| Taken the entry driving along gob-side of fully-mechanized top coal caving face in extra-thick coal seam as the research object,the two key problems about reasonable narrow space-time arrangement of roadway driving along gob-side and asymmetric deformation control of surrounding rock are systematically studied by research methods,such as site survey,laboratory experiment,theoretical analysis,and simulation calculation etc.Based on the maintenance difficulty of roadway with 30 m wide pillar in Madaotou coal mine,the necessity and difficulty of research on gob-side roadway with narrow pillar in extra-thick coal seam are put forward.The structure characteristics of overburden,rational layout time of gob-side roadway,asymmetric failure mechanism and control mechanism of gob-side roadway in extra-thick coal seam are thoroughly studied.Finally,field engineering practice is carried out.The main conclusions are as follows:(1)Based on the results of roof strata structure observation and physical mechanics properties test,it is found that coal seam with a uniaxial compressive strength of 11 MPa,developed fissures,multi-layers thin gangue and poor integrity,belongs to unstable surrounding rock.The roof sandstone with a uniaxial compressive strength of 79.4 MPa,relatively complete,hard and thick,belongs to stable surrounding rock.(2)Strong abnormal pressure appeared in the roadway with 30m wide pillar,characterized by serious subsidence in roof,heavy in floor and severe contraction in two ribs.The support structures were broken and twisted.The numerical simulation analysis showed that the peak stress of the coal pillar increased along with the thickness of the coal seam,and gradually transferred to the side of roadway.When the thickness of coal seam reached 15m,the roadway was in the area where the peak abutment pressure is severely affected,and serious plastic damage occurred around the roadway.(3)For fully mechanized top coal caving face in extra-thick coal seam,the below key strata above the gob cannot form the articulated rock beam structure,but form the structure of "cantilever beam+articulated rock beam".The mechanical model of lateral cantilever beam structure is established to calculate that the breaking position of cantilever beam structure is 16.6 m away from the gob coal face.The distribution range of internal and external stress field is determined.(4)Overburden structure above gob is closely related to coal seam thickness.When coal seam thickness is 5-10 m,stable "articulated rock beam" structure is formed above gob.When coal seam thickness is 15-20 m,the structure of "cantilever beam+articulated rock beam" is formed above gob.With the increase of coal seam thickness,the damage scope of overburden is larger,and the stability time of overburden activity is longer.(5)The movement process of overburden structure is divided into four stages,and the dynamic distribution characteristics of lateral abutment pressure in different stages are obtained.It is considered that the gob-side roadway can be in a relatively stable overburden structure and stress environment during the stable stage of abutment pressure.The abutment pressure range,peak distribution characteristics,stability time and dynamic response law of surrounding rock deformation under the influence of overburden structure movement in gob are obtained by field measurement method.Combined with solution value of time function equation from fracture cantilever beam movement to the stability,the earliest reasonable driving time of gob-side roadway is determined.(6)The bearing status of narrow pillar in extra-thick coal seam is obtained by analysis.It is considered that the plastic bearing zone in narrow coal pillar is the precondition to ensure the stability of narrow coal pillar.The occupancy rate of plastic bearing zone in different coal pillar widths and the deformation law of roadway surrounding rock are compared and analyzed.Combined with the specific geological condition of gob-side roadway,the characteristics of internal and external stress fields,the control of secondary disasters,the final width of narrow coal pillar is sured comprehensively.(7)During driving,the distortion energy release and transfer caused by decrease in bearing capacity of coal pillar makes roof on coal pillar side be in a low distortion energy state,while roof on solid coal side has a high distortion energy due to slight damage.The asymmetric distribution of distortion energy in roof is characterized by "high on solid coal side and low on coal pillar side”.(8)During mining,affected by the secondary rotary and subsidence of the key stratum,the deformation and damage degree of coal pillar and roof on coal pillar side are further intensified.The deviating stress is moved to the deep area of the solid coal side.As a result,the asymmetry between two sides is more significant.(9)In the process of mining,the average subsidence of roof on coal pillar side is obviously larger than roof on solid coal side.The maximum subsidence is located at about 500mm away from central axis of the roadway.The horizontal displacement of roof in different layers is obviously different.(10)Based on the principle of orthogonal test,the test scheme about influencing factors of roof asymmetric deformation is designed.The key influencing factors of roof subsidence and eccentricity are simulated and analyzed.The reciprocal feedbacks between these factors and roofasymmetric deformation are described in detail.(11)Aimed at the difference of mechanical properties and stress distribution of gob-side roadway surrounding rock,a new type of asymmetric anchor cable truss support structure is put forward to realize asymmetric control of roof.The mechanical analysis of the new type anchor cable truss structure is carried out.and the expression of bending moment reduction along the width direction of roof is obtained.It is show that the bending moment reduction of roof from solid coal side to coal pillar side,first increases and then decreases,and the maximum bending moment reduction deviates to coal pillar side.(12)Combined with the geological and production conditions of 5211 roadway,the support parameters are designed,and the field control ideas are put forward.The asymmetric control system of the gob-side roadway with "anchor cable and channel steel composite structure+asymmetric anchor cable truss structure+high strength and large elongation anchor cable structure in the side" is formed.It could be seen that during driving and mining,the support effect of 5211 roadway is obvious,and the stability of narrow coal pillar was ensured.The overall deformation of surrounding rock is controlled well to meet requirements of mine safety production... |
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