| In the shallow seam of Western China,double roadways with wide pillars(greater than 20 m and less than 30 m)are adopted to protect roadways.Wide pillars cause a great waste of coal resources while ensuring safe mining.How to determine the reasonable width of coal pillars is an urgent problem to be solved.The decisive factor to solve the problem is the analysis and study of the overburden rock structure above the pillars.In this paper,the structure of coal pillar and its overlying strata in shallow coal seam is taken as the research object.Combining with the engineering practice of Fengjiata Coal Mine,the spatial structure of coal pillar overlying strata after shallow coal seam mining is analyzed by means of theoretical analysis,numerical simulation and field measurement,and the reasonable width of coal pillar for roadway protection is studied.The main research results are as follows:(1)Based on the discriminant theory of key strata,the main key strata and sub-key strata of overlying strata near Fengjiata 3203 working face are determined.Based on the "S-R" stability theory of masonry beam,the structural instability of overburden bedrock in 3203 working face of Fengjiata Coal Mine is discriminated.Subcritical bedrock structure will not rotate instability and slide instability.The main key bedrock will not rotate instability to form a stable masonry beam structure,and the space of overburden structure over coal pillar will form a small "F" structure.After shallow seam mining,according to the main influencing factors of spatial structure of overlying strata on coal pillars,it is divided into the main key strata cantilever beam structure(large "F" structure)and multi-roof combination "cantilever-articulated beam" structure(small "F" structure).The stress distribution law and roof movement of these two structures on coal seam floor and coal pillar are studied by numerical simulation method.Law.(2)By analyzing the mechanical model of roof cantilever beam structure(large "F" structure),it is concluded that the smaller the roof thickness of shallow seam,the shorter the extension length of roof cantilever beam structure and the smaller the overlying load of roof,the smaller the load transferred from roof to coal pillar,the less obvious the phenomenon of stress concentration in coal pillar,the better the stability of coal pillar,and the better the maintenance of mining roadway.The mechanical relationship between "cantilever-articulated beam"(small "F" structure)of multi-layer roof is analyzed,and the calculation formula of supporting load required to maintainthe stability of coal pillar is deduced.It is concluded that the supporting stress of controlling overburden strata in coal pillar mainly consists of three parts:(a)the weight of the block that forms the cantilever beam directly overburden strata above the coal pillar;(b)the interaction force between overburden strata of coal pillar.(c)The interaction force between the key strata of roof due to mutual extrusion and friction.(3)Based on the multi-layer "cantilever-articulated beam" structural mechanics model above the coal pillar,the support stress of the overlying strata in the coal pillar of Fengjiata 3203 working face is calculated to be 6.21 MPa,and the width of the coal pillar is greater than or equal to 7.82 M.Based on the analysis of spatial structure of overburden strata on coal pillars,the joint surface between key strata and adjacent strata below the numerical calculation model is established,which improves the traditional numerical calculation model and makes it closer to engineering practice.By means of numerical simulation,the stability of coal pillars,the stress distribution law of surrounding rock and the elastic-plastic zone distribution law during double roadway excavation,primary mining and secondary mining are analyzed,and the reasonable width of coal pillars should be 12.5m is obtained.(4)Taking 3203 working face of Fengjiata Coal Mine as geological background,industrial test was carried out.Based on the improved numerical calculation model and using the numerical simulation method,the control effect of supporting parameters of auxiliary haulage roadway and haulage roadway in 3203 working face on the surrounding rock of roadway is analyzed,and the final supporting scheme is determined.The monitoring data of 3203 auxiliary haulage roadway and 3203 haulage roadway are monitored,and the monitoring data of bolt hydraulic pillow,force-measuring bolt,surrounding rock of roadway and borehole peep data are analyzed and studied.The results show that the coal pillar width determined based on the research results in this paper can ensure the safe and efficient mining of the working face,and has a guiding role in the determination of coal pillars in other similar geological conditions. |
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