矿岩稳固性可分为:a、极不稳固的:不允许有暴露面积,掘巷道时用超前支护;b、不稳定的:允许不支护的暴露面积〈50m2;c、中等稳固:50~200m2;d、稳固的:200~800m2;e、极稳固的:允许不支护的暴露面积〉800m2。
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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