It is generally known that discontinuities have a remarkable influence on the mechanical behaviour of rock masses. To further understand the fracture mechanisms of jointed rock masses, substantial ...effort has been focused on the strength anisotropy and failure characteristics of rocks/rock-like specimens containing persistent joints with different geometric parameters. However, only a few laboratory tests have considered the failure mechanism of a rock mass with 3D joints, especially for non-persistent joints with different persistence levels. In the present work, experiments on cubic rock-like specimens containing non-persistent joints (in areal extent) subjected to uniaxial compression were conducted to further investigate the influence of the joint inclination (θ) and persistence (N) on the rock mechanical properties and failure characteristics. The strength of a 3D non-persistent jointed specimen is characterized by three stages as the joint inclination angle (θ) increases from 0° to 90°. The strength of jointed specimens decreases with increasing N for all θ values, with the highest strength obtained for N = 0.42 and the lowest strength recorded for N = 0.92. Based on CT scan results, four typical fracture modes were identified: splitting, splitting + sliding, sliding, and intact failure. Overall, as the joint inclination increases, the failure mode of the specimen transforms from splitting to sliding and then to the intact failure mode. However, with decreasing joint persistence, the failure modes of some specimens will change from sliding to mixed failure (splitting + sliding).
•3D printing technology was used to prepare cuboid rock-like specimens containing 3D non-persistent joints•The strength of the jointed specimen is characterized by three stages as the joint inclination angle (θ) increases.•The strength of the jointed specimen with the highest strength obtained for N = 0.42 and the lowest strength for N = 0.92.•Four typical fracture modes were identified: splitting, splitting + sliding, sliding, and intact failure.
The peak dilation angle is an important mechanical feature of rock discontinuities, which is significant in assessing the mechanical behaviour of rock masses. Previous studies have shown that the ...efficiency and accuracy of traditional experimental methods and analytical models in determining the shear dilation angle are not completely satisfactory. Machine learning methods are popular due to their efficient prediction of outcomes for multiple influencing factors. In this paper, a novel hybrid machine learning model is proposed for predicting the peak dilation angle. The model incorporates support vector regression (SVR) techniques as the primary prediction tools, augmented with the grid search optimization algorithm to enhance prediction performance and optimize hyperparameters. The proposed model was employed on eighty-nine datasets with six input variables encompassing morphology and mechanical property parameters. Comparative analysis is conducted between the proposed model, the original SVR model, and existing analytical models. The results show that the proposed model surpasses both the original SVR model and analytical models, with a coefficient of determination (R2) of 0.917 and a mean absolute percentage error (MAPE) of 4.5%. Additionally, the study also reveals that normal stress is the most influential mechanical property parameter affecting the peak dilation angle. Consequently, the proposed model was shown to be effective in predicting the peak dilation angle of rock discontinuities.
It is well known that joints or fissures have an important effect on the failure mechanism of natural rocks. Previously, many numerical and experimental papers have been carried out to study the ...strength anisotropy and failure characteristics of jointed rocks. However, few studies have been carried out on the failure mechanism of nonpersistent jointed rock masses with different persistence, especially for nonpersistent joints in three dimensions. In the present study, the failure characteristics of a 3D nonpersistent jointed rock mass with different inclinations (θ) and persistence (K) are studied by numerical simulation. For the 3D digital elevation model (DEM), the linear parallel bond model (LPBM) and smooth-joint model (S-J) were used to model the rock-like material and joint interface, respectively. The connections between the geometric parameters of joints and peak strength are revealed. For the peak strength, the joint persistence only plays a minor role in specimens with inclinations of 0° and 90°, and its influence on strength is mainly reflected in the specimens with shear failure (θ = 45°, 60°, and 75°). Based on microcrack accumulation and evolution, four typical failure processes (shear failure, split failure, mixed failure, and intact failure) are analysed from the micro perspective. The shear stress evolution process on the 3D nonpersistent joint of the specimen with different inclinations under K1 = 0.42 was monitored by the measurement circle, and it was found that the distribution of shear stress inside the rock bridge is related to the failure mode of the specimen. For the specimens with θ = 0° and 90°, the shear stress had little change, indicating that there is slight shear slip behaviour on the joint surface. When the inclination is 45°, 60°, and 75°, the shear stress changes obviously during loading, indicating that the shear action is strong in this failure mode.
