兰渝铁路胡麻岭隧道7号竖井工区通过的第三系含水弱胶结粉细砂岩具有异常复杂的水稳性特征，掌子面自稳性差，施工进度极其缓慢，采用洞内降水措施，效果很难保证，施工风险大。根据地形地质条件，对胡麻岭隧道7号竖井工区开展地表降水试验研究，对试验成果进行计算分析，了解和掌握该第三系含水砂岩的水文地质特性，确定地表降水井的设计参数。通过对降水前后围岩、施工进度的比较，验证了降水井的设计合理。

The tunneling from No. 7 vertical shaft of Humaling tunnel on LanzhouChongqing railway crosses Tertiary watercontaining poorlycemented finesilty sandstone, which has extremely complex waterstable property. The tunnel face has poor selfsupport stability, and the tunneling progress is very slow. Furthermore, the effect of dewatering carried out inside the tunnel cannot be guaranteed. The tunneling has great risks. Experimental study is made on the dewatering made from the ground surface based on the terrain and geological conditions of the tunnel. The study results are analyzed, the hydrogeological property of the Tertiary watercontaining sandstone is obtained and the design parameters of the ground surface dewatering wells are determined.