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The presence of discontinuities (e.g. faults, fractures, veins, layering) in crystalline rocks can be challenging for seismic interpretations because the wide range of their size, orientation, and intensity, which controls the mechanical properties of the rock and elastic wave propagation, resulting in equally varying seismic responses at different scales. The geometrical characterisation of adjacent outcrop discontinuity networks allows a better understanding of the nature of the subsurface rocks and aids seismic interpretation. In this study, we characterise the discontinuity network of the Balmuccia peridotite (BP) in the Ivrea–Verbano Zone (IVZ), northwestern Italy. This geological body is the focus of the Drilling the Ivrea–Verbano zonE (DIVE), an international continental scientific drilling project, and two active seismic surveys, SEismic imaging of the Ivrea ZonE (SEIZE) and high-resolution SEIZE (Hi-SEIZE), which aim to resolve the subsurface structure of the DIVE drilling target through high-resolution seismic imaging. For fracture characterisation, we developed two drone-based digital outcrop models (DOMs) at two different resolutions (10−3–10 m and 10−1–103 m), which allowed us to quantitatively characterise the orientation, size, and intensity of the main rock discontinuities. These properties affect the seismic velocity and consequently the interpretation of the seismic data. We found that (i) the outcropping BP discontinuity network is represented by three more sets of fractures with respect to those reported in the literature; (ii) the discontinuity sizes follow a power-law distribution, indicating similarity across scales, and (iii) discontinuity intensity is not uniformly distributed along the outcrop. Our results help to explain the seismic behaviour of the BP detected by the SEIZE survey, suggesting that the low P-wave velocities observed can be related to the discontinuity network, and provide the basic topological parameters (orientation, density, distribution, and aperture) of the fracture network unique to the BP. These, in turn, can be used for interpretation of the Hi-SEIZE seismic survey and forward modelling of the seismic response.

Rock discontinuity has a crucial impact on the deformation and strength of rock masses, and thus, the clustering of discontinuities is a critical aspect of rock mechanics. Traditional clustering methods require initial cluster centers to be specified and involve a multitude of parameter calculations, leading to a complex and cumbersome process. In this paper, a novel clustering approach based on the sparrow search algorithm (SSA) is introduced to overcome these limitations. This method utilizes a sparrow population coding technique and fitness function tailored to the unique characteristics of rock discontinuity orientation data. The SSA is adeptly applied to the clustering of rock joints, and the optimal number of clusters are automatically determined via the silhouette coefficient method. This methodology was tested on artificial datasets and actual discontinuity survey results from the underground powerhouse of the Henan Wuyue Hydropower Station to evaluate its feasibility and efficacy in analyzing rock discontinuities. Comparative data analysis reveals that the proposed method outperforms classic algorithms such as FCM and KPSO in terms of clustering accuracy and stability. The proposed method stands out among various clustering methods of discontinuity orientation for its ability to achieve convergent results without user intervention, demonstrating significant practical utility. [ABSTRACT FROM AUTHOR]

Discontinuities largely influence the mechanical properties of rock joints. However, traditional discontinuity recognition methods often require manual intervention during processing. This paper proposes a new deep-learning-based method for discontinuity recognition using 3D point clouds. A neighborhood PCA-weighted oriented contraction method is proposed to extract point cloud skeletons as discontinuity intersection lines. Then an optimal color mapping (OCM) method is proposed to establish the optimal mapping relationship between 3D normal vectors and RGB, converting 3D point clouds to 2D OCM images for discontinuity segmentation. The convolutional neural network of Mask R-CNN is adopted to segment discontinuities from OCM images. Finally, 3D discontinuities can be generated from discontinuity-segmented OCM images. Forty-two rock slope image sequences and a rock slope point cloud are collected and labeled, generating 4632 OCM images including 430,613 discontinuity planes after data augmentation for training. Three cases of rock slopes and rock tunnel excavation faces are adopted for testing. The average recognition time per 3D point cloud model is approximately 12 s due to the high recognition efficiency of Mask R-CNN for 2D images. The results show the proposed method can recognize discontinuities close to manual judgements with high accuracy, good robustness to point cloud density variations, and good adaptability to different rock engineering scenarios. Highlights: An NPW-OC method is proposed to extract point cloud skeletons. An OCM method is proposed to assign 3D normal vectors with optimal RGB. OCM images are generated to assign discontinuities with different and uniform color. Deep-learning-based method is used for intelligent recognition of discontinuities. Conversion of discontinuity recognition from 3D point clouds to 2D OCM images. The results show good efficiency, accuracy, robustness, and adaptability. [ABSTRACT FROM AUTHOR]

The rock discontinuity orientation has a significant impact on the engineering stability. This paper proposes a semi-automatic method for identifying the orientation of rock discontinuities based on 3D point clouds. First, the normal of each point is calculated by k-nearest neighbors (KNN) and least squares (LS) algorithms. Then the point normal is regarded as the input parameter and the number of joint sets can be determined by density-based spatial clustering of applications with noise (DBSCAN). For each joint set, the point coordinate is considered as the input parameter, and DBSCAN is employed again to obtain the individual joint plane. Finally, the orientation of each joint plane is computed by LS algorithm, and the weighted mean of the orientation based on size is taken as the orientation of each joint set. The correctness of the method is verified by two standard geometric solid objects, and it is applied in real slope engineering to identify the orientation of rock discontinuities. The results are consistent with those observed in the field, and the differences with other methods are discussed. In comparison with the open source procedure DSE and the commercial software ShapeMetrix, the proposed method shows remarkable advantages in terms of precision and accuracy, and can preserve more point cloud information. This study extends the method of semi-automatic identification of rock discontinuities and can provide a reference for similar engineering. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) roughness of discontinuity affects the quality of the rock mass, but 3D roughness is hard to be measured due to that the discontinuity is invisible in the engineering. Two-dimensional (2D) roughness can be calculated from the visible traces, but it is difficult to obtain enough quantity of the traces to directly derive 3D roughness during the tunnel excavation. In this study, a new method using Bayesian theory is proposed to derive 3D roughness from the low quantity of 2D roughness samples. For more accurately calculating 3D roughness, a new regression formula of 2D roughness is established firstly based on wavelet analysis. The new JRC3D prediction model based on Bayesian theory is then developed, and Markov chain Monte Carlo (MCMC) sampling is adopted to process JRC3D prediction model. The discontinuity sample collected from the literature is used to verify the proposed method. Twenty groups with the sampling size of 2, 3, 4, and 5 of each group are randomly sampled from JRC2D values of 170 profiles of the discontinuity, respectively. The research results indicate that 100%, 90%, 85%, and 60% predicting JRC3D of the sample groups corresponding to the sampling size of 5, 4, 3, and 2 fall into the tolerance interval [JRCtrue–1, JRCtrue + 1]. It is validated that the sampling size of 5 is enough for predicting JRC3D. The sensitivities of sampling results are then analyzed on the influencing factors, which are the correlation function, the prior distribution, and the prior information. The discontinuity across the excavation face at ZK78 + 67.5 of Daxiagu tunnel is taken as the tunnel engineering application, and the results further verify that the predicting JRC3D with the sampling size of 5 is generally in good agreement with JRC3D true values.

