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Accurate estimation of the triaxial compression behavior of jointed coal is essential for coal mining. Few studies addressed the triaxial compression behavior of large-scale rock mass, especially with real joint geometry. We employed a numerical synthetic rock mass (SRM) method to study the triaxial compression behavior of jointed coal. Jointed-coa...
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... based on a cuboid with an aspect ratio of 2:1 by developing a PFC3D Fish program. In the Fish program, the sub-DFNs are intercepted randomly five times for each size in the jointed-coal DFN to reduce the discretization, and an average value is calculated for P 32 and P 31 . The varieties of P 32 and P 31 with the size of sub-DFNs are shown in Fig. 3. P 32 and P 31 tend to be steady when the sub-DFN size increases to 1.0 m × 1.0 m × 2.0 m, the size of which is identified as the REV for jointed coal. The REV of jointed coal is consistent with the field observation where the REV of coal is 1.0-1.5 m. (Bieniawski 1968a, b). Log-normal (0.16/0.59) Note: (1) NE means negative ...
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In this paper, the strength and deformation failure characteristics of bearing coal rock mass are related to the confining pressure, and the SAS-2000 experimental system is used to carry out uniaxial and 3, 6, and 9 MPa triaxial tests on coal rock to assess the strength and deformation failure characteristics of coal rock under different confining...
Citations
... The underground geomechanical environment of coal mines is complex and changeable, so the risk assessment of roadway roof falls should involve many factors [15][16][17]. However, the above evaluation method is too simple to correctly judge the actual situation of roof falls in coal mine roadways. ...
Roof falls in coal mine roadways are the main causes of many casualties, shutdowns and production plan delays. To understand the relationship between the influencing factors of roadway roof fall accidents and the importance ranking of the accidents, we will reduce safety accidents in coal mines. To enable the timely prediction and control of roadway roof fall risks, based on the investigation of many roadway roof fall risk factors, 12 evaluation indexes such as the roadway roof rock thickness, geological conditions and roadway section shape were selected. An evaluation index system of roadway roof fall risks is constructed. A risk degree standard of roadway roof falls is proposed. The risk evaluation model of roadway roof falls was established by using the combination weight of the analytic hierarchy process (AHP), entropy weight method (EW) and matter element extension theory. According to the principle of the maximum membership degree, the risk degree of roadway roof falls is determined. Based on Java Web, a risk assessment system for roadway roof falls was developed. We name the system Multiple Weight-Material Element Web (MW-MEW). The MW-MEW system was used to evaluate the risk degree of roof falls in the C9 return airway of the Xingu Coal Mine. Compared with the evaluation results of the AHP matter element extension model, it is found that the evaluation results of the MW-MEW system are more in line with the actual engineering conditions. The successful application of the MW-MEW system will provide new avenues for the quantitative evaluation of roof fall risks in coal mine roadways.
... The discontinuities in the rock mass separate it into numerous irregularly shaped blocks, consequently leading to the disruption of its continuity and integrity [1][2][3][4]. This, in turn, significantly impairs the mechanical properties of the rock mass, making these discontinuities the primary determinant of rock mass failure [5][6][7][8]. As such, the accurate acquisition of information concerning the development of rock mass joints assumes critical significance in assessing and ensuring rock mass stability. ...
Rock Quality Designation (RQD) is among the widely used measures of the quality of rock masses and can be derived through Monte Carlo stochastic process-based fracture network simulations. However, repeated simulations can yield variable RQD results. Here, we introduce a four-step approach that incorporates class ratio analysis to estimate the representative RQD, which includes (1) extracting the mean and confidence interval of the RQD sample, in terms of the Confidence Neutrosophic Number Cubic Value (CNNCV), (2) employing class ratio analysis to determine the thresholds of the number of virtual boreholes and that of the number of models for a given size D, beyond which the CNNCV remains substantially unchanged, (3) accepting the CNNCV at the thresholds of the number of models as the representative RQD for the model of size D (RQD(D)) and (4) determining the representative RQD (rRQD), defined as the specific value which, once D exceeds, the RQD(D) does not change significantly. The introduced approach is illustrated with a case study of an open-pit slope in China, and it was tested for its performance. The RQD calculation results of the proposed method and the traditional single-model approach exhibit differences, which diminish with increasing model sizes. At the 95% confidence level, the stable size of the RQD determined by the proposed method is 13 m, compared to 25 m for the single-model approach. This method enhances the accuracy of representative elementary volume predictions by accounting for the diversity in the simulation results of RQDs for the same size. Overall, the introduced approach offers a reliable method for obtaining RQD estimates.
