Dynamic indirect tensile tests were carried out by using a Split Hopkinson Pressure Bar (SHPB) for coal sampled from the Datong mine in China. The principal purpose was to explore the influence of ...bedding structure in the coal on its dynamic indirect tensile strength. However, to resolve some contradictions, X-ray micro CT, high speed optical imaging and a discrete element based modelling approach were combined to analyze the test results. The X-ray micro CT was used to detect the actual bedding structure in the coal; the high speed imaging captured failure patterns of the specimens with different bedding directions; and the numerical modelling was utilized to investigate the influence of different bedding structures on dynamic strength. The SHPB and numerical results illustrate that dynamic indirect tensile strength reliably correlates with impact velocity. In addition, the dynamic indirect tensile strength is not only influenced by the bedding direction but also by the roughness and discontinuity of the bedding. Based on these findings, a method is developed to further process the test data including a model to describe the dynamic indirect tensile strength of Datong coal.
•Dynamic indirect tensile tests of coal were carried out by using a Split Hopkinson Pressure Bar.•X-ray CT and discrete element approach were combined to analyze the results.•The strength was found to be reliably correlated with impact velocity.•The strength is influenced by the bedding direction, roughness and discontinuity.
Coal bump refers to a sudden catastrophic failure of coal seam and usually can cause serious damages to underground mining facilities and staff. In this circumstance, this paper focuses on the recent ...achievements in the mechanism and prevention techniques of coal bumps over the past five years in China. Based on theoretical analysis, laboratory experiment, numerical simulation and field test, the characteristics of coal bumps occurrence in China’s coal mines were described, and the difference between coal bumps and rockbursts was also discussed. In addition, three categories of coal bumps induced by “material failure” were introduced, i.e. hard roof, floor strata and tectonic structures, in which the mechanism of coal bumps induced by geological structures was analyzed. This involves the bump liability and microstructure effects on bump-prone coal failure, the mechanism of coal bumps in response to fault reactivation, island face mining or hard roof failure. Next, the achievements in the monitoring and controlling methods of coal bumps were reviewed. These methods involve the incorporated prediction system of micro-seismicity and mining-induced pressure, the distributed micro-seismic monitoring system, energy absorption support system, bolts with constant resistance and large elongation, and the “multi-stage” high-performance support. Finally, an optimal mining design is desirable for the purpose of coal bump mitigation.
Laboratory observations have shown that coal permeability under the influence of gas adsorption can change instantaneously from reduction to enhancement. It is commonly believed that this ...instantaneous switching of permeability is due to the fact that the matrix swelling ultimately ceases at higher pressures and the influence of effective stresses take over. In this study, our previously-developed poroelastic model is used to uncover the true reason why coal permeability switches from reduction to enhancement. This goal is achieved through explicit simulations of the dynamic interactions between coal matrix swelling/shrinking and fracture aperture alteration, and translations of these interactions to perrmeability evolution under unconstrained swellings. Our results of this study have revealed the transition of coal matrix swelling from local swelling to macro-swelling as a novel mechanism for this switching. Our specific findings include: (1) at the initial stage of CO
2 injection, matrix swelling is localized within the vicinity of the fracture compartment. As the injection continues, the swelling zone is extending further into the matrix and becomes macro-swelling. Matrix properties control the swelling transition from local swelling to macro swelling; (2) matrix swelling processes control the evolution of coal permeability. When the swelling is localized, coal permeability is controlled by the internal fracture boundary condition and behaves volumetrically; when the swelling becomes macro-swelling, coal permeability is controlled by the external boundary condition and behaves non-volumetrically; and (3) matrix properties control the switch from local swelling to macro swelling and the associated switch in permeability behavior from reduction to recovery. Based on these findings, a permeability switching model has been proposed to represent the evolution of coal permeability under variable stress conditions. This model is verified against our experimental data. It is found that the model predictions are consistent with typical laboratory and in-situ observations available in lietratures.
► We revealed why coal permeability switches from reduction to enhancement under free swelling. ► This switching is due to transition from local swelling to macro-swelling. ► When swelling is localized, coal permeability is controlled by the internal fracture. ► When swelling becomes macro-swelling, permeability is controlled by the external boundary.