The mechanical and hydraulic properties of rock discontinuity are anisotropic, and the main reason resides in the anisotropy of surface morphology. This study delved systematically into the anisotropic characteristics of surface morphology and its evaluation method. Among hundreds of morphology parameters, the most representative one was selected utilizing mathematical statistics and correlation analysis. Through morphology analysis of eight discontinuity specimens, the maximum deviation of roughness in each direction was proposed as an anisotropy evaluation index, serving as a bridge to establish the relationship between the anisotropic characteristics of surface morphology and shear strength. In-depth research was conducted to discuss the distribution range of controlling factors of shear strength and explore the influence of anisotropy degree and controlling factors on the prediction deviation of shear strength. Based on the aforementioned results, an anisotropy evaluation method was put forth to divide the anisotropy degree of rock discontinuity into four levels, and the prediction deviation of shear strength at each level was described in a quantitative manner. The novelty of this study is listed as follows. The existing literature lacks research on quantitative evaluation methods of anisotropic characteristics, and this study would compensate for this deficiency; this method has a valid theoretical foundation and is capable of simultaneously determining the anisotropic characteristics of morphology and shear strength; the methods concerning how to simplify surface morphology as isotropy and embed discontinuity roughness into a numerical algorithm have been proposed, respectively. These findings would provide theoretical support for the deformation control and stability analysis of rock mass in engineering. [ABSTRACT FROM AUTHOR]

The use of unmanned aerial vehicles (UAVs) for photogrammetry allows the rapid acquisition of high-resolution images of geological masses in complex landforms. However, effective analysis of the acquired image information remains a key research issue. At K158 + 837 on the Chongqing-Huaihua Railway, Baima jointed rock masses were reconstructed with high accuracy using UAV close-range photogrammetry technology, and rock discontinuities were extracted from the projected image. The proposed modeling algorithm for jointed rock masses enables the preprocessing of two-dimensional jointed rock mass slopes. Numerical simulations using the disk-based discontinuous deformation analysis method show that the discontinuity network formed by initial cutting significantly affects the subsequent crack development. Meanwhile, simulation results under different scenarios indicate the importance of the pre-reinforcement measures applied to unstable rock masses. The workflow developed based on these results can serve as a reference for the comprehensive acquisition, recognition and numerical modeling analysis of similar jointed rock masses.

The geological strength index (GSI) plays an important role in the quality evaluation and stability analysis of rock mass. Traditional methods for quantitatively estimating GSI are often subjective, time-consuming, and dangerous. This paper proposed a method for rapid and quantitative GSI estimate using 3D point clouds, which can be generated through non-contact measurement methods such as photogrammetry and Light Detection and Ranging (LiDAR). The overall methodology is as follows: (1) point clouds were acquired using a terrestrial laser scanner; (2) discontinuities were identified through artificial neural networks (ANN) and density-based spatial clustering of applications with noise (DBSCAN); (3) geometric information was extracted for the detected discontinuities; (4) GSI was estimated according to the detection and characterization of discontinuities. The proposed method was used for the Yujiashan road cut to calculate the GSI and the GSI partitioning was performed simultaneously. Three sets of discontinuity were detected in the Yujiashan road cut, and Structure Rating (SR) and Surface Conditions Point Clouds Rating (SCPC) were calculated to be 13.2 and 25, respectively. Correspondingly, the GSI was estimated to be 32, which was consistent with the results of the in-situ evaluation (rating 25–40). Furthermore, the Yujiashan road cut was divided into 17 segments, and the effect of sampling size on the GSI calculation was discussed. The application results show that the GSI of the rock mass can be obtained objectively and efficiently through 3D point clouds, which can be used as a potential alternative to the traditional method for GSI estimation. Article Highlights: A semi-automatic method was developed to determine the rockmass GSI using laser scanning. Discontinuities were detected from the 3D point cloud using a machine-learning algorithm. Four geometric parameters of detected discontinuities were extracted for GSI calculation. The scale effect of GSI calculation was investigated using the proposed partitioning algorithm. GSI estimation obtained from the proposed method matched the traditional manual surveys. [ABSTRACT FROM AUTHOR]

In tunnel construction, predicting the roughness of discontinuity is significant for preventing the collapse of the excavation face. However, currently, we are unable to use a parameter with invariant properties to quantify and predict the roughness of discontinuity. Fractal dimension D is one such parameter that be used to characterize the roughness of discontinuity. The study proposes a new method to predict the roughness of discontinuity from the fractal dimension D of outcrops. The measurement method of the coordinates of outcrops is firstly summarized, and the most suitable method of calculating fractal dimension D is then provided. For characterizing the spatial variability of fractal dimension D, the random field of fractal dimension D is discretized, and the prediction model is then established based on Bayesian theory. The proposed method is applied to one tunnel for predicting the roughness of discontinuity, and the results indicate that the relative errors of prediction are less than 1.5%. The sensitivities of correlation function and discontinuity size are analyzed. It is found that the different correlation functions have no obvious effect on the prediction results, and the proposed method is well applied to relatively large sizes of discontinuity. [ABSTRACT FROM AUTHOR]

