富水未成岩粉细砂地层是极软岩地层中非常特殊的地层，因成岩不均匀，水动力环境复杂，地层力学特性差异性大，复杂多变，岩层整体缺乏自稳性，遇水易发生溜塌、滑移、涌水涌砂、基底软化承载力下降等问题，严重阻碍施工进度，危及施工安全。为解决该地层施工难题，以便安全、快速的施工，文章以兰渝铁路桃树坪隧道出口现场施工中采用的垂直深井降水+超前真空深孔降水+轻型井点降水的3级降水方法为例，总结在该地层条件下降水的设备、工艺、工法和技术参数，基本解决了富水未成岩粉细砂地层施工降水难题，可为类似地层条件下隧道洞内降水作业提供技术参考和经验借鉴。

Waterrich immature fine silty sand stratum is a special stratum of the extremely soft rock strata with the characteristics of nonuniform diagenesis, complex hydrodynamic environment, diverse, complex and changeable strata mechanical properties, and the rock formation as a whole lacking selfstability, which are prone to the problems of sliding down, slippage, sand and water grushing, base softening and bearing capacity decreasing, etc. In this way, it brings about many disadvantages for construction and even threats for construction safety. In order to solve construction difficulties in the stratum, threelevel dewatering method, i.e. vertical deep well dewatering + advanced vacuum deep hole dewatering + light well point dewatering, used in construction of the exit of Taoshuping Tunnel in LanzhouChongqing Railway is set as an example to be studied. And then the equipment, craft, method and technique parameters for dewatering in this stratum are summarized, and thus the problem of dewatering in waterrich immature fine silty sand stratum are solved, which can provide references for tunnel dewatering in the similar stratum.