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With the increasing depth of mining, the disasters of water inrush above confined aquifers tends to increase annually, which seriously threatens the safe mining of coal seam, especially the extra-thick coal seam floor in deep mining. This research is to study the characteristics of deformation and failure of extra-thick coal seam floor in deep mining, the field measurement is carried out by the distributed fiber optic sensing technology (BOTDR) and the 3D resistivity method of studying coal field in China. The axial strain curves and resistivity distribution characteristics of the mining face at different depth under the coal seam floor are obtained by extending the working face. Through the comprehensive comparison and analysis of monitoring data in the monitoring boreholes, the characteristics of deformation and failure of extra-thick coal seam floor in deep mining can be ascertained. The results indicate that the comprehensive monitoring technology can effectively distinguish the characteristics of stress excess, distortion and failure of rock during the process of the working face mining. The influence range of mining advance stress is about 36 m, the failure depth of coal seam floor is about 12 m and the depth of mining disturbance deformation is about 26 m. The formation of the deformation and failure of the coal seam floor have obvious stratification, which are mainly influenced by the formation structure and the lithology of the rock strata. The comprehensive monitoring technology can effectively capture the fine features of the deformation and failure of the coal seam floor, which is valuable for extra-thick coal seam floor in deep mining. • In this paper, the distributed fiber optic sensing technology is used for the first time to monitor the deformation and failure of the coal seam floor. • The coal seam studied in the paper is an extra-thick coal seam and the buried depth of the coal seam is large, belonging to the extra-thick coal seam in deep mining. • The high frequency of data acquisition and the long duration of acquisition, so as to realize the dynamic monitoring research on the deformation and failure of extra-thick coal seam floor in deep mining. [ABSTRACT FROM AUTHOR]

Mining-induced fault water inrush has become the most critical disaster-causing factor of mine water disasters in mines in recent years. In near-fault coal mining, the water flowing fractures caused by mining in the roof (floor) of a coal seam may connect the fault fracture zone and induce fault water inrush. The most effective way to prevent fault water damage is to set waterproof coal (rock) pillars. However, the width of coal (rock) pillars was mainly calculated by empirical formulas in the past, and the calculated results were quite different from the actual ones. This paper determines the safe water blocking zone and inelastic zone by introducing mine pressure and confined water pressure ("double pressure"). Then, the range of the mine pressure failure zone is determined by comprehensively considering the floor strata's peak stress propagation direction and the roof strata's collapse propagation angle. The hydraulic model of roof (floor) water inrush in near-fault coal mining under "double pressure" control is established. A new method for calculating the width of waterproof coal (rock) pillars in coal mining on large normal faults is derived by considering the nature of the fault, mine pressure, and confined water pressure. Finally, the research results are applied to the width calculation of waterproof coal (rock) pillars in the Jiaxiang branch fault of Xinhe Coal Mine, and the FLAC3D software is used to conduct numerical simulation and check the calculation of the pillars' width of coal (rock) under the influence of mining. The formation mode of fracture channels in the roof (floor) of a coal seam in near-fault mining is proposed, and the failure theory of surrounding rock in near-fault mining is perfected. The research results will effectively guide the layout of the near-fault working face on the Ordovician limestone confined aquifer in North China Coalfield, liberate a large number of resources threatened by fault water damage, and provide a theoretical basis for the prevention and control of mine fault water damage under other similar geological conditions. • Fault water inrush is the result of the combined action of mine pressure and confined water pressure. • A new method for calculating the width of fault waterproof coal (rock) pillars. • The formation pattern of water inrush channel in near-fault mining under high confined water. [ABSTRACT FROM AUTHOR]

The sustainable and clean mining of coal is essential for Australia and the world as coal is a key energy source. However, with the increase of mining depth, many coalmines are facing potential coal burst hazards as deep mining always associated with high gravitational stress and complicated geology. More recently, the coal burst risk is highlighted by accidents happened at Austar and Appin coalmines in Australia. Assumedly due to long time mining history with relatively shallow mining depth, coalmines in Australia has no coal burst history and corresponding risk controlling plans, technics and equipment. This paper proposes a novel method for coal burst risk prediction based on fractal dimension analysis of AE spatial distribution. Besides, this paper introduces the mathematical analysis method of fractal dimension based on dimension calculation formula and MATLAB coding. Finally, obvious fractal dimension decrease of AE spatial distribution is observed in experimental study of coal samples with high burst propensity, which promises the feasibility of coal bursts prediction through AE monitoring. • Novel prediction method for coal burst risk mitigation. • Mathematical analysis method of fractal dimension based on MATLAB coding. • Experimental verification of fractal dimension decrease before coal failure. [ABSTRACT FROM AUTHOR]

