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Underground mine structures like mine seals or built-in-place (BIP) refuge alternatives (RA) are exposed to specific loadings like explosions because of overlying and underlying strata conditions. Previous studies worked on the vertical loading response of structures like abutments from panel extraction but not the explosion resistance upon being subjected to these vertical strata loads. In this study, two steel-reinforced concrete wall designs as seal and RA applications are simulated to examine the performance and failure analysis under abutment-loading conditions during an explosion for a coal mine model using dynamic analysis by a distinct element code (3DEC). The available abutment monitoring data by longwall stoppings was used to estimate the abutment-loading conditions. The model setup was calibrated through two validation stages: (1) the concrete wall simulation with the explosion test of steel-reinforced concrete mine seal conducted by previous researchers; and (2) the stress change at the strata by the tailgate convergence-abutment-load multiplier curve. The calibrated models are subjected to various dynamic loadings to simulate explosions. The findings demonstrate that, in addition to magnitude, the pressure versus time mode of loading significantly influences the wall response. Instantaneous loading criteria can provide more cautious measurements to assess how well such walls operate in similar situations. Except for a few cases, the deformations after a single explosion are more pronounced for the effects of subsequent second and third explosions. As a result, the changes in deformation during subsequent explosions are relatively minor compared with the initial permanent deflections. [ABSTRACT FROM AUTHOR]

With the increase in the depth and intensity of coal mining, some coal mines encounter the dual disaster threat of gas outburst and rockburst, and the composite gas-rockburst disaster has become a scientific and engineering problem that deserves more attention, and the distribution of abutment stress is of great significance for revealing the mechanism of composite dynamic disasters. This paper focuses on the influence of gas pressure on the abutment stress, establishes a theoretical model of a circular roadway and obtains the influence of gas pressure changes on the tangential stress and the radius of the plastic zone. The influence characteristic of gas pressure on the front abutment of a longwall working face is simulated based on the typical engineering geological conditions of an outburst-rockburst mine. The results show that as the gas pressure decreases, the tangential stress around the roadway increases, and the radius of the plastic zone decreases, meaning that the peak tangential stress is closer to the roadway. The peak abutment stress is negatively correlated with the gas pressure, while the distance of the peak from working face is opposite, both showing a quadratic relationship. On-site monitoring shows that as gas extraction proceeds, the frequency of mining-induced tremors increases significantly. When there is a sharp upward trend of the mining-induced tremors along with the continuous decrease of gas pressure, the risk of rockburst is high. A gas-rockburst coordination prevention and control technology system is established in Haishiwan Coal Mine based on the relationship of gas pressure changes and rockburst danger. During the gas control process, the gas pressure-microseismicity frequency is used to predict the rockburst risk and optimize the starting position and strength of destress engineering to achieve collaborative prevention and control of outburst-rockburst.

Abstract During the processing of deep mining, revealing the distribution of abutment pressure is significant for controlling stability of the entry. In this study, the abutment pressure distribution of roof-cutting coalface was investigated by FLAC3D and self-developed flexible detection unit (FDU). In the numerical simulation, the double-yield model was built to analyze the goaf abutment pressure under the fracturing roofs to maintain entry (FRME). Compared with the non-fracturing side, the peak value of the advanced abutment pressure on the fracturing side is reduced by 19.29% on average, the influence range (span) increases by 30.78% and the distance between the peak value and the working face increases by 66.7%. The goaf abutment pressure within 23m near the cutting side is significantly higher than other areas along the dip. The FDU was employed in the coalface to record the change of advanced abutment stress. And the field measured results are in well agreement with the numerical results.

A FLAC3D based model has been developed to estimate the extent of coal wall spalling for a longwall face having a working depth of 406.5 m during the main weighting period. The results obtained from the model helped in characterizing two key phases of main weighting, namely, peak stress, and main fall period based on vertical stress concentration factor, percentage of yield zones, and extent of yielding ahead of the face. The maximum extent of coal wall spalling of 2 m was observed during the main fall period. Spalling is mainly concentrated near the upper-middle and the top sections of the face. [ABSTRACT FROM AUTHOR]

Abstract With the increase in cover depth of the coal seam, more and more rock bursts have occurred in deep coal roadways. The conventionally used passive preventive measures are not sufficient to reduce rock burst hazards in deep coal mines. To address this problem, this study firstly evaluated the abutment stress applied to the roadway surrounding rock by constructing an abutment‐stress‐transfer model after roadway excavation. Then, according to different positions of roadways in the coal seam, roadways are classified into three types: shallow‐buried roadway with hard surrounding coal/rock; deep‐buried roadway with soft surrounding coal/rock; and medium‐deep–buried roadway with medium‐hard surrounding coal/rock. Stress criterion and energy criterion for rock burst occurrence were proposed according to the three types of roadways. The rock bursts that occurred in stope 32 of Gucheng coal mine in China agree well with the calculation results. Finally, an active rock burst preventive approach (long borehole destress drilling) is suggested before intense supporting measures are adopted during roadway construction to prefracture the coal seam or competent strata. The efficiency of this method at different excavation stages is evaluated by examining borehole drill cuttings. The whole roadway was excavated without any rock burst after the borehole destress drilling method was implemented.

