On the roof of coal bed are consists of the mudstones in dark-gray or grizzle which will expand met water and wet air, with higher expansion stress and coefficient.

 
  • 煤层顶板为深灰色和灰色泥岩,该岩石遇水和潮湿空气产生膨胀,膨胀应力和膨胀率大。
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