墙体周边掏槽(岩巷、锚喷、砌碹巷道除外),要见硬顶、硬帮,要与煤岩接实,掏槽浓度符合通风设施建筑标准,四周要有不少于m的裙边。
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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