Research On The Stability Of"Backfill Body-Immediate Roof"Combined Bearing Structure In Contructional Backfill Coal Mining

Backfill mining is an important green mining method for controlling overburden failure and surface subsidence because of its advantages of high recovery rate and protecting environment.However,backfill mining is not widely used in coal mine at present,the essential reason is the high cost,which is caused by that the volume of backfill materials is large,the workload of material transportation is large,the backfill technology is complex and inefficient.At the same time,backfill mining is also facing the challenge that a lot of underground space is wasted since a large number of backfill materials occupy the goaf of coal seam.Professor Feng Guorui of Taiyuan University of Technology puts forward the idea of constructional backfill mining?CBM?in view of the problems and challenges.CBM is a green mining method using the"backfill material of solid waste recycling"adapted to local conditions,the"constructional backfill body"scientifically analyzed,and the"constructural backfill technology and equipment"which are complete researched and developmented to adapt to the"tectonic stress and mining induced stress"and maximize using the"available underground space".Through establish a stable composite bearing structure of"backfill body-immediate roof",CBM can improve the bearing capacity of backfill body and exert the maximum self-bearing capacity of immediate roof,reduce the cost of backfill coal mining,and provide convenient conditions for the development and utilization the underground space of coal mine,which can effectively promote the further development of backfill mining in the field of coal mine.In view of this,the stability of the"backfill body-immediate roof"composite bearing structure is studied in this paper.The failure characteristics and bearing mechanism of unconfined backfill body and reinforced backfill column are analyzed through laboratory mechanical experiments,which provide the basis for the selection of mechanical parameters of constructional backfill body.The deformation characteristics of immediate roof under strip and column constructional backfill mining are analyzed by establishing the mechanical model of"backfill body-immediate roof"composite bearing structure,and the maximum backfill spacing is obtained according to the immediate roof breaking.The influence law of different factors on the stability of"backfill body-immediate roof"composite bearing structure is analyzed by numerical simulation experiment,and reasonable values of the factors are obtained.The control effect on the pressure appearance and strata movement of CBM is verified by similar simulation test,which verified the bearing effect of"backfill body-immediate roof"composite bearing structure.The main research contents and conclusions are as follows:?1?The uniaxial compression experiments of cube specimens of standard size?150 mm×150 mm×150 mm?and large size?800 mm×800 mm×800 mm?show that there is a core bearing area in the unconfined backfill body.In the compression process,the core bearing area evolves from elastic core area to residual plastic bearing area gradually.The start breaking stress of specimen surface is about 28%of uniaxial compressive strength?UCS?.The start breaking stress of core bearing area is about 64%of UCS.The maximum bearing capacity of the core bearing area is about 2 times of the UCS.The residual bearing capacity of the core bearing area is larger than 80%of the UCS of cubic specimens.The internal stress curves and internal deformation curves of unconfined backfill body are divided into four stages:slow growth stage,rapid growth stage,rapid decline stage and slow decline stage.There are three turning points of the four stages that can be used as the failure precursor piont,critical instability point and starting point of residual bearing capacity or deformation,respectively.?2?By comparing with the backfill column without stirrups,it is found that the generation and expansion of vertical cracks is limited and the shear strength of backfill column is improved by stirrups,and an effective bearing area is formed in the middle of the 800mm×800mm×1600 mm backfill column with stirrups in the failure process under uniaxial compression.The effective bearing area improves the compressive bearing capacity and ductility of backfill column,and prevents the sudden instability of backfill column effectively.It shows that the difference of internal stress is the fundamental reason for the instability of backfill columns,and the difference of internal deformation is the direct reason for the instability of backfill columns.The stability of backfill columns in constructional backfill engineering can be monitored by internal stress and deformation.?3?Combining elastic foundation theory,the finite beam model on Winkler elastic foundation of"backfill strip-immediate roof"composite bearing structure is established.The deformation characteristics of the immediate roof filled with backfill strip are analyzed through the initial parameter method.According to the engineering geological conditions of Xinyang Mine,the critical spacing of strip constructional backfill when immediate roof is broken in Xinyang Mine is 11.95 m with the strength of backfill body is 10 MPa and backfill rate is 51%.The effects of backfill spacing,backfill strip width and lacking distance of roof-contact on the stability of composite bearing structure are studied by FLAC^3D numerical simulation.The results show that it will cause tensile failure in the middle of the goaf and shear failure at the edge of the backfill strip when the spacing of backfill strips is larger than 5m.The stress concentration will occur at the edge of backfill strip when the width of backfill strip is more than 4 m,which will cause the shear failure of the top angle of backfill strip.The immediate roof will separate from the overlying strata at the edge of the goaf when the lacking distance of roof-contact is 200 mm.Comprehensive analysis shows that the reasonable spacing of backfill strip is 2⁵m,the optimum width of backfill strip is ahout 2 m and the lacking distance of roof-contact should be not more than 150mm.The similar simulation test shows that the strip costructional backfill has good structural stability and the bearing capacity of the immediate roof is obviously enhanced in Xinyang Mine when the width and spacing of backfill strips are 2.5m.The stable composite bearing structure of"backfill strip-immediate roof"is formed.?4?Combining elastic foundation theory,the medium thick plate model of Winkler elastic foundation of"backfill column-immediate roof"composite bearing structure is established.The deformation characteristics of the immediate roof filled with backfill column are analyzed based on the Hu Haichang's theory.According to the engineering geological conditions of Xinyang Mine,the critical spacing of column constructional backfill when immediate roof is broken in Xinyang Mine is 3.23 m with the strength of backfill body is 20 MPa and backfill rate is 25.45%.Further more,the effects of backfill spacing,backfill column width and lacking distance of roof-contact on the stability of composite bearing structure are studied by FLAC^3D numerical simulation.The results show that it will cause tensile failure in the middle of the goaf and shear failure at the edge of the backfill column when the spacing of backfill columns is larger than 3m.The stress concentration will occur at the edge of backfill column when the width of backfill column is more than 3 m,which will cause the shear failure of the top angle of backfill column.The roof and backfill column in middle of goaf are seriously damaged,and the composite bearing structure of“backfill column and immediate roof”loses stability when the lacking distance of roof-contact is 150 mm.Comprehensive analysis shows that the reasonable spacing of backfill column is 1³m,the optimum width of backfill column is about 2m and the lacking distance of roof-contact should be not more than 100mm.The similar simulation test shows that the column costructional backfill has good structural stability and the bearing capacity of the immediate roof is obviously enhanced in Xinyang Mine when the width and spacing of backfill columns are 2.5m.The stable composite bearing structure of"backfill column-immediate roof"is formed.