| Most of the mining areas in Western China operate in terms of shallow mining techniques,while some mining areas such as the Hujirt mining area in Inner Mongolia have begun to work in terms of mining deep-buried coal resources.In the Hujirt mining area,there is a hard roof above the main coal seam,and most of the coal pillars in working face are designed to be 20–40m wide.As the mining depth increases,the existence of a hard roof,the influence of the operational layout,and the frequency and intensity of rockburst in the working face gradually increases,which poses an immense challenge to the development of mining deep-buried coal resources and represents a significant risk to the safety of workers.Therefore,we take the working face in the 3-1 coal seam whose buried depth is more than 600 m as the engineering background.The characteristics of rockbursts,triggering mechanisms,and blasting mitigating mechanisms in the working face with a hard roof and wide pillars were analyzed by taking actual field measurements,utilizing theoretical analysis,numerical simulation,laboratory testing,and practical verification of the results.The main findings are presented as follows:(1)The analysis of field monitoring data indicated that there are three types of 2 rockburst characteristic in the working face,namely small periodic strata behavior,large periodic strata behavior,and special strata behavior.In addition,according to the characteristics of the working face in the area where a rockburst occurs,working face with lateral suspended multilayer roof structure and working face with residual suspended roof structure in goaf.(2)The calculation models combining both the dynamic load of instability of the impact loading structure and the static equilibrium load of the impact bearing structure were determined,and the expressions of dynamic load energy for three periods were derived.Finally,the energy instability criterion for the rockburst bearing structure was obtained based on the results of the dynamic load model.(3)The numerical simulation revealed the stope structure evolution in the Menkeing Coal Mine,where three loading strata displaying different characteristic were found in LW3102.Through orthogonal simulation testing,the main control law of the change in each component of the working face structure on the stress distribution in the working face was determined.(4)Based on test results,the stress was reduced and the energy was released in sandstone samples after blasting,which means that blasting in the confined sandstone medium had the effect of inducing the release of energy and reducing the load.The effect was primarily accomplished by the dynamic blasting load and blast fracturing,and the stress reduction of the samples was found to be greater under higher load conditions,while the value was found to be smaller under lower load conditions.(5)A numerical calculation model that considers the influence of the blasting load was constructed.The influences of charge quantity,pressure conditions,and the changes in the blasting medium on blasting inducing energy release effect were analyzed.The law of energy release with the variable parameters and the inducing energy effect of the blasting load was verified effectively.(6)By analyzing the stress distribution around the working face with regard to different roof cutting borehole parameters,three specific types of roof pre-cutting hole parameters were optimized,including roof pre-cutting holes on a previously constructed roadway section,boreholes in the working face,and boreholes above the coal pillar.The field practice of deep-hole blasting in the roof was successfully performed,and the improvement in the effect of roof blasting on the stress environment and rockburst behavior of the working face was verified. |
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