We explore microstructure-related effects of loading direction and specimen size on the anisotropy of uniaxial compressive strength in coal. We measure uniaxial compressive strength on coal samples ...of four different diameters (25 to 75 mm) and with varied dip of the bedding plane with respect to loading direction (0°, 15°, 30°, 45°, 60° and 90°) including characterizing the variation of microstructures in the specimens by X-ray imaging. The uniaxial compressive strength for each specimen size exhibits a unique U-shape curve against the angle of anisotropy (Jaeger, 1971). The degree of the strength anisotropy decreases with increasing specimen size, principally due to the enhanced microstructural volume of a larger specimen. The contribution of microstructures on uniaxial compressive strength differs for different orientations of loading relative to anisotropy. The rate of decline of the average uniaxial compressive strength with increasing specimen size is greatest when loading is parallel to bedding (anisotropic angle of 0°) and is the most moderate at 45°. An empirical equation relating specimen diameter with uniaxial compressive strength is proposed and verified against the experimental data. Based on this equation, the UCS of coal samples with different orientations relative to the anisotropy are predicted for the limiting sizes of zero and ∞. The anisotropy of the scale effect is also explored. Meanwhile, it is verified that a cosine relation between anisotropy angle and uniaxial compressive strength is applicable to coal specimens of different sizes. This demonstrates that the strength anisotropy in coal will remain constant when the specimen diameter is larger than a critical threshold. Based on these two observations/equations, a universal equation relying on the minimum strength angle, friction angle and Weibull coefficients is proposed and verified against experimental data to describe the relationship among anisotropy angle, specimen size, and uniaxial compressive strength in coal.
•Microstructure-related effects of loading direction and specimen size on the strength anisotropy in coal were explored.•An equation relating specimen diameter with the UCS is proposed and verified.•A universal equation describing the correlation of anisotropy angle, specimen size, and UCS is developed and verified.
Coal bump is sudden catastrophic failure of coal and usually causes serious problems to underground coal mining. The acoustic and thermal infrared radiation (TIR) characteristics of bump-prone coal ...under uniaxial compression and cyclic loading conditions are investigated in this study. The monitoring system consists of a TIR observation system, a stress and strain measurement system and an acoustic emission monitoring system. Precursory information for impending failure of coal samples is collected, including TIR, acoustic emission and strain precursors. Moreover, the response speed and the sensitivity of different monitoring systems during the cracking events in the samples are studied under different loading conditions. The microstructure of coal sample is analyzed by X-ray computerized tomography (CT) and scanning electron microscope (SEM). The experiment results show that 0.9
σ
c (
σ
c is the compressive strength) and 0.82
σ
c can be regarded as the stress-caution-points for the failure of bump-prone coal under uniaxial compression and under uniaxial cyclic compression, respectively. They are much higher than 0.79
σ
c as the stress-caution-point for the normal coal samples. It is also indicated that the temperature at coal surface or in the coal mass should be monitored at the perimeter of the site with anticipated collapses. It can be a valuable tool to obtain the precursors for mine disaster control such as coal bumps, gas outburst and water inrush.
Due to strain energy releases sharply, fault slip is often accompanied with sudden stress change around fault structures, especially on the fault surface. In this study, the slippage of a reverse ...fault structure induced by coal mining in Yima mining area, Henan province, China is studied by employing physical and numerical simulation. The stress evolution around this fault is analyzed to further understand the mechanism of coal bursts around the fault structures. The results show that the normal stress is relatively greater than shear stress on the fault surface, and the normal and shear stress on the fault surface near the coal seam are greater than that far away from the coal seam. It is interesting to find that the variation of normal stress and shear stress on the middle of fault is opposite in the physical model. To in-depth understand this phenomenon, the numerical simulation method is employed to investigate the stress variation and dynamic energy evolution on the fault surface before, during and after fault slip by applied different horizontal stress on the lateral surface of model. The results show that the opposite variation of normal and shear stress is induced by the greater horizontal stress. This phenomenon can be considered as a precursory information for major fault slip. The horizontal stress is the main driving force of the fault slip induced by the coal seam mining. In this study, it is also suggested that the existence of extra-thick conglomerate rock and reverse fault in the high horizontal stress environment could cause large area of roof movement and provide the dynamic pressure source for the occurrence of coal bursts in Yima mining area.
•The reason why the horizontal loading is the main driving force of the fault slip has been found out.•The fault slip precursory information in terms of fault stress is found out.•The dynamic environment of coal bursts from combined effect of extra-thick rock and F16 fault is studied.•The reason why the coal burst accidents occur so frequently in Yima mining area is analyzed.