The variations in the electric property of loaded rocks are essential in understanding the rock dynamics and fracturing process. Decades of laboratory experiments have revealed different behaviors of ...stress‐stimulated electric current due to the effects of rock types, loading modes, and detection methods. These different behaviors result in difficulties in revealing the underlying physics of electric current in rock and adequately explaining the wide variety of electric precursors measured before rock failure or geohazards. In this study, cubic‐and conical‐shaped diorite specimens were specially designed and produced to experimentally investigate the characteristics of pressure‐stimulated rock current (PSRC) in the process of loading rock specimens to failure. We measured a particular phenomenon of diorite PSRC variation with pressure; that is, PSRC remained nearly stable until the applied stress reached 83%–98% of the failure strength. A remarkable step‐like increment in PSRC was uncovered, and drastic oscillations with maximum amplitudes of several hundreds of nA occurred 1 s prior to abrupt rock failure. A holistic mechanism that includes positive hole activation, field emission of electrons, crack charge separation, and moving charged dislocation was applied to interpret this particular phenomenon. We found that these mechanisms contribute comprehensively rather than individually to the evolution of PSRC. This study provides an improved understanding of the underlying physics of PSRC and the variation in rock electric properties, which is also helpful to understand the abnormal electromagnetic phenomena observed before earthquakes.
Plain Language Summary
Many kinds of electric precursors of rock fracturing or rock failure have been experimentally revealed in the past four decades. The behaviors of stress‐stimulated electric currents in rock are influenced by the loading modes and current detection methods; thus, different mechanisms have been proposed accordingly. By uniaxially and partly compressing cubic‐and conical‐shaped diorite specimens to failure, we revealed the particular and significant variations in rock current before rock failure and found that such behaviors were attributed to a combination of several mechanisms rather than a single mechanism. This study exhibits the potential use of dynamic signal detection of pressure‐stimulated rock current and possible precursor identification of rock fracturing.
Key Points
Pressure‐stimulated rock current (PSRC) increases in a step‐like way at high stress levels
PSRC oscillates with maximum amplitudes of several hundreds of nA just before rock failure
Positive hole activation, crack charge separation, and field emission of electrons comprehensively contribute to the PSRC variations
•Compressive-shear tests were conducted on transversely isotropic rock with different inclinations.•The micro-scale fracturing process of transversely isotropic rock with different inclination are ...analyzed.•The effects of bedding plane strength and spacing are analyzed.
The failure and mechanical behavior of transversely isotropic rock are significantly affected by the original bedding planes. Until now, few studies have been performed to investigate the influence of the geometrical and mechanical parameters of the bedding planes on the fracture characteristics of transversely isotropic rocks under planar shear fracture loading conditions. For this purpose, experimental and numerical compression-shear tests on double-notched specimens are conducted to investigate the fracturing characteristics of transversely isotropic rock under planar shear fracture loading. The experimental study that focuses on the influence of bedding plane inclination on fracture load, fracture pattern and AE evolution, and six inclination angles is conducted in this study. Based on the flat joint contact model (for the rock matrix) and smooth joint contact model (for the original bedding plane) in PFC2D (particle flow code), the microscale fracturing process of transversely isotropic rock with different inclinations is simulated and analyzed. The results show that the inclination has an important influence on the fracture load and fracture pattern, and the maximum and minimum fracture loads are obtained for specimens with inclination angles of 30° and 60°, respectively. Moreover, the strength and spacing of the original bedding planes also play an important role in fracture loads. Higher bedding plane strength and wider bedding plane spacing result in higher fracture loads. In addition, with a moderate inclination angle, transversely isotropic rock with higher bedding plane strength tends to form cracks that cut through the rock matrix. However, with the decrease in the bedding plane strength, more fractures form along the bedding planes.
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•The theoretical relationship between MWD data and second-order elastic modulus is deduced.•The method of obtaining tunnel acoustic wave velocity vector is established.•A method for ...estimating in-situ stress based on MWD data and acoustic waves was proposed.•This offers a novel approach for the rapid measurement of tunnel in-situ stress.
The rapid determination and analysis of in-situ stress are crucial for ensuring the stability and design integrity of underground engineering endeavours. This study introduces an innovative approach to swiftly estimate tunnel in-situ stress by leveraging drilling jumbo Measurement While Drilling (MWD) data in conjunction with acoustic wave information. Initially, a quantitative model linking MWD data to the second-order elastic modulus is established through a mechanical model of the drilling jumbo’s percussion and rotation for rock fracturing, coupled with Hertzian elastic contact theory. Subsequently, a field-measurement scheme for acquiring acoustic wave information within the tunnel is devised, and an estimation method is devised to determine the magnitude and direction of in-situ stress by fitting the wave velocity ellipsoid. Twenty comparative tests across various tunnel sections were conducted to validate the approach. The measurement errors for maximum horizontal principal stress, minimum horizontal principal stress, and direction of the maximum horizontal principal stress were found to be 3.81 MPa, 2.82 MPa, and 11°, respectively, with average errors of 2.16 MPa, 1.61 MPa, and 5.6°. These findings affirm the efficacy of the proposed method in accurately characterizing the distribution of in-situ stress magnitude and direction.
