Experimental Study on Damage Characteristics of Coal Samples under True Triaxial Loading and Dynamic Unloading

AbstractCoal bursts are one of the formidable hazards in underground coal mines, yet it is still not fully explored due to the complex stress environment that exists during mining. To better understand the bursting mechanism of coal under real-time mining conditions, it is necessary to develop an experimental method capable of reproducing in situ stress and loading-unloading paths of coal in in situ conditions. In this study, a self-developed true triaxial testing system was used to investigate the damage and failure characteristics of coal samples under true triaxial loading and dynamic unloading conditions. Acoustic Emission (AE) monitoring was used to capture the fracturing of the loaded coal. Passive Velocity Tomography (PVT) and Cumulative AE Energy Density (CAEED) were used to analyse damage evolution characteristics of the coal samples under true triaxial loading conditions. A high-speed camera was used to record the failure of the coal samples when the minimum principal stress σ3 was suddenly unloaded. It was found that continuous coal damage occurred primarily during the true triaxial loading period. The peak number of AE events in the coal samples increased and then dropped as σ1 levels increased. High and low wave velocity zones in the coal samples represent regenerations of the high-density zone and fracture emergence, respectively. Significant energy release zones transferred and expanded as the triaxial loading level increased. Under significant triaxial loads, the coal samples failed and were severely damaged, and the dynamic unloading of σ3 caused ejections of coal fragments at low velocities. The outcome of this study provides in-depth understanding of the failure mechanism of coal under in situ conditions

Mine earthquake mechanism of extremely thick strata based on focalmemchanism analysis

In the Ordos mining area of North China, the Jurassic coal seam is commonly overlain by extremely thick cretaceous sandstone strata. Based on the seismic displacement field and relative moment tensor inversion method, this paper investigates the mine earthquake mechanism and roof fracture characteristics of coal seams under extremely thick strata. In this study, by using seismic source groups as units and microseismic stations as objects, the source mechanism of mine earthquakes is inverted by constructing an inversion matrix, which greatly improves the inversion efficiency and accuracy. The focal mechanism of large-energy mine earthquakes located in solid coal section and goaf section under extremely thick strata is calculated. The characteristics of roof fracture evolution and the influence of vibration wave radiation are quantitatively analyzed. The results show that the extremely thick strata of Cretaceous goaf start to break when the panel below retreats along goaf. During this period, the surface subsidence increases rapidly, and the significant mine earthquakes with energy above 100 kJ begin to occur, and the mine earthquake distribution is more concentrative than that in the solid coal stage. Most of the large-energy mine earthquakes are located behind the goaf, which is closely related to the breakage and slide of the thick roof. Roof tension rupture accounts for more than 85% of the mine earthquakes. During mining along goaf, the seismic sources tend to expand upward, and more shear slip ruptures are presented compared with that in the solid coal mining stage. The strike of the fracture surface has a similar direction or an angle of “X” with the advancing direction. However, compared with that in the solid coal stage, the angle of “X” during mining along goaf increases from 30° to 45°. For the source rupture surface, the dip angle of between 0°−30° accounts for about 50% of the total. The failure type of roof is dominated by horizontal separation tension and roof rotation compression. The generated P waves mostly propagate to the goaf below, which causes a minor influence on the mining face. Only a small part of the P waves propagate to the surrounding working face, which may pose a great impact on the working face. The prevention and control strategy on the extremely-thick-strata type mine earthquakes can be conducted by weakening the integrity of thick strata and making thick strata break in tiers, aiming to reduce the rockburst risks to the working face induced by the breakage of the whole thick strata

Numerical Investigation on Influence of Two Combined Faults and Its Structure Features on Rock Burst Mechanism

With the increase in coal mining depth, engineering geological conditions and the stress environment become more complex. Many rock bursts triggered by two combined faults have been observed in China, but the mechanism is not understood clearly. The focus of this research aims at investigating the influence of two combined faults on rock burst mechanisms. The six types of two combined faults were first introduced, and two cases were utilized to show the effects of two combined faults types on coal mining. The mechanical response of the numerical model with or without combined faults was compared, and a conceptual model was set up to explain the rock burst mechanism triggered by two combined faults. The influence of fault throw, dip, fault pillar width, and mining height on rock burst potential was analyzed. The main control factors of rock burst in six models that combined two faults were identified by an orthogonal experiment. Results show that six combinations of two faults can be identified, including stair-stepping fault, imbricate fault, graben fault, horst fault, back thrust fault, and ramp fault. The particular roof structure near the two combined faults mining preventing longwall face lateral abutment pressure from transferring to deep rock mass leads to stress concentration near the fault areas. Otherwise, a special roof structure causing the lower system stiffness of mining gives rise to the easier gathering of elastic energy in the coal pillars, which makes it easier to trigger a rock burst. There is a nonlinear relationship between fault parameters and static or dynamic load for graben faults mining. The longwall face has the highest rock burst risk when the fault throw is between 6 and 8 m, the fault dip is larger than 65°, the mining height is greater than 6 m, and the coal pillar width is less than 50 m. The stair-stepping, imbricate, horst, and ramp fault compared to the other fault types will produce higher dynamic load stress during longwall retreat. Fault pillar width is the most significant factor for different two combined faults, leading to the rise of static load stress and dynamic proneness

