An innovative approach to the modeling of the tunnel excavation/construction process is presented. The development forms part of the integrated research project TUNCONSTRUCT (Technology Innovation in Underground Construction) of the European Union. The developed software is based on the Boundary Element Method (BEM) but this can be combined with the Finite Element Method (FEM) as required. The application of the simulation model is mainly for conventional tunnel excavation in rock.
Numerical simulation plays an important role in the design of tunnels. Simulations are usually routinely done during the design and construction phase. Because of constraints in time and available resources the majority of the analyses are however plane strain approximations of the real 3-D tunneling process. This involves simplified assumptions for example to include 3-D effects at the tunnel face and restrictions with respect to the virgin stress field and geological features which have to be aligned to the plane under consideration in this case. Three-dimensional analyses are rarely carried out and then only by specialists consultants. However for the numerical simulation of NATM tunnel construction it is very important to model the sequential construction process. Tunneling in weak ground and near surface for example often requires a complicated sequence involving temporary support and this is definitely a 3-D process. A plane approximation of this construction sequence is not very meaningful. The aim of the work reported here, which forms part of the integrated projectTUNCONSTRUCT, is to make 3-D analysis capabilities for design and during the construction accessible to non-specialists tunnel engineers. This is done to assist in the often difficult decisions with respect to changing the designed procedures in response to actual ground behavior. A significant improvement in user friendliness and efficiency is achieved by moving away from domain based methods currently in use, such as the Finite Element or Finite Difference Methods and turning to a boundary based method. However the method is currently in a state of development that makes it unsuitable for tunneling. In the following we will shortly introduce the Boundary Element Method and the modifications that are necessary in order to adapt it to tunneling.
The boundary element method (BEM) is very rarely used for tunneling. In fact a 3-D BEM program that includes all the essential features for tunneling does not exist at the moment and this is the aim of our development. There is insufficient space here to show details of the method and the reader is referred to (Beer, 2006) for a description. The difference in Figure 1. Volume and surface based models for a tunnel. the discretisation effort between domain and boundary base method is shown in Figure 1. It can be seen that for the BEM the mesh generation is much easier than for volume based method because only the surface of the excavated tunnel has to be discretised. Indeed, since no elements exist in the rock mass result planes are required to display the stresses in Figure 1.