Large-scale discontinuities can significantly affect the mechanical properties of rock masses. However, the tensile behavior of rock discontinuities is often less investigated. To study the statistical characteristics of failure strength and fracture characteristics of rock discontinuities, Brazilian disc tests were conducted on limestone specimens with a single natural discontinuity at different load-discontinuity angles (β). In this study, β=0° and β=90° correspond to the discontinuity parallel and perpendicular to loading direction, respectively. The results show that Brazilian failure strength (BFS) can reasonably represent the tensile strength of rock with discontinuities, by comparing the BFS and tensile stress in the disc center at peak force. The two-parameter Weibull distribution can capture the statistical BFS characteristics of rock discontinuities parallel to loading direction (β=0°) and at different load-discontinuity angles (β≠0°). All specimens with discontinuity at different load-discontinuity angles show more plastic deformational behaviour than intact rock specimen. With increasing β, the mean BFS of limestone with discontinuity increases before reaching a plateau at β=45°. The single plane of weakness theory best explains the BFS of fractured limestone with β. Only a specific segment of pre-existing rock discontinuity could affect the fracture process. When β=0°, interfacial cracks and alternative cracks formed. When β≠0°, mixed failure mode with shear and tensile failure occurred, particularly when β=30° and β=60°. The findings can contribute to better understanding the failure and fracture characteristics of rock with discontinuities, particularly the interaction of pre-existing discontinuities with stress-induced fracturing.

The use of unmanned aerial vehicles (UAVs) for photogrammetry allows the rapid acquisition of high-resolution images of geological masses in complex landforms. However, effective analysis of the acquired image information remains a key research issue. At K158 + 837 on the Chongqing-Huaihua Railway, Baima jointed rock masses were reconstructed with high accuracy using UAV close-range photogrammetry technology, and rock discontinuities were extracted from the projected image. The proposed modeling algorithm for jointed rock masses enables the preprocessing of two-dimensional jointed rock mass slopes. Numerical simulations using the disk-based discontinuous deformation analysis method show that the discontinuity network formed by initial cutting significantly affects the subsequent crack development. Meanwhile, simulation results under different scenarios indicate the importance of the pre-reinforcement measures applied to unstable rock masses. The workflow developed based on these results can serve as a reference for the comprehensive acquisition, recognition and numerical modeling analysis of similar jointed rock masses. [ABSTRACT FROM AUTHOR]

Digital geological survey methods have become supplementary approaches for traditional geological survey in the last two decades. In this paper, the Unmanned Aerial Vehicle (UAV)‐based photogrammetry technology is used to obtain the 3D point cloud model of rock outcrops. The clustering algorithm is used to automatically identify the rock discontinuity parameters. However, the obtained 3D point cloud model with high resolution often has a huge point data which usually poses a great challenge to the computational efficiency of the automatic identification. In fact, too‐dense point cloud data may not be necessary for cases when the rock mass is relatively intact. The optimal point cloud resolution, which balances the accuracy and efficiency, depends on the degrees of fragmentation of the rock mass under investigation. For a model with the same resolution, large‐size discontinuities may have quite a few redundant point cloud data that are of little use to improve the identification accuracy whereas small‐size discontinuities may not be properly identified due to insufficient number of data points. In this paper, the influence of the degree of fragmentation of rock mass on the identified results was investigated. The uniform grid method was adopted to sparse the 3D point cloud model. The optimal point cloud resolution for different discontinuities was suggested. The applicability and feasibility of the proposed approach were verified via three illustrative examples of typical rock slopes. [ABSTRACT FROM AUTHOR]

Copyright of Chinese Journal of Computational Mechanics / Jisuan Lixue Xuebao is the property of Chinese Journal of Computational Mechanics Editorial Office, Dalian University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

To analyze the shear characteristics and mesoscopic failure mechanism of irregular serrated rock discontinuities, a great deal of interview samples of irregular serrated structures were made by 3D printing technology, and laboratory shear tests were carried out on them under different normal stresses. At the same time, PFC numerical simulation software is used to establish relevant models to study the evolution of microcracks and the distribution characteristics of the force chain on the rock discontinuity during the shear process. The results show that the shear mechanical properties of irregular serrated rock discontinuities are affected by normal stress, undulating angle, and undulating height. The shear strength increases with the increase of normal stress and undulating height, and decreases with the increase of undulating angle. The numerical simulation results show that the irregular structural surface cracks under different undulation angles, which first start at the near force end serration root on both sides and further evolve to the adjacent serrations, while the irregular structural surface cracks under different undulation heights, which first start at the serration root with the lowest height and expand to the adjacent serrations. At the same time, the number of cracks increases with the increase of normal stress and the force chain is mainly distributed near the sawtooth surface. The force chain is more concentrated near the near force end sawtooth and at the tip and root of the rest of the sawtooth. At the same time, the direction of the force chain is approximately perpendicular to the force surface of the sawtooth. The research results are helpful in further understanding the shear mechanical properties and differences of irregular serrated rock discontinuities. [ABSTRACT FROM AUTHOR]

Dynamic disturbance is an important influence factor that causes the sliding instability of rock discontinuity and then triggers strong dynamic geological disasters, such as earthquakes and rock bursts. In order to study the mechanical response and disaster-causing mechanism of rock discontinuity under dynamic disturbance loading, a multifunctional shear test apparatus for rock discontinuity under dynamic disturbance loading (DDST-1800) is independently developed. The test apparatus is composed of normal and shear loading units with quasi-static loading, various normal boundary conditions, dynamic disturbance loading with complex and wide-frequency waveform (0.5−50 Hz), and wide-range shear velocity loading (0.000 1−10 mm /s) functions, pendulum impact system, cyclic shear box unit, data acquisition and control units. This apparatus can conduct the following typical tests by programmed operations: a monotonic or cyclic direct shear test of rock discontinuity under Constant Normal Load (CNL), Constant Normal Stiffness (CNS), Unloading Normal Load (UNL), Variable Normal Stiffness (VNS), and Dynamic Normal Load (DNL) boundary conditions with different shear velocity, superimposed dynamic disturbance shear test of rock discontinuity in both normal and tangential directions of rock discontinuity under pre-set normal and shear stress conditions, velocity and stress step tests, and pendulum hammer-driven impact test of rock discontinuity under pre-set normal and shear stress conditions. Preliminary experimental studies were carried out and the results verified the stability and accuracy of the new apparatus. The research and development of the new apparatus have important scientific significance and engineering value for clarifying the dynamic response characteristics of rock discontinuity under dynamic disturbance loading and deeply understanding the triggering effect and activation instability mechanism of strong dynamic geological disasters such as earthquakes and rock bursts related to rock discontinuity.

