Abstract:

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.

Key words: highway tunnel, poorlycemented powderfine sandstone, excavation scheme, secondary support, supporting timing, doublelayer support