顶板厚沙土层；
The sand stratum in old river bed is important factor affecting stability of dam.
古河道砂层的存在，是影响围坝稳定的主要环境因素。
It is difficult for mined tunnel with shallow cover to cross water-rich sand stratum.
穿越富水砂层一直是浅埋暗挖隧道的难点。
It is difficult for mined tunnel with shallow cover to cross water-rich sand stratum.
穿越富水砂层一直是浅埋暗挖隧道的难点。
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