Research On Overburden Movement And Fracture Field Evolution Under The Coal Pillar In 8309 Working Face Of Tongxin Mine

Datong mining area is a typical double-system coal seam mining area in China.The upper Jurassic coal seam has been largely mined,leaving a large amount of coal pillar resources.Besides,a large number of harmful gases such as goaf water and CO accumulated in the gob area.With the exploitation of 3-5# extra thick coal seam in the carboniferous system,the overburden damage scope will be greatly increasing.The mining fracture will be different from the conventional mining situation,especially when the mining face is pushed to the area near the coal pillar,the mining fracture field will be further developed under the concentrated stress of the coal pillar.And once the mining fracture connects with the overlying goaf,it is very easy to induce the discharge disaster of water and gas.With the mining geological conditions of 8309 working face of Tongxin coal mine in Datong mining area as references,the approaches such as indoor experiment,analog simulation,numerical simulation and theoretical analysis were adopted to study the laws of overburden fracture and fracture field evolution under the coal pillar in the paper,and the main research results were shown as follows:(1)The main sandstone,conglomerate and argillaceous rocks account for 75.16%,17.29% and 7.55% respectively in the dual system of coal seam.The stratum is mainly comprised of thick and hard rocks,and their compressive strength is in the following order: fine sandstone > medium sandstone > siltstone > sandy mudstone > mudstone >coarse sandstone > gravel bearing coarse sandstone.There are two key layers in the overburden,the low-key layer and the high-key layer,which have an important influence on the overburden fracture and fracture field evolution.(2)The results of analog simulation and numerical simulation showed that the overlying coal pillar has no effect on fracture of the low-key layer,but has certain effect on the fracture of the high-key layer;the stratum fracture and fracture field development between the coal pillar and the high-key layer are the major layer for the water and gas discharge disaster.Under the function of concentrated load,the coal pillar forms a downward floor failure slip surface at both sides,and the upward main fracture channel is formed after the high-key layer is fractured,and then the connection between two fractures will lead to the water and gas discharge disaster;the wider the coal pillar is,the less likely the water and gas discharge disaster will occur.(3)The distinguishing conditions of dangerous area of water and gas discharge disaster under mining under coal pillar in the process of exploration.It was verified by field measurement that water and gas discharge disaster is not incurred under the coal pillar during the initial pressure period of 8309 working face;while when the coal pillar is 861 m away from the cutting point,it will greatly affect the safe mining of the face,as a small amount of gas leakage will occur.