Strata behaviors in some extra thick coal seams with fully-mechanized mining are fierce. In some circumstances, some hydraulic supports were still crushed even though their working resistance had been sharply increased beforehand comparing normal supports in medium-thick coal seams. Based on field measurement, numerical simulation, laboratory simulation experiments and theoretical analysis, the movement regularities of roof strata in extra thick coal seams with fully-mechanized mining was studied in this article. The studies show that: if an extra thick coal seam is mined in one single time, the thicker the coal seam is, the bigger is the vertical and horizontal movement range of the surrounding rocks, and scope and peak value of the abutment pressure increases obviously. The concepts of "immediate roof" and "basic roof" in extra thick coal seams are different from those in medium-thickness coal seams. The rock strata above the extra thick coal seam would form a structure like "inverted-step cantilever group-articulated rock beams". Such structure breaks periodically with the advancing of the working face, where roof weighting comes regularly in "small cycle - big cycle". Different from high cut mining (3.5~7.2 m. mining height), physical and mechanical features of the top coal in top coal caving mining (=7.2 m. height) play a key role in strata behavior. Under roof deformation pressure, the top coal was condensed to "a rigid coal structure". Some roof strata deform the top coal while exerting no pressure on the hydraulic supports and they are called "strata without deforming pressure". While the other part above such strata is called "strata with deforming pressure" since it exert deforming pressure to hydraulic supports via "rigid coal structure". Based on the above knowledge, the lower limit of working resistance of hydraulic support in high cut mining and top coal caving for extra thick coal seams are proposed.

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