Numerical modelling of in situ stress conditions as an aid in route selection for rail tunnels in complex geological formations in South Germany
The paper describes three-dimensional numerical modelling studies of in situ stress distributions in complex geological conditions. The modelling was intended to augment and generalise extensive hydraulic fracturing stress measurements carried out to assist in selecting the optimum alignment of an approximately 14 km long tunnel, part of a proposed new rail link between Stuttgart and Augsburg, Germany. The numerical model includes specific representation of seven different geological layers and six geological faults with throws of up to 30 m. Results indicate complex and variable three-dimensional in situ stress conditions along the tunnel routes. This is confirmed by the field measurements. Stress conditions are characterised by strong inhomogeneity and anisotropy with a maximum to minimum principal stress ratio of up to 4:1. The numerical model indicates a large change in orientation of the quasi-horizontal maximum principal stress direction along the tunnel route. This is also observed in the measurement results. Based on the stress profiles from the model, the tunnel routes can be subdivided into four and five sections in each of which the stress conditions are approximately uniform. An initial assessment has been made of the necessary support measures and problems that may be anticipated during tunnel construction by determining a factor of safety for a circular tunnel of a certain diameter in each of the sections defined above.
Keywords: Tunnel, Stress field, Numerical methods, Distinct element method, 3DEC