Simulational And Experimental Study On The Development And Evolution Of Mining Overlying Strata Fracture Field

Affected by the mining effect,there form a fracture field in the overlying strata.These fractures are connected with each other,which can provide space for gas migration and storage of gas.In order to distinguish the gas enrichment area and gas migration channel and to solve the problem of gas disaster based on the distributional characteristics of fractures in the overlying strata,it is necessary to study the evolutional law of mining fracture field in the overlying strata.Using similar material physical experiment,FLAC3 D software and UDEC software respectively to the law of overlying strata movement and caving,stress field and fracture distribution characteristics.The main conclusions are as follows:(1)Using the physical similarity experiment to study the law of overlying strata movement and caving.The fracture evolutional process of “development-open-close” is analyzed with the separation rate curve,and the fracture evolutional process is divided into three stages: the single peak stage,the hump stage and the whole compaction stage.Using the digital photogrammetry technology to analyze the distributional characteristics of fractures in the overlying strata,the trapezoid fracture zone and the fracture compaction zone were obtained.In the mining process,the broken fracture near the side of open cut experienced the process of "developing-expansion".Due to the development of the broken fracture near the side of the working face,a new fracture will be developed along with the continuous advance of the excavation distance,and the broken fracture will experience two stages,the first stage(stopping period of excavation): "developing-expansion".The second stage(period of excavation): the dynamic process of "expansion-closing ".The fracture of overlying strata in the middle position of the mined area has experienced the process of "development-opening-closing".(2)Using the FLAC3 D software to study the evolutional law of the vertical stress.Due to the influence of mining,the pressure relief zone and stress concentration zone are formed in a certain range.When the mining distance is short,the "arch" pressure relief zone is formed in the overlying and underlying strata.With the increase of mining distance,the pressure relief range increase.When the roof contacted the floor,the transfer of force was formed in mined out area and the stress in the middle of mined area began to recover.Due to the supporting effect of coal wall on both sides of the working face,the stress transfer to both sides of the working face,and the stress concentration area is formed on both sides of the working face.According to the characteristics of vertical stress distribution,the stress evolution process of overlying and underlying strata can be divided into two stages:(a)Before touched the floor: Original stress area-Stress concentration area-Stress relief area;(b)After touched the floor: original stress area-stress concentration area-stress relief area-stress recovery area.(3)Using the UDEC software to study the fracture distribution characteristics.Analyzes the evolution process of “deformation-separation-instability collapse” in the overlying strata.When the mining distance is short,the simply supported beam is formed between the overlying strata and coal walls at both ends of the working face.There are fractures In the middle of overlying strata.With the advancing distance increasing,the strata separation is increasing.Once the strata limit is exceeded,the broken fractures were formed on the both sides of the working face,and the factures began to close along with the advancing distance in the middle of the mined area.