Overburden Strata Movement And Mine Pressure Regularity Of Gob-side Entry Retaining By Roof Cutting In Close Coal Seam

Gob-side entry retaining by roof-cutting and pressure-relief(GERRC),an emerging method of coal-pillar-free mining in recent years.It is an effective way to mining realize safe,efficient,and rescue valuable coal resources.In recent years,it has been successfully applied in many mines with different geological conditions.With the rapid development of coal mining technology,the mining intensity of coal is also increasing year by year.Thus,the coal resources with good occurrence conditions in many mines are gradually becoming scarce.Therefore,the research and application of GERRC in the close-coal seam are of great significance to the sustainable development of coal mining.Based on this,this paper comprehensively uses the methods of theoretical analysis,numerical simulation calculation,physical similarity simulation test,and industrial test to systematically study the method for determining the closecoal seams,explains the principle and key technologies of roof-cutting and roadway formation,reveals the control mechanism of surrounding rock for GERRC in the closecoal seam and clarifies the movement characteristics of the overlying rock and the mine pressure law of the GERRC in the close-coal seam.Meanwhile,the roof structure and mechanical stability mechanism of GERRC in close-coal seam are explored.The optimal parameters of the roof cutting are determined,and the corresponding surrounding rock control methods are put forward.Furthermore,the engineering tests are carried out in Taiping Coal Mine in Yunnan Province.The results are as follows:(1)The geological overview of the Taiping Coal Mine is introduced.Based on the limit equilibrium theory,the bearing characteristics of the coal pillar after the upper coal seam is mined are analyzed.The support stress function model of the coal pillar is established.The support stress distribution function model in the floor under the coal pillar is proposed.By using the dual indexes of the stress concentration coefficient and lateral pressure coefficient of the bottom floor under the coal pillar to judge the closecoal seam.Taking the engineering geological conditions of the Taiping Coal Mine as an example,the judgment results show that the stress concentration coefficient of the coal pillar floor of the upper coal seam can reach 1～2.5 at the lower coal seam,which is a typical short distance coal seam.(2)The movement process of the overlying strata in the close-coal seam by GERRC was analyzed.The control mechanism of the surrounding rock in the closecoal seam with GERRC was proposed.The results show that the subsidence of the basic roof strata and the height of the key layer is reduced.The displacement of the key layer of the upper coal seam is controlled.Thus,the coordinated control of the upper and lower key layers is realized.According to the roof structure characteristics of the close-coal seam with GERRC,the mechanical model of the composite structure of the upper "masonry beam" and lower "short arm beam" is established.The mechanical analysis and its stability conditions are proposed.The research shows that increasing the filling density of the mined-out area by the crushed gangue through reasonable roof cutting parameters is the key to the simultaneous stability of the roof composite structure of the lower "short arm beam" and the upper "masonry beam".(3)3DEC numerical simulation software is used to study the overburden deformation and stress distribution characteristics of the close-coal seam with GERRC.By comparing the parameters of roadway caving shape,overlying strata displacement,the stress distribution of entry surrounding rock,and secondary movement of the key layer of the upper coal seam when mining with GERRC and without GERRC,the control mechanism of the surrounding rock of close-coal seam with GERRC is verified.The results show that the roof-cutting increases the height of the caving zone utilizes the crushing and expansion characteristics of the caving gangue to support the interlayer basic roof.Thus,it reduces the subsidence of the interlayer basic roof and its overlying strata,eliminating the influence of the second movement of the stabilized structure of the upper coal seam on the surrounding rock of the roadway in the lower coal seam.Therefore,the coordinated control of the upper and lower key layers of the close coal seam mining with GERRC is realized.