Simulation Study On The Development Law Of Water Flowing Fracture Under The Combined Action Of Jurassic And Carboniferous Coal Seams

The common coal-bearing strata are Jurassic and Carboniferous-Permian strata in China.The Jurassic and Carboniferous coal seams exist simultaneously in the same mining area and the co-mining of Jurassic and carboniferous coal seams is rare and the coal seams in the Datong mining area have the characteristics of “Jurassic and Carboniferous coal seams coexist,multiple coal seams and hard roof”among them.After the mining of the Jurassic coal seam,the goaf and coal pillars were left,forming a large-scale mining fissure development zone.After the mining of the Carboniferous coal seam,the secondary fault of the overburden was caused.The mining conditions are more complicated than before because the failure characteristics of the overlying strata are coexisting in the formation of multiple structures formed by the joint action of the Jurassic and Carboniferous coal seams.In this paper,the development law of water flowing fracture under the combined action of mining of Jurassic and Carboniferous coal seam is revealed by means of material simulation and numerical simulation methods and compare and analyze with empirical formulas and field observation results which based on the analysis of mine hydrogeological conditions and roof rock structure of the mining of Jurassic and Carboniferous coal seam in Majiliang Coal Mine.Main achievements are as follows:(1)Through analyzing the hydrogeological and structural characteristics of the rock strata,it is concluded that the water content of Jurassic is weak,and only the water in the goaf may cause water damage to the no.3 coal seam mining.The overburden rock of the carboniferous coal seam shows the characteristics that the upper coal seam roof to the lower part of the shanxi formation of the Permian is easy to produce water flowing fracture,and the upper coal floor of the shanxi formation of the Permian to the Jurassic 14-2 is easy to produce deformation and not conducive to the expansion of water flowing fracture.At the same time,the upper water-insulating layer can play a good role in water isolation.(2)Through material simulation,it can be seen that with the advance of the working face and the periodic failure of overburden,when the mining distance of the working face is approximately equal to the width of the working face,the height of water flowing fracture is the largest.The water flowing fracture development height of no.14-2 coal seam is about 44~48m,and that of no.3 coal seam is about110~130m.From the model apparent resistivity distribution diagram and inversiondiagram,it can be seen that the model goaf has an obvious high resistance area,and it can be judged that the height of water flowing fracture is about 100 m.(3)Through the numerical simulation study,the mining of Jurassic coal seam will form a stress concentration area on both sides and in the middle of the goaf and transfer to the roof of the carboniferous coal seam,resulting in the increase of fracture height and density in the mining process of the carboniferous coal seam.When the Jurassic coal seam is fully mined,the water flowing fracture height is 135 m.When the Jurassic coal seam is mined in half,the water flowing fracture height is 140 m.When only no.3 coal is mined,the water flowing fracture height is 120 m.After the mining of no.3 coal seam in carboniferous system,secondary subsidence.The maximum secondary subsidence of no.14-2 coal overburden is 1.81 m,which will aggravate the expansion of ground fractures and surface subsidence.(4)Comprehensive analysis of material simulation and numerical simulation results that the Jurassic formation during mining of stress concentration will lead to the carboniferous coal bed mining cracks formed in the process of "closed",cause the Jurassic and carboniferous coal bed mining goaf secondary bending down,lead to the Jurassic coal seam by compaction in the overburden strata fracture "activation",in which the impact on the transverse crack is more noticeable,influence on the longitudinal crack is small.(5)According to hard rock strata in the calculation formula of the fracture height experience,in-situ observation,material simulation and numerical simulation methods such as comparative analysis,empirical formula of the water guide fracture height is small,field measurement and simulation study of water fractured zone height consistent,determine the carboniferous coal strata lead water crack height is 150 m,split ratio 24.27,corresponding to the measured coal seam thickness of 6.18 m,water fractured not through Jurassic carboniferous coal strata and goaf unicom,to ensure the mine safety production.In this paper,the study on the development of water flowing fracture under the joint action of Jurassic and Carboniferous coal seam mining has certain theoretical significance and practical value for the prevention and control of roof water damage in the study area.