Three-Dimension FEM Analysis of Large Cross-Section Tunnel in Collapsible Loess Constructed by CRD Method

The Fenghuangling Tunnel is located within collapsible loess and has a cross-section area of 170 m² for excavation. To overcome the difficulty in excavation, the Cross Diaphragm (CRD) Method was adopted to construct this tunnel. This paper presents a three-dimensional numerical analysis using a finite element method program (PLAXIS 3D Tunnel) to simulate the tunnel's excavation process. The effect of rainfall was considered in the analysis. The analysis yields the settlement of the ground surface and subsidence of the tunnel arch, which were compared with the values measured in the field. In addition, axial forces and bending moments of the lining structure and axial forces of the rock bolts were obtained from the analysis. The results show that the bolts within a certain range of the tunnel arc top play a trivial role. The computed safety factors indicate the most vulnerable stage of the entire system during tunnel excavation and also reflect the significant impact on tunnel excavation from rainfall.