短語詞組短語longwall actual mining 走向長壁回采longwall advancing mining system 長壁采煤法longwall advancing mining stope 長壁式回采工作面longwall advancing mining workings 長壁開采法longwall shrinkage mining method 留礦全面法longwall advancing mining stoping 長壁采礦法longwall advancing mining slicing 長壁分層采礦longwall advancing mining retreating 長壁后退式回采longwall advancing mining mining 長壁式開采 返回 longwall mining