Deep mining commonly causes redistribution of the stress-strain field for coal seam floor strata. The study of this distribution law was of great significance to analyzing the characteristics of floor deformation and failure and predicting the risk of floor water inrush. In this paper, the damage of a coal seam floor with strong water pressure in the Junggar Basin in western China was examined, and the stress analytical formula for every point of floor confined aquifer was constructed using a stress model in the direction of coal seam strike. Combined with Mohr-Coulomb failure criterion, the maximum depth of floor failure was 17.5 m. According to geological and stratigraphic data of the stope floor, a 3D numerical model of fluid-structure coupling was built. Simulation results showed that under a 1.5 MPa water pressure, the maximum floor failure depth was 18 m. The internal stress of the coal-rock interface and soft rock increases significantly, and the weak coal seam intensifies the transfer of stress and strain at depth to some extent. Water pressure had a certain influence on floor deformation and failure. The joint simulation results of stress-seepage-fracture were closer to the real stope measurements. In the boreholes, two types of sensors, distributed fiber and electrodes were arranged. When the coal floor broke, the compressive strain increased with a peak of 4152 με. Meanwhile, the apparent resistivity of the rock seam reached 500–700 Ω•m. The comprehensive analysis showed that: the floor failure depth in the monitoring area of the 61,303 working face was −18 m, and the floor failure disturbance depth was −35 m. The results presented in this contribution can provide reference for future examinations of coal and rock interbedded floor failure over deep confined water. • Study on mining failure of deep interbedded coal and rock floor. • The analytical formula of stress at any point of floor is derived. • The influence of weak coal seam on stress transfer is studied. • Combined application of borehole strain and apparent resistivity imaging. [ABSTRACT FROM AUTHOR]

A better understanding of geological structures, stress regimes, and rock burst risks around longwall mining panels can allow for higher extraction efficiency with reduced safety concerns. In this paper, the stress change of rock mass was first examined by using ultrasonic technique into laboratory-scale rock samples. Subsequently, the active and passive seismic velocity tomograms were simultaneously applied into two study cases with field-scale. Similar characteristics can be found between the active and passive tomography results. More specifically, in the first case, a geological discontinuity was clearly indicated by a linear image in both active and passive seismic tomography results. The results of the second case suggest that seismic tomography can be used to infer stress redistribution, and assess rock burst hazard or locate high-seismicity zones. Ultimately, comparisons have been made between the results of active and passive seismic tomography. Active tomography is found to be better applied in accurately detecting stress distribution and geological structures prior to the extraction of longwall panels, while passive tomography has advantages in continuously monitoring the stress changes and assessing rock burst potential during the mining of longwall panels. This study is expected to increase the safety and efficiency of the underground mining. [ABSTRACT FROM AUTHOR]

Abstract: Prediction of potential methane emission pathways from various sources into active mine workings or sealed gobs from longwall overburden is important for controlling methane and for improving mining safety. The aim of this paper is to infer strata separation intervals and thus gas emission pathways from standard well log data. The proposed technique was applied to well logs acquired through the Mary Lee/Blue Creek coal seam of the Upper Pottsville Formation in the Black Warrior Basin, Alabama, using well logs from a series of boreholes aligned along a nearly linear profile. For this purpose, continuous wavelet transform (CWT) of digitized gamma well logs was performed by using Mexican hat and Morlet, as the mother wavelets, to identify potential discontinuities in the signal. Pointwise Hölder exponents (PHE) of gamma logs were also computed using the generalized quadratic variations (GQV) method to identify the location and strength of singularities of well log signals as a complementary analysis. PHEs and wavelet coefficients were analyzed to find the locations of singularities along the logs. Using the well logs in this study, locations of predicted singularities were used as indicators in single normal equation simulation (SNESIM) to generate equi-probable realizations of potential strata separation intervals. Horizontal and vertical variograms of realizations were then analyzed and compared with those of indicator data and training image (TI) data using the Kruskal–Wallis test. A sum of squared differences was employed to select the most probable realization representing the locations of potential strata separations and methane flow paths. Results indicated that singularities located in well log signals reliably correlated with strata transitions or discontinuities within the strata. Geostatistical simulation of these discontinuities provided information about the location and extents of the continuous channels that may form during mining. If there is a gas source within their zone of influence, paths may develop and allow methane movement towards sealed or active gobs under pressure differentials. Knowledge gained from this research will better prepare mine operations for potential methane inflows, thus improving mine safety. [Copyright &y& Elsevier]

Mining-induced seismicity is one of the most dangerous physical phenomenon's accompanying coal and ore extraction in underground mines and often resulting in rock burst. In many cases, this seismicity can be grouped together, forming spatial clusters of events related to some structures in mines such as faults or pillars. However, as shown in this paper, the usual seismicity in coal mines is mainly related to current mining process and there are no distinct spatial or temporal groups of seismic events, but rather they form continuous cloud of events related to mining activity. Therefore, we introduce new spatio-temporal metric together with Ward's minimum variance method to obtain hierarchical clustering analysis of mining-induced seismicity. Constructing metric in this way made it possible to assess seismic hazard concurrently in space and time. The clustering method is utilized to a case study of mining-induced seismicity from coal mine with high seismic hazard and where high energy mainshock occurred. In each cluster, magnitude distribution of seismic activity is calculated, showing in some cases, a significant departure from Gutenberg-Richter distribution of seismic events, which can be important in seismic hazard analysis. As a measure of accuracy of each cluster locations we have introduced bootstrapping procedure. We have shown that location errors of seismic activity affect each cluster center location of the order of a few tens of meters. The presented cluster analysis of mining seismicity provides new measures to determine group of seismic sources and high stressed areas in the rock mass in mines allowing for determination of seismic and rockburst hazard. • new clustering method developed to analyze mining seismicity. • seismic and rockburst hazard assessment in space and time concurrently. • analyzed clusters indicate the location of seismic sources in high stressed rocks. • influence of location errors of seismic events on the accuracy of cluster location. • cumulative energy distribution violate Gutenberg-Richter law. [ABSTRACT FROM AUTHOR]