Abstract: Abstract: The overburden failure of fully mechanized sub-level caving for extremely thick coal seam has its particularity, and needs systemic and deeply researches. Based on mine and terrain condition of Chenjiagou coal mine in Huating mine area, choosing the device that is underground, high pressure water in the upward slant deep drilling, interior permanent magnet gas transmission, double-ends plugging, and pressure balance leakage measuring. Testing the height of fractured zone of 3201、3202 face of Chenjiagou coal mine, the maximum ratio of the height of fractured zone to the mining height is 11.39. Combination with numerical simulation and surface fissure observation, the paper infers the calculation formula of the height of safety coal and rock pillars under water body, which is suitable for fully mechanized sub-level caving of extremely thick coal seam of Chenjiagou coal mine. The research results provide the reliable basis for safety mining under water body in Huating mine area.

Key words: observation by the upward slant deep drilling, ratio of the height of the fractured zone to the mining, fractured zone, mining under water body