Study On Movement Of Surrounding Rock And Influence Of Mining Sequence In Mining Middle Abandoned Coal Seam With "Upper Piller-lower Longwall" Residual Areas

The whole layer of the abandoned coal seam in “Upper pillar-Lower longwall” composite mining areas reserves occur widely,objectively,in order to improve the recovery rate of coal resources,realize repeated mining in the composite residual mining area is an imperative topic.In the dual mining influence of “upper-lower” coal seam mining,the laws of upper coal floor movement,lower coal floor deformation and the stress distribution between the rock strata in “Upper pillar-Lower longwall” composite mining areas and the residual coal mining single mining area compared with the large difference,the existing research results are not applicable to the emergence of new issues.Less experience and the current central composite residual mining area whole layer of abandoned coal can be used for reference at home and abroad,there is no systematic theoretical research and complete technical solutions to the scientific guidance for safe production.Therefore,thorough and systematical study on the strata of "stress distribution-movement and deformation-structure evolution-failure characteristics" in mining middle abandoned coal seam with“Upper pillar-Lower longwall” composite mining areas research is urgent and important.The middle abandoned coal seam No.7 with “Upper pillar-Lower longwall”composite mining areas in Baijiazhuang coal mine,which belongs to Xishan Coal Electricity Group,is selected as the research object.The research methods include field investigation,theoretical analysis,similar simulation and numerical simulation.The influence of upper pillar-lower longwall mining onstress distribution,moving deformation and structure evolution are explored firstly.At the same time for this type of composite residual are in two different forms,analyzes the different effect of mining sequence for strata of middle residual coal,finally got the optimal mining sequence.The research on the middle layer of the abandoned coal safe and efficient mining provides theoretical basis to further enrich.The main research contents and results are as follows:(1)The goaf overburden is in the lower stress area after the lower coal seam mining,its final form is "unloading arch" effect,mainly concentrated in the lower layer between the middle strata,middle residual coal stope rock to a certain extent the pressure relief effect.The vertical stress variation law of overlying strata in longwall goaf is different,and the smaller the vertical distance of the lower coal seam and strata,the larger the peak value of supporting pressure in these strata.After a final steady state of lower coal seam mining,rock structure extended up to the upper coal seam,non fractured rock zone throughout the entire upper layer between layers,and from the central to the coal seam floor did not occur step broken,keep the macro continuity and integrity.According to the theoretical analysis of the damage range of the lower coal seam longwall mining in the vertical direction of the mining face,it can be seen that the exploitation of the lower coal seam has not caused damage to the middle coal seam.(2)When the upper coal seam use longwall pillar mining method,mining the band number more than 4,the influence of coal pillar pillar mining range formed basically stable,which shows that the lower rock damage range is no longer expanding;when the strip number is more than 5,pillar mining areas the lower strata "stress recovery" phenomenon disappeared.The pressure relief effect,basically stable.In the process of mining upper coal seam with pillar mining method,the central residual coal rock strata stress distribution in the horizontal direction stress increasing zone and stress decreasing zone appearalternately,and the vertical distance of strata and upper coal seam is smaller,which has greater the degree of stress concentration,the stress concentration phenomenon is significant in the middle coal seam.After mining upper coal seam with pillar mining method,the upper coal seam roof and floor rock layer is affected by the deformation showed distribution of high middle and low on both sides looks like wave.According to the theoretical analysis of the damage range of the coal seam in the upper coal seam mining,it can be seen that the residual coal in the middle is in the range of damage,which caused by the upper coal seam mining,the middle coal seam roof needs real time monitoring.(3)Mining the middle of the abandoned coal seam caused influence about the stress distribution of coal seam,as follows: mining the middle coal seam different distance have different pressure relief effect,different rock layers and the vertical stress is still in "wave" distribution.It can be reflected from the displacement data,central coal seam roof subsidence reaches the maximum value of the position is located in the upper coal seam goaf center column axis position.The process of mining middle coal seam,the plastic zone is mainly distributed in the upper layer and lower layer of rock between the rock lithology soft strata,in the stope roof the plastic deformation occurs significantly,its basic damage range throughout the upper layer of rock.(4)Through the analysis of mining upper and lower coal seam caused influence to the middle coal seam about stress distribution,movement and deformation,damage range..movement and deformation-structure evolution under the condition of two different mining sequence.the conclusion can be drawn in such engineering problems,optimal mining sequence is “firstly mining upper coal seam-secondly mining lower coal seam-finally mining middle coal seam”.