Research On The Stress Relief And Fissure Evolution And Seepage Flow Characteristics For The Lower Protective Seam Mining With Large Interburden

Understanding gas issues from the mining point of view, based on key strata theory,combining stress fields, fissure fields and gas laws during the mining process, attainingsimultaneous extraction of coal and gas is a hot topic of mining disciplinary research. Thispaper, with coal seam being high gassy and low permeability, applying theoretic analysis,conducting laboratory testing, utilizing physical modeling and numerical simulation andengaged with on-site measuring, has systematically and comprehensively studied the laws oflower protective seam regarding fully mining-induced destressed height, coal and rockpermeability characteristics under various stress conditions, ultra distance mining inducedstresses of upper protected seam, fissure evolution and gas seepage flow (permeability) law,thereby having brought forward protection-range parameters of destressed ultra upperprotective seam as well as principles and methodology of destressed gas drainage andpermeability enhancement completed by destressing in the coal mine. This research hasenriched the theory of mining lower protective seam, has provided theorem for ascertainingextraction scope for ultra distance lower protective as well as destressed gas drainage. Thispaper has achieved research results and innovation as follows:First, expansion ratio3%was regarded the critical value for fully destressed coal, thustheoretically concluding the methodology for mining the lower seam with respect to fullydestrssed height. The key strata location and mining height turned out the paramountparameters that affect the maximum expansion and angle of relief.Second, the permeability experiment which contains coal samples with gas shows that,permeability of coal sample is closely related to rupturing process while being lagged behind.Simulation of hydrostatic pressure less than20MPa lower protective seam mining, when thevertical stress reaches around30MPa unloading, the maximum permeability of coal samplewould increase only36%, indicating that the seam destressing and fissure evolution areimportant factors affecting gas flow.Third, jointly utilising physical models and numerical simulation, detressing distributionmodel and destressing protection range have been obtained for large distance lower protective seam, namely, downhill angle of relief,73degrees, uphill angle of relief,76degrees, angle ofrelief in stike,63-76degrees and the maximum destressed height being over160m, therebyproviding a basis for coal and gas outburst control and management in terms of lowerprotective seam mining at a large distance.