Abstract: Approximate horizontal soft-hard inter-bedding rock is equivalent to the transverse isotropy rock，and its material parameters are deduced，such as density，elastic modulus，poisson ratio，etc． With the Wenjiaya tunnel in the Guang-Nan expressway as the analysis object，according to the parameters of transverse isotropic surrounding rock，the plane strain finite element model is established． And combining with the field test data，the primary support’s stress state in the process of the step cut method of the approximate horizontal layer weak rock is analyzed． Research shows that after the excavation of the upper step，the primary support is forced by the axial pressure，and vault and arch wall waist bend inward，while arch back bends outward; after the excavation of the lower step，the primary support continues to be governed by the axial pressure，and vaults and inverted arch invert inwards，while arch back and wall waist bend outward． Research conclusions correspond with the phenomenon of vault settlement and arched bottom uplifting．

Key words: mountain tunnel, transverse isotropy, finite element method, approximate horizontal rock, field testing

摘要： 将近水平软硬互层围岩等效为横观各向同性岩层，推导其密度、弹性模量、泊松比等材料参数。以广南高速公路文家垭隧道为分析对象，根据横观各向同性围岩参数建立平面应变有限元模型，结合现场测试数据分析近水平软弱围岩台阶法施工过程中初期支护的受力状态。研究表明: 上台阶开挖后初期支护受轴向压力，拱顶和拱墙腰向内弯，而拱腰向外弯; 下台阶开挖后，初期支护继续受轴向压力，拱顶和仰拱向内弯，拱腰和拱墙腰向外弯。研究结论与拱顶沉降和拱底隆起现象相符合。

关键词: 山岭隧道, 横观各向同性, 有限元法, 近水平岩层, 现场测试