Response Of Overburden Structure And Its Stability Around The Longwall Mining Face Area In Steeply Dipping Seam

The steeply dipping seam is defined by the coal seam a pitch between 35°~55°and has complex occurrence conditions. The reaction of the overburden structure and support–surrounding rock system in the steeply dipping seam to mining is very different from that in flat or moderately pitched coal seam. Studies on the structural stability in the overburden around the mining face in steeply dipping seam has great significance to enrich the practice and theory in mining the steeply dipping seam.Numerous past practices, experimental research and theoretic analysis indicate that the strata movement occurs in the middle of working face first, then at the top and lastly at the bottom along the seam inclination direction. Such sequential changes and imbalance features of failure and strata movements occur in the immediate roof, main roof and top strata. The"Three-zones"features are obvious in middle and top areas along inclination direction at a higher horizons. No intact"three-zones"can be found in the bottom area.Incline masonry structure is formed at the roof in working face along the inclination direction. There is incline masonry structure active region in middle and top areas along the inclination direction, and a transition area in the centre and lower areas, and stability region at the bottom. Asymmetrical"shell structure"is formed at the top strata of mining stope and its magnitude is mainly determined by the mechanical characteristic of working face overburden strata, mining room size and so on. The incline masonry structure and asymmetrical"shell structure"are the special structures for minig steeply dipping seam. Their stability dedicates rational strata control measures to choose.The interaction between support and incline masonry structure can be divided into uprightness-pressing, inclination-pushing, anti-inclination-pushing, back-pushing and strike-pushing functions. These compressive actions can cause stability supports problems to face, downslide-stability, inclination-instability, and concluded six instability criterions. Localized instability of shell structure is the main factor to lead the overall structural instability in the stope. The instability modes are followed: the shell roof instability took tensile destruction as the predominent way, shell shoulder instability is initialized by compression-shear failure predominently,and shell bases composite instability occurs in tensile and compression-shear modes predominently.The complete"shell"and destruction"incline masonry"are interpreted as load-bearing structure around the coal mining face in steeply dipping seam. Between the"incline masonry"has extrusion and constraint effect on"shell", and"shell"has extrusion and loading impact effect on"incline masonry", the instability impacting action of"shell structure"to working face is relieved by the hinge function of"shell"and"incline masonry structure". Load-bearing structure instability types can be comprised of uprightness-pressing instability, pushing instability, composite instability and so on. The instability possibility can be described by the instability coefficient, the macroscopic stress F of"shell structure"to working face supporting and stability of load-bearing structure are concluded by using energy conservation principle and masonry beam theory, and also five criterions are obtained: uprightness-pressing instability, forward pushing instability, reverse pushing instability, strike pushing instability and composite instability.According to field practices and experimental research, the overburden structure working face 25112 is analyzed by the theory of overburden structure stability around the coal face. The roof pre-blasting scheme and the support system anti-skid measures are recommended. Field tests show that the theory and the experimental analysis matched well. Good technical and economic benefits are realized.