Abstract The calculation of abutment stress has always been the most important topic in safe underground mining and design. In this paper, based on the formation mechanism of abutment stress, combined with the mechanical properties of actual strata and the theory of elastic foundation beams, a structure model of overlying strata with an elastic foundation is established, and a new theoretical calculation method of abutment stress is proposed. The new calculation method takes into account the interaction of the overlying key strata and the effect on the transmission of the front abutment stress and then analyzes the stress patterns of multilayered key strata under the combined action of the bending moment, shear force, and nonuniform load and its transmission to the underlying strata. In addition, based on field observations of panel 10 305 in the Xinglongzhuang coalmine, the influence range, peak position, and variation trend of the front abutment stress before and after breaking of the overlying key strata are studied and verified. The results show that the influence range of the abutment stress remains unchanged after KS1 breaks (eg, from 0 to 180 m), and its peak position shifts forward from 8 to 19 m, while the influence range (0‐180 m) and its peak position (6‐8 m in front of coal wall) remain unchanged after KS2 breaks; the abutment stress decreases, and the maximum decrease is 3.9 MPa in the range of 0‐22 m ahead of the coal wall after KS1 breaks, while the maximum decrease in the abutment stress is 2.1 MPa in the range of 0‐71 m ahead of the coal wall after KS2 breaks. The results of the new calculation method are basically consistent with the field observations and the existing theoretical results, showing that the method has good adaptability and reliability and can provide a new theoretical basis for on‐site design and construction practices.

Abstract The principles of mining pressure in extra‐thick coal seams are different from those of medium‐thick and thick coal seams. Field investigation, laboratory test, and simulation analysis methods were used to study the correlation between the gob‐side entry (GSE) stability and coal pillar width during longwall top coal caving mining in extra‐thick coal seams. The test site is in Datong City, Shanxi Province, China. The mining pressure in N5209 tailgate with coal pillar of 30 m is severe based on the field investigations during the N5209 panel retreat. Global constitutive double‐yield and strain‐softening models were built to study the abutment pressures and plastic failure ranges of coal seams with different thicknesses and coal pillar widths. The simulation results show that the GSE with an 8‐m coal pillar is destressed and minimal GSE deformation occurs. In addition, the abutment stress in the coal pillar decreases with increasing coal seam thickness. The field tests indicate that the GSE with an 8‐m‐wide coal pillar is stable in extra‐thick coal seams. This study provides a reference for the coal pillar design in coal mines with similar geological and mining conditions.

The concentration of abutment pressure acting on coal seams induced by mining is a key factor to trigger rock burst. Understanding of abutment pressure or stress concentration is fundamental in preventing and controlling rock burst. The influence on abutment pressure fluctuation caused by the inhomogeneity of coal seams needs to be considered, but it is difficult to obtain by the present usual ways such as acoustic transmission, electromagnetic wave transmission, etc. In this article, the relationship between the amount of cuttings drilled in a coal seam and stress level was analyzed by considering the effect of drilling cutting expansion, and the drilling cutting test was carried out in Xinglongzhuang Coal Mine, Shandong Energy Ltd. It is found that the amount of cuttings drilled is positively related to the degree of stress concentration in both the plastic fracture zone and elastic zone. The amount of drilling cuttings is closely related to the roof weighting. In addition, the irregular fluctuation of drilling cuttings is an approximate map of distribution of stress concentration because of the non-uniformity of cracks and other defects in the coal seam. In order to meet the need of rock burst prevention by accurate pressure relief in high-stress zones, enough boreholes are needed. [ABSTRACT FROM AUTHOR]

As the cover depth of coalmines in China increases, large protective coal pillars are widely used to support the overlying strata during mining. After the longwall panels on both sides of the protective pillar are mined, the protective pillar becomes an island pillar. The high static and dynamic stress accumulated in a protective island pillar may cause overall rockburst hazards in the gateway between the protective island pillar and the longwall panel being mined. Based on a rockburst accident that occurred in the Longyun coalmine, Shandong province, China, this study elucidates the occurrence mechanism of this type of rockburst. Firstly, a mathematical model is proposed to evaluate static stress distribution in the protective coal pillar. The calculation results indicate that large elastic energy is accumulated in the large protective pillar and overall rockburst hazards is very likely to be induced. To reduce rockburst risk, rockburst index is defined and long-borehole de-stress measures were adopted in this study. Then, dynamic stress evolution in the protective island pillar during mining the longwall panel was analyzed using PFC2D. The results show that the peak stress in the protective pillar increased continuously from 28 to 67 MPa with the working face advancing, indicating high rockburst risk in the protective pillar. The results of the mathematical model and numerical analysis agreed well with the rockburst accident that occurred in Longyun coalmine. This study paves the way for the design of large protective pillars based on overall rockburst prevention in deep coalmines. [ABSTRACT FROM AUTHOR]