Digital photogrammetry is widespread in site investigation of rock discontinuities. However, existing methods often rely on multi-frame synthesis techniques and calibrating references to achieve three-dimensional (3D) modeling and orientation analysis of rock discontinuities, which limits the rapid application of digital photogrammetry techniques onsite. To address this challenge, this paper introduces a hybrid machine learning architecture called OneShot-DisconNet. This method first utilizes deep neural networks trained on the MegaDepth dataset to predict single-layer depth map from monocular images. Subsequently, multi-viewpoint depth fields and textures is reconstructed on the layered depth images (LDIs) using Partial Convolutions (PConv), enabling point cloud modeling of rock surfaces. Additionally, a novel uncertainty-based multi-population genetic algorithm-driven backpropagation (UB-MPGA-BP) neural network is developed based on the MegaDepth dataset to enable true-scale mapping of 3D point clouds without references. Finally, by addressing the limitations of the Density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm in handling outliers and coplanar point clouds, the robustness of the structural analysis results has been enhanced. Case studies in the Beiyi Section of Shilu Iron Mine and the Rockbench database demonstrate that the proposed method is comparable in performance to Lidar-based digital photogrammetry technology, enabling true-scale mapping and orientation analysis of rock discontinuities in various scenarios. [ABSTRACT FROM AUTHOR]

Energy is often dissipated and released in the process of rock deformation and failure. To study the energy evolution of rock discontinuities under cyclic loading and unloading, cement mortar was used as rock material and a CSS-1950 rock biaxial rheological testing machine was used to conduct graded cyclic loading and unloading tests on Barton's standard profile line discontinuities with different joint roughness coefficients (JRCs). According to the deformation characteristics of the rock discontinuity sample, the change of internal energy is calculated and analyzed. The experimental results show that under the same cyclic stress, the samples harden with the increase in the number of cycles. With the increase of cyclic stress, the dissipated energy density of each stage gradually exceeds the elastic energy density and occupies a dominant position and increases rapidly as failure becomes imminent. In the process of increasing the shear stress step-by-step, the elastic energy ratio shows a downward trend, but the dissipated energy is contrary to it. The energy dissipation ratio can be used to characterize the internal damage of the sample under load. In the initial stage of fractional loading, the sample is in the extrusion compaction stage, and the energy dissipation ratio remains quasi-constant; then the fracture develops steadily, the damage inside the sample intensifies, and the energy dissipation ratio increases linearly (albeit at a low rate). When the energy storage limit is reached, the growth rate of energy dissipation ratio increases and changes when the stress level reaches a certain threshold. The increase of the roughness of rock discontinuity samples will improve their energy storage capacity to a certain extent. [ABSTRACT FROM AUTHOR]

In this paper, the shear mechanical behavior of rock discontinuity during direct shear test was studied by using Particle Flow Code. The feasibility of bond-debonding method and smooth-joint contact method to simulate the shear mechanical behavior of planar discontinuity was analyzed. The random distribution of particle size and position introduces some micro-roughness, which causes the non-actual stress concentration during the shear process using bond-debonding method. Particles on both sides of discontinuity are prone to cross the discontinuity during shearing, the unreasonable setting of contact type leads to non-actual stress concentration on the discontinuity with smooth-joint contact method. Therefore, it is proposed to adopt the discrimination method of contact and discontinuity intersection, and the discrimination method of particle groups on both sides of discontinuity, to overcome the disadvantages of smooth-joint contact method. Finally, through the direct shear test simulation of planer discontinuity with the friction angle of 57°under normal stress 3, 5, and 7 MPa, the feasibility of the discrimination method of particle groups on both sides of discontinuity was verified.

In tunnel construction, predicting the roughness of discontinuity is significant for preventing the collapse of the excavation face. However, currently, we are unable to use a parameter with invariant properties to quantify and predict the roughness of discontinuity. Fractal dimension D is one such parameter that be used to characterize the roughness of discontinuity. The study proposes a new method to predict the roughness of discontinuity from the fractal dimension D of outcrops. The measurement method of the coordinates of outcrops is firstly summarized, and the most suitable method of calculating fractal dimension D is then provided. For characterizing the spatial variability of fractal dimension D, the random field of fractal dimension D is discretized, and the prediction model is then established based on Bayesian theory. The proposed method is applied to one tunnel for predicting the roughness of discontinuity, and the results indicate that the relative errors of prediction are less than 1.5%. The sensitivities of correlation function and discontinuity size are analyzed. It is found that the different correlation functions have no obvious effect on the prediction results, and the proposed method is well applied to relatively large sizes of discontinuity.

An automatic recognition of discontinuities in borehole images is a desirable way to overcome the inefficiency and inconsistency inherent in the conventional method of manual annotation. The rough borehole walls and prominent noise failed the application of existing recognition methods to borehole images taken in the coal-measure strata. This paper presents a novel approach to automatically convert coal-measure strata borehole images into identified discontinuity maps. The developed procedure formed an integrated feature representation for the borehole image through combining the color information of image and the textural features generated from multi-channel filtering. Image regions containing discontinuities are then separated from other regions by implementing fuzzy c-means clustering on the acquired feature representation. The identification of discontinuities is finally accomplished by searching for four predefined patterns (named topographic model) on the intensity transection of image regions. The proposed method is proven to be superior in the respects of noise suppression, discontinuity positioning, and recognition completeness.

