Analysis of Surrounding Rock Macro Stress Arch-Shell of Longwall Face in Steeply Dipping Seam Mining

A steeply dipping seam is one with its dip angle in the range of 35° and 55°. Mining of such coal seams is recognized to be difficult in the mining industry. Based on the results of physical simulation experiments and numerical simulation of steeply dipping seam mining, the mechanics characteristic of surrounding rock Macro Stress Arch-Shell (MSAS) is analyzed. According to the analysis, the MSAS is a high stress form which bears the main load and is formed under the action of rock self-organization after stress distribution. It is a spatial distribution form of intensity envelope curve under a strength criterion and has geometry shape asymmetry and stress distribution heterogeneity. The analyzing model and coordinate system of MSAS is established, then configuration equation is derived and stability criterion is given. There are three failure forms of the key parts of instability: tensile instability in shell roof, compress-shear instability in shell shoulder and composite instability in shell base. According to the instability process of key parts of the MSAS, instability mode of the MSAS can be divided into shell base-roof (shoulder) instability mode and shell roof (shoulder)-base instability mode. The results enrich the mining theory of complex coal seams and provide a theoretical basis for safe and efficient mining in steeply dipping seam.