(4)Based on the engineering geological conditions of the Taiping Mine,a physical similarity model test of the close-coal seam with GERRC is carried out.The results show that after roof pre-splitting,the immediate roof on the cutting side of the seam preferentially collapses,then the collapsed gangue fills the goaf and supports the basic roof between layers.Therefore,the amount of subsidence of the basic roof between layers is reduced.The height of the key layer of the lower coal seam decreased to the position of the end of the cutting seam.Therefore,the stable structure of the upper coal seam is not affected by the mining disturbance of the lower coal seam.At the same time,the roof-cutting blocks the stress transfer path between the roof of the goaf and the "short arm beam".Under the load-bearing effect of the basic roof between layers,the roof of the entry forms a pressure relief area,which effectively increases the stability of the surrounding rock of the roadway.(5)Through stress monitoring and infrared thermal imaging technology,the stress evolution process of the surrounding rock of the close-coal seam with GERRC is analyzed.According to the stress evolution of the surrounding rock during the mining process,the surrounding rock of the roadway is divided into the advanced influence area,the dynamic pressure influence area,and the stable roadway area.Meanwhile,the corresponding mechanical model of entry surrounding rock is established to theoretically analyze the support resistance in the roadway.Provide theoretical guidance for the design of the support scheme.(6)Theoretical analysis of the bearing characteristics of the surrounding rock of the entry under different roof structures of close-coal seams with GERRC is carried out.The relationship between the roadway bearing capacity and roof cutting parameters under different roof structures is established.The results show that increasing the height of the roof-cutting can effectively increase the filling density of the gob with the gangue.Thereby increasing the support force of the gangue on the basic roof,and effectively improving the self-supporting performance of the upper and lower key layers.The roof-cutting angle mainly affects the load characteristics of the short-arm beam structure of the roadway.When the roof-cutting angle increases,the crushed and expanded gangue not only supports the basic top of the interlayer but also produces a normal support force on the roof-cutting seam surface of the short-arm beam structure.The deformation of the roadway roof is reduced.However,the increase of the roofcutting angle will increase the length and weight of the short arm beam structure,which is not conducive to the stability of the surrounding rock of the roadway.The optimal parameter of the roof cutting angle is finally determined to be 15°.(7)Aiming at the movement characteristics of the overlying rock in the close-coal seam with GERRC,the control strategy of the roadway surrounding rock is proposed,which mainly includes the precise cutting seam control technology of the basic roof between the layers,the asymmetric pressure-yielding support technology of the short arm beam structure,and the dynamic pressure bearing temporary support technology for the impact stage of the roadway dynamic pressure,and the gangue support technology for the gangue in the goaf side.Design the key parameters of GERRC,formulate a monitoring plan for surrounding rock stress and deformation,and apply it to the 110301 working face of Taiping Coal Mine.According to the on-site monitoring results,the main stage of the stress change of the surrounding rock is when the roadway is behind the working face of 10 m to 50 m.At this stage,affected by the mining disturbance of the working face,the roof moves violently,the roadway is subjected to large forces,and the surrounding rock of the roadway is deformed,the amount of deformation increases rapidly.The deformation of the roadway roof and floor gradually tends to be stable after the lag of 160 m in the working face.(8)The monitoring points in the original rock stress area,the coal pillar support stress affected area,and the roadway under the goaf area are analyzed.The results show that the overlying rock structure has a significant impact on the rock pressure law and the appearance of the rock pressure around the roadway.The stress of the roadway in the area affected by the coal pillar support stress is 14.7% higher than that of the original rock stress area and 39% higher than that of the roadway under the goaf.The deformation of the surrounding rock of the roadway also reaches the maximum value under the influence area of the coal pillar support stress.The maximum vertical distance of the roadway is 559 mm,and the horizontal distance is 492 mm,which fully meets the reuse requirements of the next working face.The research results show that the technology of GERRC successfully solves the problem of surrounding rock control in close-coal seam mining,realizes non-coal pillar mining,and improves the recovery rate.It is of great significance to the safety,efficiency,and sustainable development of mines.It also opens an innovative path for the same type of close-coal seam mining.Hence has a broad application prospect.