为确定隧道穿越软硬不均地层区段中隔壁台阶法向三台阶七步开挖法的工法转换时机，以某地下公路隧道为工程背景，考虑掌子面与软硬地层分界面不同相交位置关系，应用强度折减法对不同工况下隧道安全系数进行计算，获得掌子面与地层分界面相交位置改变时隧道安全系数的变化规律，通过数值计算，分析不同工况下隧道支护结构的变形及应力特征，进而确定隧道开挖及支护方法转换的合理时机。研究结果表明： 1）当基岩覆盖拱顶厚度达4 m后，隧道安全系数增长速率减慢并很快进入稳定状态； 2）基岩覆盖拱顶厚度达3 m后，支护结构变形及应力基本进入稳定状态； 3）确定当基岩覆盖拱顶厚度为4 m时由中隔壁台阶法转换为三台阶法，并通过工程实践对此转换时机的可靠性进行了验证。

The tunnel safety factors under different construction cases are calculated by means of strength reduction method. The variation rules of tunnel safety factor vs. relationship between rock and soil interface and tunnel working face are obtained. The deformation and stress of tunnel support structure under different construction cases are analyzed by means of numerical calculation; and then the excavation method conversion time is decided. The study results show that: 1) The variation speed of tunnel safety factor becomes slow when the distance between crown roof and siltite is larger than 4 m. 2) The deformation and stress of support structure becomes stable when the distance between crown roof and siltite is larger than 3 m. 3) When the distance between crown roof and siltite reaches 4 m, it is the time for construction method conversion.