Research On Mechanism And Surrounding Rock Controlling Of Gob-side Entry Retaining Under Thick-Layer Composite Roof In Deep Coal Mine

At present, with the coal mining propelling to the deeper levels underground at the rate of20-30m each year, our coal mines face "three high situations and one disturbance", namely, high stress, high ground temperature, highland mild high karst water pressure and strong mining disturbance. The status seriously restricts the safe and efficient mining in depth. The geological condition of the surrounding rock is complicated. In the control over the surrounding rock, the thick layer composite roof strata is difficult to control. The thick-layer composite roof in the course of gob side entry retaining is subject to severe bed separation, which together with the severe deformation of the filling body, can lead to roof accident. This accident, like the water inrush, coal and gas outburst, is regarded as the main engineering disasters in the deep coal mining.In order to achieve safe and efficient mining and rapid succession in the deep coal mines, and solve gas restriction on coal mining, An approach is to release pressure and drain the gas in the fracture of the coal seam. Specifically speaking, on the premise of conducting uphole and downhole drilling to extract the fracture gas in the adjacent coal seam and adopting y-shaped ventilation, I carry out no-coal-pillar gob-side entry retaining at the edge of low-stress area of the goaf. Such way can both solve the gas overrun problem in upper corner of the working face, and offer favorable conditions for the stability of the surrounding rock of the gob-side entry retaining.The characteristics of the gob-side entry retaining under the condition of thick-layer composite roof in deep coal mine is as follows. In the process of working face advancing, the coal mining causes the strata to be deformed and fractured, and induces "three areas along horizontal direction and three zones on vertical direction", namely, the low-stress area at the edge of goaf of gob-side entry retaining, increasing-stress area and the original rock stress area in the coal body of gob-side entry retaining, and the caving zone, fractured zone and bend submerged zone in turn on the vertical direction. The overburden strata movement of gob-side entry retaining forms the dynamic evolution of the "internal and external" stress arch. The numerical analysis of FLAC3D is applied to analyze the "large" structure of the surrounding rock in the gob-side entry retaining formed by the fractured stratum within the "external" stress arch in the overburden strata. When the working face advances to the working face length orso, the stress arch of overburden strata expands to the highest; with the working face pushing forward, the expansion height of the stress arch basically remains unchanged, and the arch foot of" big "stress arch transfers to the deep coal body of gob-side entry retaining. Surrounding rock of gob-side entry retaining under thick composite roofs lies in the "small" structure formed by the movement of overburden rock. The arch feet of inner stress arch transfer to the depth of coal body more slowly than outer stress arch. The fracture of the "big" structure induces great deformation of the "small" structure. Whether the deformation is coupled or not is directly related to the supporting structure, parameter and time-space relationship of the surrounding rock of gob side entry retaining.The roof of coal seam is mostly composite roof in deep coal mines, and thin-layer composite roof is more easily to be controlled. The surrounding rock of gob side entry retaining under thick-layer composite roof appears to be whole roadway-section pressure, and the degree of deformation of the surrounding rock of gob-side entry retaining is obviously stronger than that in shallow coal mines, and in addition, the deformation takes on prominent subarea cracking, obvious imbalance and continuous rheological behavior. Its main forms of damage are flexure, shear, tensile and compression. The thick layer of composite roof of gob-side entry retaining has plastic abscission layer, interlayer bending and interlayer abscission layer caused by composite rocks dip offset. The roof and floor is found to have obvious increasing span effect. The ordinary bolt and anchor cable can hardly anchor thick-layer composite rock in deep hard-roof rock, which makes it difficult for the surrounding rock of gob-side entry retaining to form the stable arch structure, and the instability is highly probable under the dynamical impact of "big" structure.The strata of thick-layer con posite roof can be roughly divided into three kinds, such as the entirely soft rocks, the upper soft and lower hard stratum, and the upper hard and lower soft stratum. For different types of composite roof, the difficulty of the surrounding rock of gob-side entry retaining varies. The subsidence of the abscission layer is different. Through the orthogonal numerical analysis, the composite roof with entirely soft rock is proved to reach the maximum, the upper hard and lower soft stratum takes second place, the upper soft and lower hard stratum is the least.This paper applies the critical layer theory of strata control and elastic-plastic theory to build mechanical models of the movement of main roof and thick-layer composite roofs, and to analyze the motion of the key block of main roof and the deformation of the composite immediate roof. It concludes the deflection formula of composite roof, and analyzes the mechanics mechanism of deformation of floor heave. FLAC3D is used to analyze evolution characteristics of "inside and outside" stress arch of overburden rock, the stress of surrounding rock and the evolution of displacement fields. The orthogonal analysis reveals that the different types of composite roof, the width and strength of filling body, the strength of coal body, different supports in gob-side entry retaining and the compaction stiffness goaf have an effect on the stress and deformation of the surrounding rock under thick-layer composite roof, and laboratory analog simulation analysis shows the stress and deformation of the surrounding rock under thick composite roof, which provides a train of thought on controlling the deformation of surrounding rock of gob-side entry retaining.The paper, finally, according to the temporal and spatial characteristics of the deformation of surrounding rock of gob-side entry retaining of thick-layer composite roof proposes the non-symmetry control and boltgrouting control technology. Sub-partition reinforcement is completed on the key position in the course of gob-side entry retaining, and multistep stereoscopic anchoring supporting is completed on thick-layer composite roof, and forms multistep supporting structure with a certain thickness and bearing strength in roadway of gob-side entry retaining under thick-layer composite roof. the technology can effectively control the separation and deformation of thick-layer composite roof, and in the mean time strengthen the support of lane-side coal body, ensure the proper strength and width of the filling body, reduce the composite roof increasing span brings adverse effects, and help to prevent deformation instability of thick-layer composite roof. The application of bolt-grouting support in the bottom floor and the two horns of gob-side entry retaining can effectively control floor heave of gob-side entry retaining, and avail to the stability of "small" structure of surrounding rock of gob-side entry retaining under thick-layer composite roof in deep coal mine.