by a discrete fracture network is correlated to the GSI system through a quantitative factor of rock block size. The strength and modulus are then derived from the GSI/Hoek-Brown system and used as references for a series of synthetic rock mass models. A comparative study shows that when the predominant joints are steeply inclined in the rock mass subjected to axial loading, the empirical GSI/Hoek-Brown system provides reliable mechanical properties. When the joints are less favorable to shear sliding, the GSI/Hoek-Brown system underestimates the rock mass strength. Cracking characteristics are also investigated in selected SRM models, where few cracks are simulated when inclined joints are inserted in the rock mass and low confining stresses are applied. This research finally shows the method to construct continuum rock mass models using the numerical parameters of the SRM models. The numerical results suggest that the global failure zone is thin when the post-peak is elasto-brittle, and the failure zone is much wider when the post-peak is elasto-plastic. The involvement of the additional failed zones is responsible for the post-peak ductility. The paper is meant to explore new methods to derive high quality geo-data for jointed rock mass. • We used rock block size to establish a linkage between DFN and rock mass structure in the GSI/Hoek-Brown system. • The effects of joints were attributed to the mechanical differences between joints and intact rocks. • The synthetic rock mass model and the GSI/Hoek-Brown system converged well when the joints favour shear sliding. • We built equivalent continuum models for jointed rock mass on the basis of the discrete synthetic rock mass models. [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]