Study On Large Scale Roof Caving And Control Technology In Hard Roof Working Face Under Pillar In Jinhuagong Mine

The Datong mining area is a typical hard roof coal seam mining.However,due to the coal seam group mining,the goaf and coal pillars of the upper coal seam are often left above the working face of the lower coal seam,resulting in a roof failure of the hard roof in the lower coal working face.Papers on the basis of 12^#coal seam in Jinhuagong coal mine 8204 working face mining conditions,using the field measurement,theoretical analysis,physical simulation and numerical calculation,the paper analyzes the failure characteristics of overburden strata and overburden in the working face of coal seam group.The migration law,the distribution law of the fracture field and the evolution process of the stress field reveal the mechanical mechanism of the large roof under the action of the upper coal pillar,and the control technology for controlling the large-scale roof collapse is given.The research results are as follows:1)Using on-site investigation and analysis,the main factors affecting the large-scale end face of the hard roof working face are determined by the stress concentration caused by the coal pillars left over from the coal seam,which changes the failure movement law of the roof.Through theoretical calculation,the maximum influence depth of concentrated stress under the coal pillar of 7^#coal seam of Jinhuagong Mine is 80m.2)FLAC^3D simulation was used to analyze the 7^#coal seam section stability of coal pillar and the influence depth of bottom plate.The research shows that the damage depth on both sides of the coal pillar under the mining action is 4m,the coal pillar does not have large-area damage,and has good bearing capacity;The stress concentration factor on the coal pillar is 3.2,and the maximum vertical stress in the coal pillar range is 15MPa.3)The effect of the upper coal pillar on the fracture of the overlying strata of the hard roof coal face was studied by the method of physical similarity simulation.Found that the hard roof is under the action of the concentrated stress of the coal pillar,the development of direct top cracks reduces the stability of the top surface of the working face.The basic top breaking block is reduced to form a"short masonry beam".It is easy to form a cutting top along the end face,thus causing a large-area roof fall accident.4)UDEC numerical simulation analysis is used to simulate the influence of coal pillars on the surrounding rock stress of hard roof working face.It is concluded that under the influence of coal pillars,the elastic energy between the rock layers continuously accumulates and gradually increases under the pressure of the leading support.However,when the old roof（key layer）breaks,it will increase sharply,thus inducing a large range of roof fall in the working face.According to the mechanical mechanism of the hard roof face roof fall under the action of the upper coal pillar,the key to determining the control of large-area roofing is to increase the fracture step of the old roof to improve the stability of its structural motion and to reduce the accumulation of elastic energy generated by stress concentration.According to the actual conditions of the site,the comprehensive management measures of“one excavation,two protection and three pressure relief”were formulated.The monitoring of the working resistance of the working face bracket shows that the technical method can effectively improve the stability of the old roof structure,thereby weakening the degree of mine pressure,which is conducive to the comprehensive treatment of roof fall under this condition.