With the increase in cover depth of the coal seam, more and more rock bursts have occurred in deep coal roadways. The conventionally used passive preventive measures are not sufficient to reduce rock burst hazards in deep coal mines. To address this problem, this study firstly evaluated the abutment stress applied to the roadway surrounding rock by constructing an abutment‐stress‐transfer model after roadway excavation. Then, according to different positions of roadways in the coal seam, roadways are classified into three types: shallow‐buried roadway with hard surrounding coal/rock; deep‐buried roadway with soft surrounding coal/rock; and medium‐deep–buried roadway with medium‐hard surrounding coal/rock. Stress criterion and energy criterion for rock burst occurrence were proposed according to the three types of roadways. The rock bursts that occurred in stope 32 of Gucheng coal mine in China agree well with the calculation results. Finally, an active rock burst preventive approach (long borehole destress drilling) is suggested before intense supporting measures are adopted during roadway construction to prefracture the coal seam or competent strata. The efficiency of this method at different excavation stages is evaluated by examining borehole drill cuttings. The whole roadway was excavated without any rock burst after the borehole destress drilling method was implemented. [ABSTRACT FROM AUTHOR]

The calculation of abutment stress has always been the most important topic in safe underground mining and design. In this paper, based on the formation mechanism of abutment stress, combined with the mechanical properties of actual strata and the theory of elastic foundation beams, a structure model of overlying strata with an elastic foundation is established, and a new theoretical calculation method of abutment stress is proposed. The new calculation method takes into account the interaction of the overlying key strata and the effect on the transmission of the front abutment stress and then analyzes the stress patterns of multilayered key strata under the combined action of the bending moment, shear force, and nonuniform load and its transmission to the underlying strata. In addition, based on field observations of panel 10 305 in the Xinglongzhuang coalmine, the influence range, peak position, and variation trend of the front abutment stress before and after breaking of the overlying key strata are studied and verified. The results show that the influence range of the abutment stress remains unchanged after KS1 breaks (eg, from 0 to 180 m), and its peak position shifts forward from 8 to 19 m, while the influence range (0‐180 m) and its peak position (6‐8 m in front of coal wall) remain unchanged after KS2 breaks; the abutment stress decreases, and the maximum decrease is 3.9 MPa in the range of 0‐22 m ahead of the coal wall after KS1 breaks, while the maximum decrease in the abutment stress is 2.1 MPa in the range of 0‐71 m ahead of the coal wall after KS2 breaks. The results of the new calculation method are basically consistent with the field observations and the existing theoretical results, showing that the method has good adaptability and reliability and can provide a new theoretical basis for on‐site design and construction practices. [ABSTRACT FROM AUTHOR]

The principles of mining pressure in extra‐thick coal seams are different from those of medium‐thick and thick coal seams. Field investigation, laboratory test, and simulation analysis methods were used to study the correlation between the gob‐side entry (GSE) stability and coal pillar width during longwall top coal caving mining in extra‐thick coal seams. The test site is in Datong City, Shanxi Province, China. The mining pressure in N5209 tailgate with coal pillar of 30 m is severe based on the field investigations during the N5209 panel retreat. Global constitutive double‐yield and strain‐softening models were built to study the abutment pressures and plastic failure ranges of coal seams with different thicknesses and coal pillar widths. The simulation results show that the GSE with an 8‐m coal pillar is destressed and minimal GSE deformation occurs. In addition, the abutment stress in the coal pillar decreases with increasing coal seam thickness. The field tests indicate that the GSE with an 8‐m‐wide coal pillar is stable in extra‐thick coal seams. This study provides a reference for the coal pillar design in coal mines with similar geological and mining conditions. [ABSTRACT FROM AUTHOR]

