main roof collapse
... main line track 干线 main roof collapse 老顶垮落 main roof 织顶 ...
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顶煤的运移与垮落，尤其是内部微观裂隙的扩展，与其直接顶和老顶岩层的活动规律密切相关。
The movement and breakage of top coal, especially its internal micro-fracture extension, are closely related to the movement regularity of its immediate roof and main roof.
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