Study On Overburden Strata Movement And Load Transfer Mechanism Of Gob-side Entry Retaining By Roof Cutting In Shallow Coal Seam

Shallow buried coal seam in Shendong mining area is characterized by shallow buried depth,thin bedrock and overlying sand layer.After mining in the lower working face,the movement characteristics of rock strata are different from those of deep buried coal seam.With the popularization and application of pillar-free mining by cutting roof into roadway,pillar-free mining can be realized and coal resources can be fully exploited.Based on the engineering background of Halagou coal mine in Shendong Mining area,this paper uses theoretical analysis,physical model test,numerical simulation and field test to study and analyze the overburden movement and load transfer law of shallow seam cutting roadway in Halagou.First of all,according to the characteristics of shallow coal seam,thin bedrock and thick soil layer,combined with theoretical analysis.After fully mechanized mining in the working face,the overburden movement structure model of shallow coal seam cutting into roadway is established.The development process is divided into four stages: caving zone development stage,fracture zone rapid development stage,fracture zone metastable stage and overburden stability stage,and the overburden movement characteristics of the four stages are summarized.According to the characteristics of overlying rock movement in shallow coal seam,a load transfer model of surrounding rock in shallow coal seam cutting roadway is proposed,and two different stress forms of short arm beam in the process of leaving roadway are analyzed.The physical model of overlying rock movement in shallow coal seam cutting roadway is constructed.The displacement field and strain field of stope in the process of overlying rock movement are analyzed with XTDIC digital speckle technology,and it is concluded that the damage of overlying rock movement stratum in shallow coal seam cutting roadway presents asymmetric trapezoidal fracture,which can be divided into asymmetric simply supported beam failure and rock beam rotation deformation failure through the dividing line of rock failure form.The main structure of rock mass is statically indeterminate rock beam structure and masonry beam structure.The displacement nephogram and the maximum principal strain cloud evolution diagram show that the goaf gangue is in a process of continuous compaction,which gradually forms an effective support structure and at the same time makes the outline of the caving area of overlying rock present an asymmetric trapezoid.Finally,the overlying rock movement reaches a stable state and forms a triangular area,which provides effective support for the overlying rock.The settlement at the top of the model is reduced by the full filling and swelling of the gangue.The relationship between fracture height and advance distance verifies the four stages of overburden movement of shallow seam cutting into roadway.By using UDEC discrete element numerical simulation software,a numerical calculation model was established based on the 12201 working face of Halagou Project.The motion law of overburden rock was basically consistent with the model experimental results,which verified the rationality of the model experimental results.The fracture zone of overburden rock was in asymmetric step form,and the cracks in the topsoil layer and rock layer formed an asymmetric hyperbolic curve.Based on the analysis of the law of mine pressure development and the evolution law of the main stress field,the load transfer law of the four stages of overburden by cutting the roof of shallow buried coal seam into roadway is obtained:(1)In the caving zone development stage,the roadway is in the relief zone,and the overlying strata load is transferred to the side of the coal pillar.(2)The asymmetric load transfer area of "coal pillar-masonry beam-coal pillar" is formed in the rapid development stage of the fissure zone.(3)In the metastable stage of the fissure zone,the crushed gangue in the goaf is compacted into a bearing structure,and the masonry beams transfer loads to each other,forming the asymmetric load transfer area of "coal pillar-masonry beam-crushed gangue".The surrounding rock of the roadway is composed of "coal pillar-roof cutting short arm beam-coal pillar" load balancing body,which together with the upper load transfer area forms the overburden load balancing system of shallow coal seam cutting roadway.(4)The overburden movement reaches a stable state in the load stabilization stage.The evolution of overburden load path can be divided into three processes:(1)During the opening of roadway,pressure relief arch is formed.(2)Asymmetric trapezoidal load transfer of "coal pillar-masonry beam-coal pillar".(3)Asymmetric trapezoidal load transfer of "coal pillar-masonry beam-broken gangue".The above research results are applied to Halagou 12201 working face to carry out field industrial experiments.Combined with the actual situation of the site,the design of Halagou12201 return roadway was carried out.The design of key parameters of the roof pre-splitting slit included the calculation of the height of the cutting top,the design of the slit Angle and the spacing of the slit holes,and finally achieved a good effect of retaining the roadway.According to a large number of field measured results,the basic roof structure of masonry beam is formed above the direct roof(2.0 ～ 2.5)times the thickness of the coal seam)in the process of overlying rock movement of shallow buried coal seam cutting into roadway.There are two deformation forms of sliding instability along the vertical of the basic roof and turning instability along the coal wall as the fulcrum.According to the observation of surface rock movement and fracture,the surface deformation in the 110 mining area is obviously better than that under the traditional longwall mining condition,and the fracture width is 1～3mm.According to the mine pressure monitoring,compared with that of the 121 working method,the weekly pressure step distance in the top cutting area of 110 working method is increased by 18m～22m,an increase of about2 times,and the maximum periodic pressure is reduced by 10-11 MPa,a reduction of 20%,which verifies the rationality of load transfer law.