FDNet: Knowledge and Data Fusion-Driven Deep Neural Network for Coal Burst Prediction

Coal burst prediction is an important research hotspot in coal mine production safety. This paper presents FDNet, which is a knowledge and data fusion-driven deep neural network for coal burst prediction. The main idea of FDNet is to extract explicit features based on the existing mine seismic physical model and utilize deep learning to automatically extract the implicit features of mine microseismic data. The key innovations of FDNet include an expert knowledge indicator selection method based on a subset search strategy, a mine microseismic data extraction method based on a deep convolutional neural network, and a feature deep fusion method of mine microseismic data based on an attention mechanism. We conducted a set of engineering experiments in Gaojiapu Coal Mine to evaluate the performance of FDNet. The results show that compared with the state-of-the-art data-driven machines and knowledge-driven methods, the prediction accuracy of FDNet is improved by 5% and 16%, respectively

Comprehensive early warning of rock burst utilizing microseismic multi-parameter indices

Rock bursts have become one of the most severe risks in underground coal mining and its early warning is an important component in the safety management. Microseismic (MS) monitoring is considered potentially as a powerful tool for the early warning of rock burst. In this study, an MS multi-parameter index system was established and the critical values of each index were estimated based on the normalized multi-information warning model of coal-rock dynamic failure. This index system includes bursting strain energy (BSE) index, time-space-magnitude independent information (TSMII) indices and time-space-magnitude compound information (TSMCI) indices. On the basis of this multi-parameter index system, a comprehensive analysis was conducted via introducing the R-value scoring method to calculate the weights of each index. To calibrate the multi-parameter index system and the associated comprehensive analysis, the weights of each index were first confirmed using historical MS data occurred in LW 402102 of Hujiahe Coal Mine (China) over a period of four months. This calibrated comprehensive analysis of MS multi-parameter index system was then applied to pre-warn the occurrence of a subsequent rock burst incident in LW 402103. The results demonstrate that this multi-parameter index system combined with the comprehensive analysis are capable of quantitatively pre-warning rock burst risk. Keywords: Rock burst, Microseismic (MS) monitoring, Multi-parameter indices, Comprehensive early warnin

Statistical analysis of distribution patterns of coal seams in fold zones in Northwest China

The mechanisms for rock bursts occurrences in fold zones are complex, and the redistribution of in-situ stresses is closely related to the complexity of the structures. Analysis of the geomorphology of fold structures and changes of coal thickness can help identify zones prone to rock bursts to improve safety and productivity in coal mines. This study investigated the distribution characteristics of fold structures in coal seams in fold zones in four mines in northwest China. Geometrical characteristics of fold structures in coal seams and changes of coal thickness were analysed, based on comprehensive evaluation indexes, such as the length–width ratio of folds, interlimb angle, ratio P1 of projected width of fold limbs to that of the hinge zone, curvature ratio P2, the maximum curvature and amplitude. The statistical analysis of the four coal mines shows that the length–width ratio of folds changed from 0.78 to 2.03 and the maximum curvature of cross sections of folds was less than 0.04. The curvature ratio of cross section of a fold in the structure was no more than 1.4 and the interlimb angles of cross sections of 89% of folds were larger than 150°. Gentle fold structures were dominant and the specific geological morphologies were domes or basins. The isopleth of coal thickness above the coal mines showed a fluctuation trend similar to the contour line of the floor of coal seams. The coal thickness in an anticline area was smaller than that in the neighboring syncline area. Therefore, the overall variation of coal thickness in the mining areas was likely to have a relation with the direction of the regional principal stress. Keywords: Rock burst, Fold structure, Distribution pattern, Changes of coal thickness, Principal stres

Evolution Characteristics of Seismic Detection Probability in Underground Mines and Its Application for Assessing Seismic Risks—A Case Study

Seismic hazards are typical mining hazards causing dynamic failure of coal and rock mass, which greatly threatens the safety of personnel and equipment. At present, various seismic analysis methods are used to assess seismic risks but their accuracy is significantly limited by the incompleteness of seismic data. The probability of detecting earthquakes (PDE) method has been proven as a powerful means for retrieving missed seismic events and enhancing the seismic data integrity in mines. However, to date, the reliability of the results of the PDE method has not been assessed and the highly integrated seismic data have not been linked with the actual hazard potential. To fill these gaps, this paper investigated the impacts of the seismic data volume used for calculation and the modification of the layout of sensors on the reliability and robustness of the PDE method. The event counts and seismic energy were compensated using the PDE method, correlated with strong seismic events. The results indicated that the compensated seismic data presented higher accuracy in locating future hazardous events than before. This research provides references on enhancing the performance of seismic analysing methods for seismic risk assessments