The instability caused by the fault slip is one of the main factors that induce the occurrence of coal bursts. To in-depth understand the mechanisms responsible for coal bursts during the underground ...coal seam mining, a mechanical model used to calculate the stress distribution on fault surface and illustrate the fault slip characteristics is proposed by considering the mining-induced stress. In order to verify the validity of this model, a set of comparative analyses with experimental results which have been published in the previous studies are carried out. The results show that the normal and shear stress distribution on the fault surface calculated by the mechanical model is approximately consistent with the experimental stress results. According to the calculated stress distribution on the fault surface, normal and reverse faults formation mechanism are explained by this mechanical model when the lateral pressure coefficient is greater than 1.0 and less than 1.0. Meanwhile, fault potential energy is proposed to research the energy storage and further explain the mechanism of fault slip. In addition, the coal bursts occurrence induced by fault slip is also discussed according to the engineering background and this mechanical model.
•A mechanical model is proposed by considering the mining-induced stress.•The formation mechanism of fault structure is explained by this mechanical model.•Fault potential energy is proposed to demonstrate the mechanism of fault slip.•The initiation position of fault slip during the coal seam mining is identified.•Coal bursts induced by fault slip is discussed according to this mechanical model.
The tensile failure characterization of dry and saturated coals under different impact loading conditions was experimentally investigated using a Split Hopkinson pressure bar. Indirect dynamic ...Brazilian disc tension tests for coals were carried out. The indirect tensile strengths for different bedding angles under different impact velocities, strain rates and loading rates are analyzed and discussed. A high-speed high-resolution digital camera was employed to capture and record the dynamic failure process of coal specimens. Based on the experimental results, it was found that the saturated specimens have stronger loading rate dependence than the dry specimens. The bedding angle has a smaller effect on the dynamic indirect tensile strength compared to the impact velocity. Both shear and tensile failures were observed in the tested coal specimens. Saturated coal specimens have higher indirect tensile strength than dry ones.
This study presents an integrated approach for field tests and numerical investigations to assess the risk of coal bumps. This approach produces a stress-relief technology using boreholes to mitigate ...risk during the extraction of an island longwall panel. The field tests were conducted in an island longwall panel in the Tangshan coal mine in the city of Tangshan, China. In these tests, roadway roof displacement and electromagnetic radiation (EMR) of roadways in the panel were investigated to determine the zones of intensive roof deformation. A numerical model FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions) was established to understand the results of the field tests and to map the zones in the panel with a high risk for coal bumps. The results of the field tests and the numerical modeling show that the roof deformation starts to occur at more than 30m ahead of the longwall face and the deformation starts to accelerate after a distance of 10m in front of the longwall face. Large and rapid roof deformation is considered to be an important precursor of coal bump occurrence during the extraction of an island longwall panel. Based on these results, a stress-relief technology using boreholes was investigated through numerical methods. The modeled results suggest that the abutment stress could be released by drilling boreholes in the zones prone to coal bumps. The effectiveness of the stress release increased with the borehole length and decreased with the borehole spacing.
► Determination of the zones where the most intensive roadway roof deformation occurs. ► Location of bump-prone zone in island panel and coal pillar ► Verification of the roof deformation results of field tests ► Investigation of the dynamic mechanical state of coal pillar during coal mining ► Application of the stress-relief technology by boreholes drilled into in roadway rib
This study presents a numerical investigation on the dynamic mechanical state of a coal pillar and the assessment of the coal bump risk during extraction using the longwall mining method. The present ...research indicates that there is an intact core, even when the peak pillar strength has been exceeded under uniaxial compression. This central portion of the coal pillar plays a significant role in its loading capacity. In this study, the intact core of the coal pillar is defined as an elastic core. Based on the geological conditions of a typical longwall panel from the Tangshan coal mine in the City of Tangshan, China, a numerical fast Lagrangian analysis of continua in three dimensions (FLAC
3D
) model was created to understand the relationship between the volume of the elastic core in a coal pillar and the vertical stress, which is considered to be an important precursor to the development of a coal bump. The numerical results suggest that, the wider the coal pillar, the greater the volume of the elastic core. Therefore, a coal pillar with large width may form a large elastic core as the panel is mined, and the vertical stress is expected to be greater in magnitude. Because of the high stresses and the associated stored elastic energy, the risk of coal bumps in a coal pillar with large width is greater than for a coal pillar with small width. The results of the model also predict that the peak abutment stress occurs near the intersection between the mining face and the roadways at a distance of 7.5 m from the mining face. It is revealed that the bump-prone zones around the longwall panel are within 7–10 m ahead of the mining face and near the edge of the roadway during panel extraction.
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