Purpose: Three-dimensional finite element analysis was used to evaluate the effect of vertical and angular misfit in three-piece implant-supported screw-retained fixed prostheses on the biomechanical response in the peri-implant bone, implants, and prosthetic components. Materials and Methods: Four three-dimensional models were fabricated to represent a right posterior mandibular section with one implant in the region of the second premolar (2PM) and another in the region of the second molar (2M). The implants were splinted by a three-piece implant-supported metal-ceramic prosthesis and differed according to the type of misfit, as represented by four different models: Control = prosthesis with complete fit to the implants; UAM (unilateral angular misfit) = prosthesis presenting unilateral angular misfit of 100 ?m in the mesial region of the 2M; UVM (unilateral vertical misfit) = prosthesis presenting unilateral vertical misfit of 100 ?m in the mesial region of the 2M; and TVM (total vertical misfit) = prosthesis presenting total vertical misfit of 100 ?m in the platform of the framework in the 2M. A vertical load of 400 N was distributed and applied on 12 centric points by the software Ansys, ie, a vertical load of 150 N was applied to each molar in the prosthesis and a vertical load of 100 N was applied at the 2PM. Results: The stress values and distribution in peri-implant bone tissue were similar for all groups. The models with misfit exhibited different distribution patterns and increased stress magnitude in comparison to the control. The highest stress values in group UAM were observed in the implant body and retention screw. The groups UVM and TVM exhibited high stress values in the platform of the framework and the implant hexagon, respectively. Conclusions: The three types of misfit influenced the magnitude and distribution of stresses. The influence of misfit on peri-implant bone tissue was modest. Each type of misfit increased the stress values in different regions of the system. [ABSTRACT FROM AUTHOR]

The concentration of abutment pressure acting on coal seams induced by mining is a key factor to trigger rock burst. Understanding of abutment pressure or stress concentration is fundamental in preventing and controlling rock burst. The influence on abutment pressure fluctuation caused by the inhomogeneity of coal seams needs to be considered, but it is difficult to obtain by the present usual ways such as acoustic transmission, electromagnetic wave transmission, etc. In this article, the relationship between the amount of cuttings drilled in a coal seam and stress level was analyzed by considering the effect of drilling cutting expansion, and the drilling cutting test was carried out in Xinglongzhuang Coal Mine, Shandong Energy Ltd. It is found that the amount of cuttings drilled is positively related to the degree of stress concentration in both the plastic fracture zone and elastic zone. The amount of drilling cuttings is closely related to the roof weighting. In addition, the irregular fluctuation of drilling cuttings is an approximate map of distribution of stress concentration because of the non-uniformity of cracks and other defects in the coal seam. In order to meet the need of rock burst prevention by accurate pressure relief in high-stress zones, enough boreholes are needed.

Determining a suitable distance between two adjacent panels' working faces (DBTF) is an important issue that should be considered in underground mining methods. A suitable DBTF causes a decrease in stresses on the pillar, reduces the width of the pillar, and controls the ground subsidence. The main purpose of this study was to determine the suitable DBTF in shortwall mining of the Tabas central coal mine. For this purpose, at the beginning, an algorithm is presented for numerical modeling. In this algorithm, front abutment stresses first and then side abutment stresses are evaluated on the chain pillar. Numerical modeling is performed by finite-difference method software in different distances of P5 and P6 panels' working faces (5-45 m). The result of this research shows that the front abutment stress on the chain pillars is not different for a distance of 25 m or more. Moreover, by increasing the distance up to 25 m, the side abutment stress in the vicinity of the P5 panel face is decreased on the chain pillar and is increased on the chain pillar in the vicinity of the P6 panel's working face. However, when the DBTF increases to 30 m, the lowest side abutment stresses are applied on the chain pillars and then increased for greater distance. It can be concluded that a suitable DBTF in the Tabas central coal mine is 30 m. [ABSTRACT FROM AUTHOR]

Gas How in coal seams is a complex coupling process of coal deformation and gas migration. This study investigated the coupling mechanism between coal seam abutment stress and the flow rate of gas drainage based on the field monitoring of the abutment stress and the flow rate of gas drainage at the panel 3211 of Songzao Mine in Chongqing. The results reveal that the change of measured stress experiences a low-high-low process with the distance ahead working face and reaches its peak value around 15-25 m ahead working face. The coal seam can be divided into three stress zones; relief stress zone, abutment stress zone, and recovered stress zone. As the pore and fracture structure is affected by coal seam abutment stress, the flow rate of gas drainage increases in the relief stress zone, decreases in the abutment stress zone and remains stable in the recovered stress zone. A negative exponential formula based on the coupling mechanism between coal seam abutment stress and the flow rate of gas drainage is presented to calculate the flow rate of gas drainage. [ABSTRACT FROM AUTHOR]

Abstract: A numerical modeling approach is proposed for predicting the progressive caving behaviour of strata and performance of powered roof support in a given geo-mining and strata condition. The study suggests that face convergence slope of 75mm/m of face advance may be considered as the maximum permissible value for selection of optimal capacity support for safe working in a given strata condition. The study also reveals that en masse caving of strata is responsible for dynamic loading leading to collapse of supports if they are not designed to discharge the fluid at the desired rate. [Copyright &y& Elsevier]