含水弱胶结砂岩具有黏结力低、水稳性差和流塑变形大等特点，导致隧道支护体系变形在时间及空间维度上表现出不均匀性、非线性及非对称性，继而产生围岩坍塌、支护失效等工程问题。结合某公路隧道建设背景，采用“先让后治”的原则，补强初期支护强度并提供二次衬砌施作条件，提出双层初期支护设计方案，并通过数值技术仿真分析三台阶七步、CRD和双侧壁导坑3种工法下拱顶沉降、水平收敛及塑性区的分布特征。结果表明： 1）在成岩差、易流砂地层下，CRD工法为此类地层120 m2断面软弱围岩的最优开挖方法； 2）二次支护施作的最佳时机应在上台阶右半幅断面开挖前； 3）在有条件的前提下宜将四部交叉中隔壁法演化为六部中隔壁法施工。

Watercontaining poorlycemented powderfine sandstone embodies the characteristics of low adhesive strength, poor stability and serious plasticized deformation, which may lead to the nonuniform, nonlinear and asymmetrical deformation of the tunnel support in time and space, and in the end may lead to collapse and failure of the structure. In the paper, case study is made on a highway tunnel located in watercontaining poorlycemented powderfine sandstone. Doublelayer support is proposed to strengthen the primary support and to provide the installation condition of the secondary lining on basis of the principle of “stress releasing before stress control”. Furthermore, the crown settlement, horizontal convergence and plasticized zone distribution of 3bench 7step excavation method, CRD excavation method and doublesidedrift excavation method are analyzed by means of FLAC3D. Conclusions drawn are as follows: 1) CRD excavation method is the best excavation method for the tunnel with 120 m2 crosssection area located in the described strata； 2) The best timing of secondary primary support construction is before the excavation of the right part of the top heading； 3) Under proper conditions, CRD method with four parts should be transferred to CRD method with six parts.