... Various researches in related fields have been reported. For example, the mechanical properties of the selected samples were tested by Triaxial shear test (Peng et al., 2015;Yang, 2016;Chen et al., 2021;Wang and Gao, 2022), which obtained its properties under specific conditions; In practical engineering, the rock mass is usually in three unequal stress combinations. Therefore, many scholars have also carried out researches on the mechanical properties Yin et al., 2019), deformation characteristics (Yin et al., 2018;Zhang et al., 2019) and energy characteristics (Luo et al., 2019;Su et al., 2019;Ding et al., 2022) of the samples under the true triaxial test conditions. ...
There are many types of coal seams in China, and the mining of protective layers will cause different rates of stress reduction in protected coal seams at different intervals. Therefore, experiments were conducted at different unloading rates to explore the strength, deformation, and energy characteristics of coal. Research findings: the AE (acoustic emission) signal of the coal body before unloading has a small range of changes and similar characteristics. After unloading begins, because of the different development rates of internal crack in the coal body under different unloading states, the AE signal of the coal body varies at different unloading rates. The maximum stress increases exponentially with the increase of unloading rate. It was found that the higher the unloading rate, the easier and earlier the coal sample is to be damaged. And it was discovered that the dissipated energy of the coal sample in the elastic stage is extremely low, and a large amount of total energy is converted into elastic energy and stored inside the coal sample. The dissipation energy increases during the plastic stage, while the trend of increasing elastic energy slows down. After the peak stage, the dissipated energy rapidly increases and the elastic energy decreases.
... Based on the macroscopic phenomenological damage mechanics, they established the stress-strain constitutive equation considering the strain-softening of rock. Later, scholars began the research and exploration of the elastoplastic damage constitutive model of rock like materials [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Zhou and Zhu [20] established an elastoplastic damage constitutive model of rock materials based on irreversible thermodynamics, which takes into account two plastic deformation mechanisms: plastic friction and plastic porosity. ...
... Thus, compared to UDEC, PFC can better capture and reflect the crack initiation, propagation and intersection of rock specimens and exhibits a fast computing efficiency (Saadat and Tah 2019;Huang et al. 2019;Armin et al. 2023). The parallel bonded model (PBM) in PFC has been widely used to simulate the particle contact of rock matrix (Wang et al. 2018a, b;Wang and Gao 2022). Although this model can closely reproduce the mechanical behavior of rock material, some nonnegligible defects still remain, such as the low ratio of uniaxial compressive strength (UCS) to uniaxial tensile strength (UTS) and the linear strength envelope of the rock material. ...
Rock failure is the result of internal crack initiation (CI), propagation and intersection under external loading. This paper carried out a series of experimental tests and numerical simulations that aim to reveal the failure mechanical behavior, crack development, AE event attributes and failure characteristics of oil shale under various confining pressures. The results suggest that the peak and residual strength of the oil shale under triaxial compression are both enhanced with increasing confining pressure. Splitting failure and diagonal shear failure are the main macrofailure modes of the specimens under uniaxial and triaxial compressive condition. Shear cracks play a dominant role in the rock failure process especially under high confining pressures. The AE event rate shifts from concentration to dispersion with the change from uniaxial to triaxial compression. The cumulative number of cracks, the cumulative AE event counts before the peak strength and the CI stress all first increase sharply and then increase gradually with increasing confining pressure. However, the ratio of the cumulative numbers of shear cracks to tensile cracks before the peak strength increases at a fairly steady rate as the confining pressure increases. These findings are helpful to evaluate the stability and safety of rock mass under confining pressure, and could also provide a basis for the design of rock mass reinforcement.
... Their mechanical properties depend on the geometric and mechanical properties of particles and bonding, which better reflect the geometric characteristics of discrete materials in discontinuous media and explain the nonlinear deformation and failure of the rock mass. Therefore, PFC simulation software is widely used in research related to coal and rock mass fragmentation [38,48,[61][62][63]. The particle flow dispersion element method is adopted in this research to establish a numerical model of sandstone and analyze the internal fracture development law of sandstone samples under different moisture contents in the loading process from the meso level. ...
In order to better understand the failure process of water-bearing rocks, samples of water-bearing sandstone were tested uniaxially. The failure process and the development of internal cracks were studied through the evolution characteristics of dissipated strain energy and particle flow simulation. In this study, we found that: (1) The presence of water in sandstone results in a reduction in energy storage capacity as well as strength. (2) The dissipated energy ratio curve of sandstone samples and simulated samples’ internal fracture development curve has obvious stages. The dissipated energy ratio turning point and the rapid fracture development point are defined as the failure precursor points of sandstone samples and simulated samples, respectively. In both sandstone samples and simulated samples, the ratio between failure precursor stress and peak strength remains almost unchanged under various water conditions. (3) The ratio of fracture to dissipated energy (RFDE) of sandstone is proposed, and interpreted as the increased number of cracks in the rock under the unit dissipated. On this basis, the fracture initiation dissipated energy (FIDE) of sandstone under different water cut conditions is determined, that is, the dissipation threshold corresponding to the start of the development of sandstone internal cracks. (4) The analysis shows that RFDE increases exponentially and FIDE decreases negatively with the scale-up in moisture content. Further, high moisture content sandstone consumes the same dissipative strain energy, which will lead to more fractures in its interior. The research in this paper can lay a theoretical and experimental foundation for monitoring and early warning of rock engineering disasters such as coal mining, tunnel excavation,slope sliding, and instability.
