为确定实际工程中大跨度隧道的变形基准，结合特征曲线法讨论了公路隧道极限位移的定义，给出基于三台阶七步法的初期支护极限位移确定方法；找出极限状态下各控制点的位移关系，即预警位移、容许位移和极限位移；基于三车道隧道设计洞形，采用连续体弹塑性有限元模 型，按同一围岩级别中的物理力学参数分位值组合计算，分析了不同埋深条件下的三车道初期支护极限位移，并进行t 分布统计，得出基于工法的三车道公路隧道初期支护相对极限位移基准值；对比分析同一围岩、同种工法在不同埋深及相同埋深条件下，隧道围岩表面的位移变化规律；研究同种工法、相同埋深条件下，弹性模量、黏聚力、泊松比、内摩擦角与位移曲线的关系；对比分析基于三台阶七步开挖法的现场监测数据，得出围岩变形的规律基本上符合计算结果的判断，即该方法的合理性得到验证。

In order to determine the deformation datum of large-span tunnel in practical engineering, the definition of limit displacement of highway tunnel was discussed combined with characteristic curve method, and the method of determining the limit displacement of initial support was given based on the three-step seven-step method. The displacement relationships of the control point in the limit state were found. Those are the warning displacement, permissible displacement and ultimate displacement. The design of hole shape was calculated according to the combination of quantile value of the physical and mechanical parameters of same surrounding rock level with the use of the finite element model of elastic-plastic continuum based on three lanes. The limit displacements of the initial support of the three lanes under different burial depths were analyzed and the t-distribution statistics were analyzed. The relative displacement limit of the initial support of the three-lane highway tunnel was obtained. The variation laws of the displacement of the surrounding rock surface with same surrounding rock and same method under same or different burial depths were analyzed. The relationship among elastic modulus, cohesion, Poisson ratio, internal friction angle and displacement curve were studied under same embedding method and same burial depth. Compared with the three-step seven-step excavation site monitoring data, the law of surrounding rock deformation is basically consistent with the judgment of the calculation result. Therefore, the rationality of the method is verified.