Overburden Strata Movement And Ground Pressure Law Of Lower Seam In Close Seams

Close coal seams are widely distributed in China.Coal fields in China tend to have one or dozens of minable seams,with the interlayer distance ranging from tenths of meters to hundreds of meters,which is mostly mined in a descending way.The mining of upper coal seam has influences on the integrity and stability of its overlying strata,and causes the floor strata to damage to a certain degree.Therefore,the mining of lower seam will face shattered roof and abutment pressure caused by upper mined space and coal pillar.Thus,the movement and failure of overlying strata is different from that of single seam mining,and is apt to suffer from roof falling and caving,and pressurizing of hydraulic support.However,most research done by academic society in recent years is mainly focused on roadway layout and its support,and research on overlying strata movement and failure of lower seam in close seams and its influence on the working face and roadway is yet not clear.Hence,to realize safe and efficient production of close coal seams,it's necessary to study overlying strata structure and movement law durng the mining of close seams and its influence on the lower seam's rock pressure.This thesis chooses working face 25301 in Shaqu Coal Mine of Huajin Coking.inc as the research object,and floor damage of 4th seam,overlying strata structure of lower seam,loading of hydraulic support and overlying strata movement is systematically studied by lab test,field practice,theorogical analysis and numerical simulation.The main research content is as follows:(1)The floor damaging process of coal 3+4th and the repeated mining of coal 5th is analysised by theorogical analysis and numerical simulations,and the scope,shape,development process and depth of floor damage area is thoroughly studied.And the result shows that floor damage is caused by mining induced pressure,which lead the original fracture in floor strata to propagate and open,and the damage area is usually in inverted triangle-shaped or ladder-shaped.The result lay the foundation for the definition of close seams,mutual influence of coal seams and floor water invasion.(2)Interlayer structure for close coal seams is divided into three categories,ie,without basic roof,with basic roof of voussior beam and cantilever beam structure,based on the strength and thichness of interlayer rock layers.And the corresponding relationship of hydraulic support and surrounding rock model for each type of interlayer structure is established based on interlayer structure model and fracture failure characteristic.The working resistence of hydraulic support calculation equation is developed as well.(3)Overlying strata moving process during the mining of coal seam 3+4th and 5th is studied using UDEC numerical simulation system.And the result shows that the overlying strata of coal seam 5th have stable sandstone layer,which server as basic roof and breaks when it reaches its tensile strength as the working face advance.And in the meantime,the equilibrium structure formed by upper basic roof slide and collapse,causing influence on the rock pressure of lower working face.Therefore,when there exists tough basic roof structure,the lower working face is influenced not only by the failure of its basic roof and the movement of upper basic roof's hinged structure as well.(4)The result of upper working face ground pressure law monitoring shows that the magnitude and scope of abutment pressure is relatively minor,and the deformation of roadway during mining process is in the range of 30mm~50mm;Periodic weighting phenomenon of lower working face is not obvious,and have little influence over the stability of the working face.The working resistence of hydraulic supports is mostly between 12 to 32 MPa,ie,1772.6 to 4726.9k N,which is below its initial resistence,4653 k N,with great redundancy.And all these indicate that the ground pressure of lower working face is mild,and so as to the influence of upper seam's overburden structure movement.