... At present, the research tools have significantly progressed (electron microscopy, tomography, etc.) and more attention is paid to the system of disturbance in the extraction of methane from the coal-bearing stratum, CO 2 utilization, choosing the direction of hydraulic fracturing shock, since the target of this method is to connect the main system of fractures (face cleat) with gas well [6][7][8][9]. ...
Evaluating the disturbance of coal beds is important both in terms of conditions of their working out and in predicting such features as methane capacity, outburst hazard and spontaneous combustion. One of the methods for evaluating coal disturbance is its research using optical microscopy. Coal consists of petrographic microcomponents (macerals) distinguished by optical properties and, most importantly, strength properties. This heterogeneity of the coal matter largely determines the nature of coal disturbance. This paper shows the impact of the macerals of the inertinite and liptinite groups on the development of disjunctive and plicative coal disturbance. The purpose of the paper is to establish the features in the formation of plicative and disjunctive disturbance of coal depending on its petrographic composition. The research is carried out with the help of a video-optical complex: MBI-11, HB 200, and Scope photo software. It is noted that the more diverse the petrographic composition of coals is, the less developed fracturing under equal other conditions coals have. The petrographic composition affects the disturbance of coals not only at the level of lithotypes but also at the level of macerals. Liptinite and inertinite prevent the development of endogenous fracturing. Liptinite and inertinite contribute to the development of exogenous fracturing. An increase in the total density of coal at high stages of metamorphism levels out the effect of the petrographic composition. Coals of low stages of metamorphism are characterized by the greatest diversity of composition. With the growth of metamorphism, the difference in strength and optical properties of the macerals is gradually leveled and coals of the C-coke, F-fat marks (CF) are characterized by the greatest fracturing. To the L mark, the composition becomes even more homogenous (features of vitrinite and liptinite fully become the same) but the overall density of coal increases, and the fracturing decreases. As long as differences remain in the strength properties of macerals (low and middle stages of metamorphism) the presence of liptinite and inertinite prevents the development of fracturing. Examples of suppression of fractures by macrospores and fusinite are given in the form of microphotographs. There are cases of interlayer slipping and slickenlines. The importance of petrographic research in evaluating both rupture and plicative disturbance of coals is emphasized.
3DEC is used herein to study the stability of a circular tunnel opening in stratified rock mass. Numerical analysis indicated that the bedding planes can influence the damage process and the distribution of secondary cracks in rock masses, creating more damaged zones. The bedding planes lead to a nonuniform distribution of water pressure behind the lining. Such nonuniform water pressure may result in localized damage in the tunnel. Water leakage occurs mainly at the top of the tunnel and on both side of the walls. Tunnels situated in vertical bedding planes are more vulnerable to water, especially for underwater projects such as water diversion tunnels and undersea tunnels, where construction in vertical bedding planes should be avoided as much as possible. The maximum deformation of shotcrete occurs at the intersection with the bedding plane, and the displacement on the upper side is greater than that on the lower side.
Understanding of continuous damage and cracking process of compressed jointed coal is critical to predict its stability. Herein, for jointed coal under uniaxial compression with bedding planes perpendicular (sample A) and parallel (sample B) to the loading direction, an acoustic emission (AE) monitoring device and a computed tomography (CT) scanner were employed to study the damage and cracking process. Furthermore, the correlations between the AE signals and internal fracture parameters were analyzed. Results show that (1) shear and tension cracks are respectively main damage forms in samples A and B; (2) there is a drop valley of variations of AE count fractal dimension (D1) over time before the stress peak, and compared with sample B, the drop valley for sample A are much wider; (3) many fractures between joint planes formed at yielding and postpeak stages, fracture fractal dimensions (D2) first increase slightly with loading and then increase significantly at yielding and postpeak stages; especially D2 of sample A is larger than that of sample B; (4) the accumulative value of D1 (AD1) increases with D2 monotonously, and an expression of porosity or connectivity of the compressed jointed coal was developed, by which the porosity and connectivity of jointed coal could be calculated. The study outcomes could contribute to predict coal stability and the variations of water and gas flowing channels in seam in underground coal mines.