The mechanism of rock fragmentation around blastholes is of prior importance in the evaluation of drilling and blasting performance in open pit and underground mines. This paper aims to numerically investigate the crack propagation mechanism around a single blasthole using the distinct element method (DEM). In this study, two specimens with different borehole diameters were considered and the effects of the stress waves on their cracking mechanism were simulated. To validate the numerical model, the length of the radial cracks around each blasthole was measured and compared against an analytical fracture mechanics model. The fractured zones around the blasthole were also compared against previous experimental tests and good agreement was observed. The effect of a single discontinuity on the crack propagation mechanism was also studied and it was found that the discontinuity normal stiffness plays a significant role in the fractured zones around the blasthole. For low values of normal stiffness, the discontinuity surface acted as a free surface, and the shock wave was significantly reflected, while at high values of normal stiffness, cracks propagate across the discontinuity surface.

To analyze the shear characteristics and mesoscopic failure mechanism of irregular serrated rock discontinuities, a great deal of interview samples of irregular serrated structures were made by 3D printing technology, and laboratory shear tests were carried out on them under different normal stresses. At the same time, PFC numerical simulation software is used to establish relevant models to study the evolution of microcracks and the distribution characteristics of the force chain on the rock discontinuity during the shear process. The results show that the shear mechanical properties of irregular serrated rock discontinuities are affected by normal stress, undulating angle, and undulating height. The shear strength increases with the increase of normal stress and undulating height, and decreases with the increase of undulating angle. The numerical simulation results show that the irregular structural surface cracks under different undulation angles, which first start at the near force end serration root on both sides and further evolve to the adjacent serrations, while the irregular structural surface cracks under different undulation heights, which first start at the serration root with the lowest height and expand to the adjacent serrations. At the same time, the number of cracks increases with the increase of normal stress and the force chain is mainly distributed near the sawtooth surface. The force chain is more concentrated near the near force end sawtooth and at the tip and root of the rest of the sawtooth. At the same time, the direction of the force chain is approximately perpendicular to the force surface of the sawtooth. The research results are helpful in further understanding the shear mechanical properties and differences of irregular serrated rock discontinuities.

Wave velocity as a nondestructive test is widely used in various fields such as mining, civil, rock mechanics and rock engineering, and dynamic elastic properties of rocks are also determined by wave velocity. Rock mass contains discontinuities with different orientation angles, numbers of sets and frequencies and the impacts of such discontinuity properties on the wave velocity have not yet been systematically investigated. In this study, 20 groups of jointed cylindrical specimens with a wide range of discontinuity orientation angles (0°, 30°, 45°, 60° and 90°), numbers of sets and frequencies were prepared, and their P-wave velocities were measured. P-wave velocity (VP) significantly increased with increasing orientation angle (θ) and decreasing the discontinuity frequency (F). Several relations were applied to analyze the results. Finally, a new empirical relation was developed to predict VP as a function of θ angle using two parameters of P-wave velocity perpendicular to the discontinuity plane (VP0°) and P-wave velocity along the discontinuity direction (VP90°) or anisotropy degree (ε = (VP90° − VP0°)/VP0°) with a good correlation of determination (R2 = 0.933), low RMSE (RMSE = 200.0 m/s) and low CV (CV = 6.7%). A good correlation between VP and F was also achieved for the test results. The intact rock elements in the complex nature of a rock mass are surrounded by discontinuities like a three dimensional puzzle. Treatment of such a rock mass system using cement grouting is an attractive method to improve its quality for various purposes. All 20 groups of jointed cylindrical specimens were grouted by cement. Wave velocities of the grouted specimens were measured after 2 months. VP significantly increased after grouting for all models of jointed specimens, as wave velocity of jointed specimens improved and approached to the wave velocity of the intact rock for most cases. The discontinuity orientation of the specimens had no significant effect on the wave velocities after grouting. But the results showed that VP and VS increased little with increasing orientation angle for jointed specimens consisting only of one set of discontinuity. After grouting the jointed specimens, orientation angle and frequency of discontinuities had also similar effects on the dynamic Young's modulus (E), Poisson's ratio (ν), shear modulus (μ), bulk modulus (K) and Lamé's constant (λ). These results also showed that the dynamic elastic properties of jointed specimens significantly improved using cement grouting. [ABSTRACT FROM AUTHOR]

The sliding of rock blocks along rock discontinuities is caused by accumulated deformation and can result in disasters such as rock bursts and earthquakes. Creep along rock discontinuities leads to the accumulation of sliding deformation and random instability over time. The initial stress and deformation are important factors that strongly influence creep behavior. To investigate the influence of the initial state on the creep behavior of rock discontinuities, shear creep tests with loading–unloading shear pre-stress paths and shear creep tests with different initial stress and deformation states were conducted on artificial rock discontinuity samples prepared according to Barton's standard roughness profiles. The creep behavior characteristics were investigated in detail according to the initial stress and deformation conditions. The results show that the initial stress and deformation states, which are related to crack development and plastic deformation accumulation, strongly influence creep behavior. Larger initial stress and deformation produce lower creep deformation, creep rates, and accelerated velocities. Samples with higher joint roughness coefficients produce more noticeable creep because more asperities provide additional space for creep to occur. A limit curve is also proven to exist, which allows the stress–deformation coordinate system to be divided into stable and unstable regions. When the current stress state of a sample is insufficient to drive the formation of further cracks or deformation in the rock discontinuity, the deformation and stress remain stable and no further creep is observed. [ABSTRACT FROM AUTHOR]