Spectrum sharing, as an approach to significantly improve spectrum efficiency in the era of 6th generation mobile networks (6G), has attracted extensive attention. Radio Environment Map (REM) based ...low-complexity spectrum sharing is widely studied where the spectrum occupancy measurement (SOM) is vital to construct REM. The SOM in three-dimensional (3D) space is becoming increasingly essential to support the spectrum sharing with space-air-ground integrated network being a great momentum of 6G. In this paper, we analyze the performance of 3D SOM to further study the tradeoff between accuracy and efficiency in 3D SOM. We discover that the error of 3D SOM is related with the area of the boundary surfaces of licensed networks, the number of discretized cubes, and the length of the edge of 3D space. Moreover, we design a fast and accurate 3D SOM algorithm that utilizes unmanned aerial vehicle (UAV) to measure the spectrum occupancy considering the path planning of UAV, which improves the measurement efficiency by requiring less measurement time and flight time of the UAV for satisfactory performance. The theoretical results obtained in this paper reveal the essential dependencies that describe the 3D SOM methodology, and the proposed algorithm is beneficial to improve the efficiency of 3D SOM. It is noted that the theoretical results and algorithm in this paper may provide a guideline for more areas such as spectrum monitoring, spectrum measurement, network measurement, planning, etc.
•Cyclic F-T treatments can lead to degradation in mode fracture toughness.•The experimental results of Keff/KIC agree well with the results of MMTS criterion.•The F-T treatments will lead to rough ...fracture paths and fracture surfaces.
For engineering projects in cold regions, the failure behaviour of jointed rocks is affected by freeze–thaw (F-T) cycle action. To reveal the fracture characteristics of rock under the influence of cyclic F-T treatments, fracture tests on semicircular specimens with different F-T cycle numbers were conducted. The fracture toughness values in different modes were investigated by SCB specimens with different crack inclinations (0°, 5°, 15°, 30°, 45° and 54°). The experimental results show that cyclic F-T treatments can lead to degradation in three kinds of fracture toughness values. Specifically, the value of KIC (fracture toughness in mode I) decreases by 3.1, 7.2, 15.0 and 24.3% after 20, 40, 60, and 80 F-T cycles, respectively. When the number of cycles increases from 20 to 80, the value of KIIC (fracture toughness in mode II) of the sandstone decreases by 8.43, 12.0, 18.8 and 27.9%. For the specimen with crack inclinations of β = 5°, 15°, 30° and 45°, after 80 F-T cycles, the Keff (effective fracture toughness) value decreases to approximately 83.7, 78.3, 79.1 and 76.7% of the initial value, respectively. The mixed-mode fracture toughness ratios Keff/KIC were calculated and compared with the theoretical solution. The experimental results of Keff/KIC agree well with the theoretical results calculated based on the MMTS criterion. Moreover, after F-T treatments, the fracture paths of the SCB specimens are more tortuous than those in specimens without F-T treatment. In addition, the fracture surface morphological characteristics of the SCB specimen were obtained by a 3D laser scanner. Overall, the fractal dimension value increases gradually with additional F-T cycles. This indicates that the F-T treatments lead to rough fracture paths and fracture surfaces.
•Compressive-shear tests were conducted on double-notched sandstone rock specimens with different freeze–thaw cycles.•The influence of F-T treatment on the mode II fracture toughness are ...analyzed.•The 3D surface topography and fractal dimension value of fracture surface was obtained.•The 3D JRC value for 3D fracture surfaces was calculated.
For jointed rock masses in cold regions, freeze–thaw cycles cause damage to the rock matrix and degradation of the strength parameters. To further understand the deterioration mechanism of freeze–thaw cycles, substantial efforts have been devoted to the investigation of the failure mechanism of rock after freeze–thaw treatments. Until now, little attention has been given to the degradation of fracture toughness and the change in the fracture surface morphology of rocks after cyclic freeze–thaw treatments, especially for planar shear fracture characteristics. In this research, experiments on double-notched sandstone specimens after different freeze–thaw cycles subjected to compression-shear tests were conducted to further investigate the effect of the freeze–thaw treatment on the mode II fracture toughness and fracture surface morphological characteristics. The results show that freeze–thaw cycles have an important role in the mode II fracture toughness and that the fracture toughness shows an obvious decreasing trend with an increase in freeze–thaw cycles (N). Moreover, the 3D surface topography was reconstructed by using 3D laser scanner scanning. Parameters such as fractal dimension, asperity height and slope angle show that the fracture surface becomes rougher with increasing N. In addition, based on the 2D contour lines extracted from fracture planes, the 3D joint roughness coefficient (JRC) values of 3D fracture surfaces were calculated. The results indicate that more freeze–thaw cycles will produce rougher fracture surfaces.
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