为了解决隧道穿越土砂分界地层的围岩稳定性问题，以蒙华铁路阳城隧道为研究对象，采用现场试验、室内试验及数值试验等研究方法对土砂分界地层围岩稳定性进行探究，重点探讨地层种类及其厚度变化对围岩变形、塑性区扩展及支护结构受力的影响。结果表明： 1）相对单一地层，土砂分界地层会影响洞周应力分布，分界面处围岩容易先出现塑性区，且土砂分界地层围岩变形性质与软弱的全砂岩地层更为接近； 2）在土砂分界地层分界面处会产生较大的水平收敛，易出现拉应力； 3）土砂分界地层中，随着砂层所占比例的增大，拱顶沉降和洞周收敛不断增大，且分界面所在台阶处支护结构的应力发展会提前达到更高的峰值； 4）当分界面出现在上台阶时，围岩会产生极大的塑性变形，支护结构承受相对较大的应力，是土砂分界地层中最不利的分界情形。

The stability of surrounding rock in soilsand strata in Yangcheng Tunnel on Menghua Railway is studied by field test, laboratory test and numerical simulation; and the influences of strata types and thicknesses on surrounding rock deformation, plastic zone and supporting structure stress are emphatically discussed. The results show that: （1） Compared to single stratum, the soilsand strata would affect the distribution of stress around the tunnel, the plastic zone is easier to appear in surrounding rock around soilsand interface, and the deformation properties of the soilsand strata are much closer to soft and weak sand stratum. （2） There is a larger horizontal convergence and tensile stress at the interface of soilsand strata. （3） As increase of the proportion of sand, the settlement of tunnel crown top and surrounding increases, and the stress of supporting structure of bench at the interface would develop to a higher peak in advance. （4） When the interface locates in top heading, large plastic deformation would occur to surrounding rock, and the support structure would bear much larger stress, which is the worst situation.