老顶悬露达到极限跨距时,老顶断裂形成三铰拱式的平衡,已破断的岩块回转失稳或滑落失稳, 工作面支架呈现受力普遍加大现象,即称为
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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