Research on rock slopes has attracted considerable attention in the fields of infrastructure construction and disaster prevention. The search for an efficient and reliable method for discontinuity collection is an important task for various studies of jointed rock slope. Capturing high-precision and complete rock exposure images based on unmanned aerial vehicle (UAV) multi-angle and nap-of-the-object photogrammetry technology is proposed to realize the construction of a millimeter-level 3D point cloud for the high and steep slope. This study develops a new automatic methodology for discontinuity identification and interpretation. A Delaunay triangulation network is created for connecting adjacent points in the point cloud to calculate normal vectors and search for neighbor points efficiently. Combined with the modified region-growing algorithm, an automatic procedure which identifies discontinuities and acquires geometrical parameters efficiently and accurately is established. The proposed approach is applied to the high-steep rock slope on the northern bank of the Sequ Grand Bridge in the Changdu area of Tibet. Compared with the results from the manual survey, the proposed method is more reliable and exhibits high accuracy for discontinuity identification and interpretation on rock exposure. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Central South University is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The deformation process and failure mechanism of rock mass with increased density of initial joints subjected to confined stress state are investigated in this study using discrete element method (DEM). A numerical model of standard size granite samples is developed and validated using experimental data for both intact and jointed rocks. The micro-parameters of the rock material are first determined, and the effects of the rock discontinuity on strength, deformability, stress–strain relationship, and failure modes are then investigated at the macro-scale level. Analyses are also performed to examine the tensile and shear crack distributions, fragmentation characteristics, particle kinematics, and energy dissipation to advance the current understanding of the deformation processes and failure mechanisms of jointed rock masses. The microscopic evolutions in the fabric and force anisotropy during loading and distributions of contact forces provide insights into the influence of increasing initial jointing on the macroscopic deformational behavior of the rock. The results show how the deceleration in the growth of fabric and contact force anisotropies develops and confirms that the increase in initial jointing and the associated changes in microstructure can restrain the development of anisotropy, thereby reducing significantly the strength of the rock samples. [ABSTRACT FROM AUTHOR]

Discontinuities have huge impact on civil and mining engineering. To understand the spatial features of discontinuities, it is common to group them into different sets based on orientation. In this paper, a new algorithm is introduced for the identification of discontinuity sets. The new algorithm is developed by combined fuzzy C-means algorithm with variable length string genetic algorithm. In the new method, the number of discontinuity sets is not the necessary input parameter any more. This method is robust, global optimal and totally automatic. To verify its validity, the new method was firstly applied to an artificial data as well as a published data. For artificial data set, the assignment error rate is only 7.4%. For published data set, only 2 discontinuities are assigned to wrong sets. The results indicate that the new algorithm is better than fuzzy C-means algorithm and comparable with other common methods. Afterwards, the new method was utilized to analyze the orientation data sampled at an underground storage cavern site. The new method determines that the ideal number of sets is 3. The new method provided satisfactory results, which confirm its effectiveness and convenience. [ABSTRACT FROM AUTHOR]

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Shear creep is one of the most important mechanical behaviors of rock discontinuities. The creep mechanism and prediction of starting point of the accelerating creep stage are vital for establishing the creep model and predicting creep failure. In this study, a series of multi-step creep tests are conducted. The three creep stages of shear creep tests are investigated in detail, and a method for predicting the accelerating creep stage is proposed. Distinct nonlinear and local fluctuations caused by cracking are observed in the creep curve. To describe the transition creep stage and steady creep stage, an empirical creep model is established, and the creep characteristics related to the joint roughness coefficient (JRC) and the normal stress are explored in detail using the model's parameters. The creep process can be described as involving the JRC resistance weakening and frictional resistance compensation, and a model also established to describe this process. The frictional resistance cannot compensate for the loss of JRC resistance; consequently, creep failure occurs. The starting point of the accelerating creep stage can be predicted by combining the JRC weakening and frictional mobilization model and the empirical creep model. A new method for determining long-term strength is also proposed based on the relationships between the starting point creep deformation and the shear creep stress. [ABSTRACT FROM AUTHOR]

The shear strength of rock joints is an important property required in order to analyze the stability of rock slopes and tunnels. However, estimation of the shear strength of rock joints for in situ conditions is a complex task due to various influencing factors present in the field. Among these factors, the shear velocity or the shear displacement rate along the rock joints are important parameters which are relatively less studied since their effect is considered to be of second order compared to other factors. However, some recent studies in the literature suggest that shear velocity has a significant influence on the shear strength of rock joints, and hence the shear strength of joints estimated at low shear velocities in laboratories cannot be used under in situ conditions where the possibility of higher shear velocities exist due to the presence of different factors, such as blasting, excavation, and thermal and seismic loads. In this paper, we have addressed these issues in three steps. In the first step, an experimental study on jointed rock specimens is presented to investigate the influence of the displacement rate on the shear strength of rock joints. In the second step, a probabilistic method is developed based on the experimental results and the compiled data from the literature to estimate the in situ shear strength of joints under higher displacement rate conditions, i.e., blasting, excavation, and seismic loads from laboratory-estimated shear strength at the International Society for Rock Mechanics suggested low displacement rates. In the third step, a case study of a Himalayan rock tunnel was used to demonstrate the described approach. It was observed that the shear strength of discontinuities reduced with ncreasing shear velocity and that the rate dependency was higher for low-density rocks and under high confining stress. Further, a considerable effect was observed on the probability of failure of the rock tunnel when the effect of shear velocity was considered in the stability analysis. [ABSTRACT FROM AUTHOR]

Stress relaxation is an important time-dependent behavior of rock mass. To investigate the stress relaxation behavior and the influence of stress history and surface morphology on the stress relaxation of rock discontinuity, iso-stress cyclic relaxation tests and shear stress relaxation tests with loading–unloading stress histories were conducted on artificial rock discontinuity samples and natural rock discontinuity samples. The characteristics of the stress relaxation curve with different stress histories were investigated in detail, and the relationship between the accumulated plastic deformation and relaxation stress was established. Results revealed that the stress history caused the weakening of the stress relaxation ability. The greater the joint roughness coefficient (JRC), the more noticeable is the stress relaxation. Moreover, the stress history presents a greater effect on stress relaxation as the JRC increases. The stress relaxation exhibits a linear relationship with the plastic deformation that occurred before the stress relaxation. It demonstrates that the relaxation stress and plastic deformation are homogenous and that the elastic energy is the power of stress relaxation. The roughness or asperities of rock discontinuity provides the "space" for the stress relaxation. • The greater the joint roughness coefficient (JRC), the more noticeable was the stress relaxation. • The stress history caused the weakening of stress relaxation ability, and the roughness influenced this feature. • The relaxation stress exhibited a linear relationship with plastic deformation occurring before the stress relaxation. • The elastic energy was the power of stress relaxation, and the roughness provided the "space" for stress relaxation. • A speculative result about the limit curve was that it was a straight line. [ABSTRACT FROM AUTHOR]