Layout Pattern of Small Panel and Large Coal Pillar for Rockburst Prevention and Water Control under Extra-Thick Water-Bearing Key Strata

There is a very thick water-bearing key strata above the coal seam in the Binchang mining area. When the mining scale is large, it easily breaks and leads to rockburst with a surge of water gushing in the panel. Adopting the layout pattern of a small panel and a large coal pillar can improve the stability of the main key strata, but at present, the research on the layout pattern of a small panel and a large coal pillar under extra-thick water-bearing key strata is still not perfect. Therefore, taking the second and third panels of a mine in Binchang as the engineering background, the width of the coal pillar and the mining scale of the panel are optimized by means of theoretical analysis, field measurement, and numerical simulation to prevent rockburst and control water inflow. The results show: (1) through theoretical calculation, it is deduced that the critical width of instability of the isolated coal pillar in the current mining scale is 257 m, and the critical mining scale of breaking and instability of the main key strata in the third panel is 537 m; (2) considering the bearing capacity of the isolated coal pillar and the recovery rate of coal resources, the reasonable width of the isolated coal pillar is 210~270 m, and when the width is 200 m and 250 m, the reasonable mining scale of the third panel is 490~550 m and 640~700 m, respectively; (3) the field practice shows that the actual width of the coal pillar between the second and third panels is less than the reasonable width, and the stress concentration in the isolated coal pillar area is relatively high, so the roof deep hole blasting and large-diameter drilling in coal seam are adopted to relieve pressure. After taking pressure relief measures, the stress concentration in the isolated coal pillar area is effectively reduced, and the pressure relief effect is remarkable

Understanding the Mechanism of Strong Mining Tremors near the Goaf Area of Longwall Mining: A Case Study

Strong mining tremors (SMTs) frequently occur in super-thick strata near the goaf when mining. Since 2021, there have been three consecutive SMTs with magnitude greater than 2.0 in longwall 1208 of the Shilawusu Coal Mine. These SMTs caused mine production to be suspended for more than 290 days and affected over 100 households located on the shaking ground, and seriously threatened the safety of underground workers and restricted production capacity. Therefore, it is essential to investigate the occurrence mechanism of SMTs in super-thick strata goaf mining in order to understand the phenomenon, how the disaster of mining tremors occurs, and the prevention and control of mining tremor disasters. In this study, field observation, numerical analysis, and theoretical calculation were used to study the occurrence mechanism of three SMTs in the Shilawusu Coal Mine. The results show that the super-thick strata fracture induced by the SMTs is generally higher by one to three orders of magnitude in some of the source mechanical parameters compared to other mining tremors, and so is more likely to cause ground shaking. Field observations revealed that before and after the occurrence of SMTs, the maximum surface subsidence suddenly increased by about 0.1 m and showed a “stepped” increase, and the super-thick strata began to experience fractures. The following theoretical mechanics model of super-thick strata was established: at the goaf stage of mining, with the increase in the area of the hanging roof, the super-thick strata will experience initial and periodic fractures, which can easily induce SMTs. The relative moment tensor inversion method was used to calculate the source mechanism of SMTs, which was found to be caused by the tensile rupture resulting from the initial and periodic ruptures of super-thick strata, in addition to the shear rupture generated by the adjustment of unstable strata structures. As the mining continues on the longwall face, there is still a possibility of SMT occurrence. This paper provides some insights into the mechanism and prevention of SMT in underground coal mines

Focal Mechanism and Source Parameters Analysis of Mining-Induced Earthquakes Based on Relative Moment Tensor Inversion

Mining-induced earthquakes (MIEs) in underground coal mines have been a common phenomenon that easily triggers rock bursts, but the mechanism is not understood clearly. This research investigates the laws of focal mechanism and source parameters based on focal mechanism and source parameters analysis of MIEs in three frequent rock burst areas. The relative moment tensor inversion (MTI) method was introduced, and the way to construct the inversion matrix was modified. The minimum ray and source number conditions were calculated, and an optimized identification criterion for source rupture type was proposed. Results show that the geological structure, stress environment, and source horizon influence the focal mechanism. The tensile type sources can distribute in the roof and coal seam, while the shear types are primarily located in the coal seam. In the typical fold structure area, the difference in source rupture strength and stress adjustment between tensile and shear types is negligible, while the disturbance scale of tensile types is distinct. The shear types have higher apparent volume and seismic moment in the deep buried fault area but lower source energy. The apparent stress of the tensile types is higher than that of the shear types, representing that the stress concentration still exists in the roof after the MIEs, but the stress near the faults could be effectively released. In the high-stress roadway pillar area, the primary fracture of the coal pillar easily produces a continuous shear rupture along the dominant stress direction under the extrusion of the roof and floor. The source parameters (except apparent stress) of shear types are higher than tensile types and have higher dynamic risk. The results contribute to expanding the understanding of rock burst mechanisms and guide MIEs’ prevention