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顶板分类
在缓倾斜煤层中,根据顶板强度指数将直接顶分为不稳定、中等稳定、稳定和坚硬顶板4类, 根据直接顶厚度和采高的比值与老顶初次来压步距将老顶分为I、II、III、 IV 共4级
expl.
classification of roof
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