The objective of this paper is to study the behavior of a low thick and low depth coal seam and the overburden rock mass. The mining method is room and pillar in retreat and partial pillar recovery. The excavation met...The objective of this paper is to study the behavior of a low thick and low depth coal seam and the overburden rock mass. The mining method is room and pillar in retreat and partial pillar recovery. The excavation method is conventional drill and blast because of the small production. The partial pillar recovery is about 30% of the previous pillar size, 7 m × 7 m. The roof displacement was monitored during retreat operation; the surface movement was also monitored. The effect of the blasting vibration on the final pillar strength had been considered. Due to blasting, the pillar reduced about 20%. The consequence is more pillar deformation and roof vertical displacement. The pillar retreat and ground movement were simulated in a three-dimensional numerical model. This model was created to predict the surface subsidence and compare to the subsidence measured. This study showed that the remaining pillar and low seam reduce the subsidence that was predicted with conventional methods.展开更多
For decades, pillar recovery accounted for a quarter of all roof fall fatalities in underground coal mines.Studies showed that a miner on a pillar recovery section was at least three times more likely to be killed by ...For decades, pillar recovery accounted for a quarter of all roof fall fatalities in underground coal mines.Studies showed that a miner on a pillar recovery section was at least three times more likely to be killed by a roof fall than other coal miners. Since 2007, however, there has been just one fatal roof fall on a pillar line. This paper describes the process that resulted in this historic achievement. It covers both the key research findings and the ways in which those insights, beginning in the early 2000 s, were implemented in mining practice. One key finding was that safe pillar recovery requires both global and local stability.Global stability is addressed primarily through proper pillar design, and became a major focus after the2007 Crandall Canyon mine disaster. But the most significant improvements resulted from detailed studies that showed that local stability, defined as roof control in the immediate work area, could be achieved with three interventions:(1) leaving an engineered final stump, rather than extracting the entire pillar,(2) enhancing roof bolt support, particularly in intersections, and(3) increasing the use of mobile roof supports(MRS). A final component was an emphasis on better management of pillar recovery operations.This included a focus on worker positioning, as well as on the pillar and lift sequences, MRS operations,and hazard identification. As retreat mines have incorporated these elements into their roof control plans,it has become clear that pillar recovery is not ‘‘inherently unsafe." The paper concludes with a discussion of the challenges that remain, including the problems of rib falls and coal bursts.展开更多
Room-and-pillar mining with pillar recovery has historically been associated with more than 25% of all ground fall fatalities in underground coal mines in the United States.The risk of ground falls during pillar recov...Room-and-pillar mining with pillar recovery has historically been associated with more than 25% of all ground fall fatalities in underground coal mines in the United States.The risk of ground falls during pillar recovery increases in multiple-seam mining conditions.The hazards associated with pillar recovery in multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave.When pillar recovery is planned in multiple seams, it is critical to properly design the mining sequence and panel layout to minimize potential seam interaction.This paper addresses geotechnical considerations for concurrent pillar recovery in two coal seams with 21 m of interburden under about 305 m of depth of cover.The study finds that, for interburden thickness of 21 m, the multiple-seam mining influence zone in the lower seam is directly under the barrier pillar within about 30 m from the gob edge of the upper seam.The peak stress in the interburden transfers down at an angle of approximately 20°away from the gob, and the entries and crosscuts in the influence zone are subjected to elevated stress during development and retreat.The study also suggests that, for full pillar recovery in close-distance multiple-seam scenarios,it is optimal to superimpose the gobs in both seams, but it is not necessary to superimpose the pillars.If the entries and/or crosscuts in the lower seam are developed outside the gob line of the upper seam,additional roof and rib support needs to be considered to account for the elevated stress in the multiple-seam influence zone.展开更多
Many states rely upon the Pennsylvania 1957 Gas Well Pillar Study to evaluate the coal barrier surrounding gas wells.The study included 77 gas well failure cases that occurred in the Pittsburgh and Freeport coal seams...Many states rely upon the Pennsylvania 1957 Gas Well Pillar Study to evaluate the coal barrier surrounding gas wells.The study included 77 gas well failure cases that occurred in the Pittsburgh and Freeport coal seams over a 25-year span.At the time,coal was mined using the room-and-pillar mining method with full or partial pillar recovery,and square or rectangle pillars surrounding the gas wells were left to protect the wells.The study provided guidelines for pillar sizes under different overburden depths up to 213 m(700 ft).The 1957 study has also been used to determine gas well pillar sizes in longwall mines since longwall mining began in the 1970 s.The original study was developed for room-and-pillar mining and could be applied to gas wells in longwall chain pillars under shallow cover.However,under deep cover,severe deformations in gas wells have occurred in longwall chain pillars.Presently,with a better understanding of coal pillar mechanics,new insight into subsidence movements induced by retreat mining,and advances in numerical modeling,it has become both critically important and feasible to evaluate the adequacy of the 1957 study for longwall gas well pillars.In this paper,the data from the 1957 study is analyzed from a new perspective by considering various factors,including overburden depth,failure location,failure time,pillar safety factor(SF),and floor pressure.The pillar SF and floor pressure are calculated by considering abutment pressure induced by full pillar recovery.A statistical analysis is performed to find correlations between various factors and helps identify the most significant factors for the stability of gas wells influenced by retreat mining.Through analyzing the data from the 1957 study,the guidelines for gas well pillars in the 1957 study are evaluated for their adequacy for roomand-pillar mining and their applicability to longwall mining.Numerical modeling is used to model the stability of gas wells by quantifying the mining-induced stresses in gas well casings.Results of this stud展开更多
文摘The objective of this paper is to study the behavior of a low thick and low depth coal seam and the overburden rock mass. The mining method is room and pillar in retreat and partial pillar recovery. The excavation method is conventional drill and blast because of the small production. The partial pillar recovery is about 30% of the previous pillar size, 7 m × 7 m. The roof displacement was monitored during retreat operation; the surface movement was also monitored. The effect of the blasting vibration on the final pillar strength had been considered. Due to blasting, the pillar reduced about 20%. The consequence is more pillar deformation and roof vertical displacement. The pillar retreat and ground movement were simulated in a three-dimensional numerical model. This model was created to predict the surface subsidence and compare to the subsidence measured. This study showed that the remaining pillar and low seam reduce the subsidence that was predicted with conventional methods.
