Study On Mechanism And Control Of The Support Crushing Disaster Caused By Interactive Failure Of Upper Residual Pillars In Shallow Multiple Coal Seams

After many years of mixed mining in the upper coal seam of Shendong Coalfield,a large number of upper residual pillars were formed,which is hidden danger of hydraulic support crushing during the working face advance in the lower coal seam.Many scholars studied the mechanism of hydraulic support crushing when the working face is advanced from barrier coal pillars to the fully mechanized goaf or excavated under the bord-and-pillar mining gob,but there are few reports on the mechanism of hydraulic support crushing from barrier coal pillars to the room-pillar type goaf.This study took Huoluowan Coal Mine in Shendong Coalfield as an typical engineering case,field measurement,mechanical calculation,similarity simulation and numerical modeling were used to study the mechanism and control of the support crushing disaster as strata caves caused by interactive failure of upper residual pillars.Finally,a new roof-cutting technology by hydraulic fracturing with super-deep drillhole through upper coal pillar was put forward.(1)The mining pressure characteristics of lower coal seam working face in shallow multiple coal seams excavated under the upper residual strip coal pillar,fully mechanized goaf,unmined coal seam and bord-and-pillar mining gob were compared analyse.The results show that the hydraulic support working resistance increases quickly with the proportion of 87.94% which exceeds the normal working resistance by35 MPa during mining out of the upper strip coal pillar.The roof activity is violent and easy to cause the support crushing disaster.(2)Considering the lateral transfer effect of overburden load,a new method based on pressure arch theory was proposed to calculate the stress of room coal pillar,which overcomes the disadvantage of the result of the pillar stress is equal in bord-and-pillar mining gob by conventional method of "tributary area theory".It is concluded that the stress of room pillar near the barrier pillar is greater than that in the middle part of goaf.The stability changes of upper residual barrier coal pillar and room pillar under the influence of lower coal seam mining was studied,and the mechanism of the interactive failure of upper residual pillars in the process of combined bearing while mining out of these pillars was revealed.(3)According to the characteristics of the roof supported by residual pillars,the continuous beam model with multi-support and multi-span was established.The influence law of the lower coal seam mining on the stability of the hard roof of the upper coal seam was studied.It is revealed that the mechanism of hydraulic support crushing disaster caused by the upper hard rock strata advanced fractured in a large range was,which is resulted by combined fracture of the hard roof supported by the upper residual pillars and lower hard roof in the shallow coal seam group.(4)A physical similarity modelling on interactive failure of upper residual pillars was conducted,the experiment shows that the working face has the mining pressure characteristics of "lower hard roof periodic weighting-upper hard roof dynamic weighting-concentrated coal pillar static loading pressurization" when mining out of the upper coal pillars,the residual barrier pillar and room pillar were interactive failure under the action of high abutment stress,the upper hard stratum was advanced broken on the position of inner side of room pillar.The long cantilever of upper hard stratum the upper controlled strata were rotating and sinking synchronously,leading to the shear failure of the interlayer strata in the upper of the 3-1 coal seam,and the strong dynamic stress is easy to cause the hydraulic support crushing disaster.(5)Using the modified UDEC-Trigon model to analyze the influence of cantilever length of higher hard strata,coal seam mining height and interlayer hard strata thickness on interlayer rock shear damage.The cantilever length of higher hard strata and mining height are positively correlated with the shear damage of interbedded strata.It is necessary to reduce the cantilever length of the higher hard strata to reduce the dynamic impact of strata caving when mining out of the upper residual pillars.(6)A innovative roof-cutting technology by hydraulic fracturing with super-deep drillhole through upper coal pillar was developed,and an integrated craft of "preexploration ? precise measuring point ? super-deep drilling through upper coal pillar ? inclinometer correction ? multiple fracturing in one drillhole ? borehole observation for evaluation " was put forward and field trials was carried out.According to the measured results and the theoretical analysis,the accuracy of the mechanism of the support crushing disaster caused by the interactive failure of upper residual pillars while mining out of these pillars was proved.There are 103 pictures,20 tables and 194 references...