Three-dimensional laser scanning is an advanced technique for fracture roughness measurements. The surface roughness of fractures (discontinuities) accurately measured is of practical importance for proper evaluation of the mechanical properties of a fractured rock material. It is also appropriate to perform a more systematic study on the effect of a sampling interval on the roughness measurement accuracy. This effect was investigated based on the 3D-point-cloud data of a fracture surface acquired with laser scanning. A series of 2D profiles corresponding to twelve directions were extracted from concentric circular sampling windows of different diameters. The roughness measurement accuracy is quantified by the three parameters, viz the mean square first derivative Z2 , structure function SF, and roughness profile index Rp. The sampling interval effect was investigated for its different values by analyzing the three parameters of different profiles. It was established that SF was very sensitive, while Z2 and Rp were less responsive to the sampling interval. It exerts a much weaker influence on the rock material fracture roughness in comparison with anisotropy. [ABSTRACT FROM AUTHOR]

A detailed investigation of the characteristics of the 2011 catastrophic landslides of Khao Phanom Benja, Krabi, Thailand was conducted. The landslides on the slope faces of the high relief granitic mountain led to devastating debris flows of large rock boulders that buried houses in the foothill area. The unfavorable orientations of four major joint sets of the mountain created high kinematic instability of rock wedge slides along the stream channel direction as well as rock wedge formation on the channel walls in the eastern sides of the mountain. Weathering and joint opening in the areas of slopes and stream channels increase the vulnerability of rock slides, in particular for a prolonged heavy rainfall. The heavy rainfall and stream channel flow on slab block of rock slope erosion along stream bank caused down bank slope failure. The increasing upward water pressure caused the increase in uplift force behind the rock block and rock wedge failure. In addition, the hazard zone delineation of debris flows deposition was mapped out using a MATLAB codenamed as FLOWS. Majority of slides on the eastern slopes of the mountain were granitic rock slide modes while soil slide and surface erosion on the residual soil to completely weathered sedimentary rock were the predominant modes of failure on the western and northern slopes in lower areas at foothills of the mountain. As found in this investigation the factors influencing landslides and debris flows in 2011 at Khao Phanom Benja, Krabi, southern Thailand were not only limited to the prolonged rainfall but also to slope gradient, rock types and weathering degree, discontinuity characteristics. [ABSTRACT FROM AUTHOR]

The shear strength of rock discontinuities is known to be scale dependent, and past research has revealed that both positive and negative scale effects could be observed. It is far from trivial to predict the occurrence of the scale effect and, to date, there is still no consensus on how to satisfactorily predict the shear strength of large discontinuities. A new stochastic approach was proposed and validated at laboratory scale by the authors. The approach consists of (1) using the information available from visible traces to create synthetic surfaces via a random field model and (2) estimating the shear strength of each one of the synthetic surfaces in order to obtain a distribution of shear strength and a mean shear strength. This paper presents the first application of this new approach to a large discontinuity that was surveyed with a resolution of 1 mm and an accuracy in the order of 150–230 µm in the Pilkington reserve of Newcastle, Australia. The paper first confirms that a scale effect does exist for the surface tested, before demonstrating that the new stochastic approach produces a strength envelope that is very close to the deterministic failure criterion of the whole surface. The key conclusion of this research is that there is enough information on visible traces, if surveyed accurately, to obtain an estimate of the shear strength of the discontinuity. [ABSTRACT FROM AUTHOR]

In rockfall hazard management, the investigation and detection of potential rockfall source areas on rock cliffs by remote-sensing-based susceptibility analysis are of primary importance. However, when the rockfall analysis results are used as feedback to the fieldwork, the irregular slope surface morphology makes it difficult to objectively locate the risk zones of hazard maps on the real slopes, and the problem of straightforward on-site visualization of rockfall susceptibility remains a research gap. This paper presents some of the pioneering studies on the augmented reality (AR) mapping of geospatial information from cyberspace within 2D screens to the physical world for on-site visualization, which directly recognizes the rock mass and superimposes corresponding rock discontinuities and rockfall susceptibility onto the real slopes. A novel method of edge-based tracking of the rock mass target for mobile AR is proposed, where the model edges extracted from unmanned aerial vehicle (UAV) structure-from-motion (SfM) 3D reconstructions are aligned with the corresponding actual rock mass to estimate the camera pose accurately. Specifically, the visually prominent edges of dominant structural planes were first explored and discovered to be a robust visual feature of rock mass for AR tracking. The novel approaches of visual-geometric synthetic image (VGSI) and prominent structural plane (Pro-SP) were developed to extract structural planes with identified prominent edges as 3D template models which could provide a pose estimation reference. An experiment verified that the proposed Pro-SP template model could effectively improve the edge tracking performance and quality, and this approach was relatively robust to the changes of sunlight conditions. A case study was carried out on a typical roadcut cliff in the Mentougou District of Beijing, China. The results validate the scalability of the proposed mobile AR strategy, which is applicable and suitable for cliff-scale fieldwork. The results also demonstrate the feasibility, efficiency, and significance of the geoinformation AR mapping methodology for on-site zoning and locating of potential rockfalls, and providing relevant guidance for subsequent detailed site investigation.

• Dynamic finite element models were proposed to investigate the dynamic responses and damage mechanism of rock under blasting. • The propagation of blasting stress waves was analyzed together with the interaction of discontinuity orientation and confining stress magnitude. • The extension mechanism of the discontinuity was explored from the perspective of stress distribution and dynamic stress intensity factor (DSIF). • Rock discontinuity can weaken the crack-guiding effect of high confining stress, and high confining stress can suppress the development of radial cracks around borehole. The propagation of seismic waves induced by blasting changes significantly at rock discontinuities such as joints and faults, many studies have focused on the mechanism of wave propagation at joints. However, in deep rock masses, high in-situ stress is non-negligible, and the interaction between confining stress and discontinuity under blasting disturbance remains poorly understood. To explore the role of confining stress and discontinuity on the dynamic responses and damage mechanism of rock masses, this paper employed the dynamic finite element method and a series of numerical models were subsequently developed. The Riedel-Hiermaier-Thoma (RHT) model was used to simulate the blast-induced damage of rock. Under blasting disturbance, the propagation of stress waves and damage patterns within the rock at different discontinuity orientations and different confining stress magnitudes were presented. Around the discontinuity, the principal stress distribution and displacements were presented and the dynamic stress intensity factor (DSIF) of the discontinuity tip was obtained to analyze the extension mechanism of the discontinuity. Numerical results show that the discontinuity, the magnitude and direction of confining stress significantly contribute to the damage patterns of rock masses, and the guiding effect of high confining stress on cracking will be weakened due to the existence of discontinuity. [ABSTRACT FROM AUTHOR]

