Finite element modelling of transportation tunnels

Abstract

The aim of this thesis is to determine the ground deformation and stress distribution around highway tunnels at various stages of excavation and for several support conditions using finite element modelling techniques. When ground is excavated and material removed the subsequent redistribution of stress in the remaining surrounding material needs to be treated by one of three methods. These are the gravity difference method, the stress reversal technique and the relaxation approach. The first two methods were chosen for the simulation of excavation in this study. The tunnel data are in the form of the dimensions of the tunnel, heights of the rock layers, details of the shotcrete lining and tunnel support systems. A pre-processing program was written to transform this information into a finite element mesh in a format suitable for use by PAFEC-FE software. This enables different tunnel models and meshes to be produced with minimum error and time. The lack of adequate post-processing facilities available in PAFEC-FE dictated that computer programs needed to be written in order to reformat the textual output files and process the mesh stress and displacement outputs for graphical display using UNIRAS. In this way repeated use could be made of PAFEC-FE without time-consuming and error-prone manual retrieval of data. The tunnels were modelled at various stages of excavation and with such support provided at those stages as to allow the computed displacements to be compared with measurements made on highway tunnels in Turkey. The stresses generated in the tunnel supports and surrounding ground were also calculated to enable the possibility of damage or failure of the support structure or ground to be assessed and the selection of an optimal support system. Insertion of a support system into the model has a marginal effect on the development of rock strength around an excavation boundary.