通过对湿陷性黄土围岩监测，揭示时间应变规律，结合此规律形成一套三台阶施工工法，并总结出使用此工法施工围岩的变形特性如下：上台阶开挖后稳定7d左右，表现为沉降，在施工二台阶马口时，允许上台阶初期支护在可控范围内下沉，也是应力释放的过程；三台阶施工沉降变形规律与二台阶类似，在仰拱封闭后，围岩趋于稳定。

The rule of the timedependent strains of the tunnels in wet subsidence loess ground is revealed by means of monitoring of the wet subsidence loess ground. A kind of 3bench construction method is developed on basis of the rule of the timedependent strains. The deformation behavior of the tunnels constructed by 3bench excavation method is summarized as follows: After top heading excavation, the tunnel stability can maintain 7 days before settlement occurs; During the excavation of the second bench, the controlled settlement of the primary reinforcement structure of the top heading is allowed, which is a stress releasing process; The settlement deformation caused by the third bench excavation is similar to that caused by the second bench excavation; When the ring is closed after invert installation, the ground tends to be stable. 3bench construction is executed under the guidance of the above deformation rule in the construction of LanzhouChongqing railway and satisfactory effects have been achieved.