文摘For decades, pillar recovery accounted for a quarter of all roof fall fatalities in underground coal mines.Studies showed that a miner on a pillar recovery section was at least three times more likely to be killed by a roof fall than other coal miners. Since 2007, however, there has been just one fatal roof fall on a pillar line. This paper describes the process that resulted in this historic achievement. It covers both the key research findings and the ways in which those insights, beginning in the early 2000 s, were implemented in mining practice. One key finding was that safe pillar recovery requires both global and local stability.Global stability is addressed primarily through proper pillar design, and became a major focus after the2007 Crandall Canyon mine disaster. But the most significant improvements resulted from detailed studies that showed that local stability, defined as roof control in the immediate work area, could be achieved with three interventions:(1) leaving an engineered final stump, rather than extracting the entire pillar,(2) enhancing roof bolt support, particularly in intersections, and(3) increasing the use of mobile roof supports(MRS). A final component was an emphasis on better management of pillar recovery operations.This included a focus on worker positioning, as well as on the pillar and lift sequences, MRS operations,and hazard identification. As retreat mines have incorporated these elements into their roof control plans,it has become clear that pillar recovery is not ‘‘inherently unsafe." The paper concludes with a discussion of the challenges that remain, including the problems of rib falls and coal bursts.
文摘Room-and-pillar mining with pillar recovery has historically been associated with more than 25% of all ground fall fatalities in underground coal mines in the United States.The risk of ground falls during pillar recovery increases in multiple-seam mining conditions.The hazards associated with pillar recovery in multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave.When pillar recovery is planned in multiple seams, it is critical to properly design the mining sequence and panel layout to minimize potential seam interaction.This paper addresses geotechnical considerations for concurrent pillar recovery in two coal seams with 21 m of interburden under about 305 m of depth of cover.The study finds that, for interburden thickness of 21 m, the multiple-seam mining influence zone in the lower seam is directly under the barrier pillar within about 30 m from the gob edge of the upper seam.The peak stress in the interburden transfers down at an angle of approximately 20°away from the gob, and the entries and crosscuts in the influence zone are subjected to elevated stress during development and retreat.The study also suggests that, for full pillar recovery in close-distance multiple-seam scenarios,it is optimal to superimpose the gobs in both seams, but it is not necessary to superimpose the pillars.If the entries and/or crosscuts in the lower seam are developed outside the gob line of the upper seam,additional roof and rib support needs to be considered to account for the elevated stress in the multiple-seam influence zone.
文摘Many states rely upon the Pennsylvania 1957 Gas Well Pillar Study to evaluate the coal barrier surrounding gas wells.The study included 77 gas well failure cases that occurred in the Pittsburgh and Freeport coal seams over a 25-year span.At the time,coal was mined using the room-and-pillar mining method with full or partial pillar recovery,and square or rectangle pillars surrounding the gas wells were left to protect the wells.The study provided guidelines for pillar sizes under different overburden depths up to 213 m(700 ft).The 1957 study has also been used to determine gas well pillar sizes in longwall mines since longwall mining began in the 1970 s.The original study was developed for room-and-pillar mining and could be applied to gas wells in longwall chain pillars under shallow cover.However,under deep cover,severe deformations in gas wells have occurred in longwall chain pillars.Presently,with a better understanding of coal pillar mechanics,new insight into subsidence movements induced by retreat mining,and advances in numerical modeling,it has become both critically important and feasible to evaluate the adequacy of the 1957 study for longwall gas well pillars.In this paper,the data from the 1957 study is analyzed from a new perspective by considering various factors,including overburden depth,failure location,failure time,pillar safety factor(SF),and floor pressure.The pillar SF and floor pressure are calculated by considering abutment pressure induced by full pillar recovery.A statistical analysis is performed to find correlations between various factors and helps identify the most significant factors for the stability of gas wells influenced by retreat mining.Through analyzing the data from the 1957 study,the guidelines for gas well pillars in the 1957 study are evaluated for their adequacy for roomand-pillar mining and their applicability to longwall mining.Numerical modeling is used to model the stability of gas wells by quantifying the mining-induced stresses in gas well casings.Results of this stud
