为计算公路山岭隧道初期支护的极限相对位移值，采用霍克布朗屈服准则，依据公路隧道设计规范和实际工程经验选取断面形式、支护方式以及材料参数，采用短台阶的开挖方式，对各级围岩不同埋深下3车道公路山岭隧道拱顶、拱腰和拱脚的极限相对位移值采用FLAC3D进行三维数值模拟，并依据突变理论确定。根据对计算结果的统计分析，提出可将洞周变形控制点位明确，细化埋深范围，通过分析上下开台阶开挖对不同点位变形值的影响，且与实际施工相结合，确定出开挖步的变化对洞周位移的影响。

In order to calculate the ultimate relative displacement of primary support of mountaincrossing highway tunnel, the HoekBrown criterion is adopted; the highway tunnel standards, crosssection pattern, supporting methods and material properties are taken as references; short bench excavation method is adopted; and the ultimate relative displacements of crown top, arch springer and arch feet of the threelane highway tunnel under different grades of surrounding rocks and buried depths are simulated and calculated by software FLAC3D and are further determined by catastrophe theory. According to the statistical analysis of calculation results, it is pointed out that the deformation control points of the tunnel surrounding can be defined clearly and the buried depth can be refined; by analyzing the influences of benches excavation on deformation of different points and comparing to field construction, the influence of the excavation step on the displacement of tunnel surrounding can be determined.