This study proposes a numerical simulation procedure to evaluate the accuracy of geometric parameters of 3-dimensional models reconstructed by photogrammetry based on the Open Graphics Library. As a technical mean to obtain photogrammetric photos and data, this method has achieved the goal of controlling the parameters in the photogrammetric process at will. The proposed method is validated by several physical model experiments using the unmanned aerial vehicle Phantom 4 Real-time kinematic (RTK). Through a series of numerical experiments, this study quantitatively analyzes the error of geometric parameters of discontinuities in rock masses caused by coordinate accuracy of camera positions and shooting distance, and some quantitative findings were obtained. These results conforming to common sense also validate the proposed simulation procedure. This numerical simulation procedure has a broad application prospect in guiding and optimization of photo acquisition of field outcrop balancing the accuracy and efficiency. • A numerical simulation procedure was first developed to evaluate the AGP3MRP. • The procedure was used to evaluate the rock discontinuity geometric parameters. • Physical tests were performed and the results show that the procedure is validated. • The influence law of the photogrammetric parameters on the results was drawn. [ABSTRACT FROM AUTHOR]

The article reports on modeling roughness of rock discontinuity surfaces. It discusses studies designed to quantify the profile matching process which show that the joint roughness coefficient (JRC) values of the original ten roughness profiles correlate reasonably well with a variety of mathematical expressions of roughness. The aim is to pave the way for development of a physically based roughness quantification method that is consistent in expressing morphology and assigning ranking.

The subgrade reaction modulus for rock foundations under axial loading is investigated by model footing tests. This study focuses on quantifying a new subgrade reaction modulus by considering rock discontinuities. A series of model-scale footing tests are performed to investigate the effects of the unconfined compressive strength, discontinuity spacing and inclination of the rock joint. Based on the experimental results, it is observed that the subgrade reaction modulus of the rock with discontinuities decreases by up to approximately 60 % of intact rock. In addition, it is found that the modulus of subgrade reaction is proportional to the discontinuity spacing, and it decreases gradually within the range of 0°-30° and tends to increase within the range of 30°-90°. [ABSTRACT FROM AUTHOR]

Rock mass characterization is obviously a key element in rock fall hazard analysis. Managing risk and determining the most adapted reinforcement method require a proper understanding of the considered rock mass. Description of discontinuity sets is therefore a crucial first step in the reinforcement work design process. The on-field survey is then followed by a structural modeling in order to extrapolate the data collected at the rock surface to the inner part of the massif. Traditional compass survey and manual observations can be undoubtedly surpassed by dense 3D data such as LiDAR or photogrammetric point clouds. However, although the acquisition phase is quite fast and highly automated, managing, handling and exploiting such great amount of collected data is an arduous task and especially for non specialist users. In this study, we propose a combined approached using both 3D point clouds (from LiDAR or image matching) and 2D digital images, gathered into the concept of ''solid image''. This product is the connection between the advantages of classical true colors 2D digital images, accessibility and interpretability, and the particular strengths of dense 3D point clouds, i.e. geometrical completeness and accuracy. The solid image can be considered as the information support for carrying-out a digital survey at the surface of the outcrop without being affected by traditional deficiencies (lack of data and sampling difficulties due to inaccessible areas, safety risk in steep sectors, etc.). Computational tools presented in this paper have been implemented into one standalone software through a graphical user interface helping operators with the completion of a digital geostructural survey and analysis. 3D coordinates extraction, 3D distances and area measurement, planar best-fit for discontinuity orientation, directional roughness profiles, block size estimation, and other tools have been experimented on a calcareous quarry in the French Alps. [ABSTRACT FROM AUTHOR]

Water is an important weathering factor on rock discontinuities and in rock mass mechanical behaviour because of its chemical features such as temperature, pH or salinity which make it a 'good' candidate to rock degradation. Furthermore the increase of rainfall frequency or intensity highlights some problems on the rock slope stability analysis. This study aims to evaluate the effect of water flow on the rock slope stability and it is performed at two space scales: in situ scale and laboratory (micro scale and macro scale). It shows how water induces degradation at multi-scale (surface roughness and matrix) and thus may decrease the stability of the discontinuous rock mass. It has two main components: the effect of water-solid chemical mechanisms and the analysis of the mechanical response of the discontinuity modified by the water alteration. [ABSTRACT FROM AUTHOR]

A two-dimensional brittle creep model for rock provides insight into the initiation of shear fracture along weak discontinuities in rock. The model accounts for material heterogeneity and introduces the concept of a mesoscopic renormalization to capture the cooperative interaction between cracks in the transition from distributed to localized damage. A series of shear creep tests on rock with discontinuities were performed to simulate the initiation and propagation of crack along a pre-existing weakness under sustained shear stress and normal stress. The investigation showed that shear stress level and the normal stress level might have significant effect on the long-term behavior of rock with weak discontinuities. Moreover, a case study of rock slope instability was also investigated, where the numerically simulated instability failure of rock slope with discontinuities showed that both tensile and shear damage at the weakest elements are the trigger for the failure surface initiation in the rock slope. Once damage occurs, redistributed stress concentrations would then intensify fracture propagation and coalescence within these damage zones, leading to the progressive development of a failure surface. Moreover, failure surface extending is not only dominated by the properties and the position of discontinuities but also influenced remarkably by the complex interaction between existing discontinuities and fracture propagation. The results are of general interest because they can be applied to the investigation of time-dependent instability in rock masses, to the mitigation of associated rock hazards in rock engineering, and even to a better understanding of the physical phenomena governing the stability of rock slope. [ABSTRACT FROM AUTHOR]