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 sit u stress distributions in complex geological conditions. The modelling was intended to augment and generalise extensive hydraulic fracturing stress me asurements carried out to assist in selecting the optimum alignment of an a pproximately 14 km long tunnel, part of a proposed new rail link between St uttgart and Augsburg, Germany. The numerical model includes specific repres entation of seven different geological layers and six geological faults wit h throws of up to 30 m. Results indicate complex and variable three-dimensi onal in situ stress conditions along the tunnel routes. This is confirmed b y the field measurements. Stress conditions are characterised by strong inh omogeneity and anisotropy with a maximum to minimum principal stress ratio of up to 4:1. The numerical model indicates a large change in orientation o f the quasi-horizontal maximum principal stress direction along the tunnel route. This is also observed in the measurement results. Based on the stres s profiles from the model, the tunnel routes can be subdivided into four an d five sections in each of which the stress conditions are approximately un iform. An initial assessment has been made of the necessary support measure s and problems that may be anticipated during tunnel construction by determ ining a factor of safety for a circular tunnel of a certain diameter in eac h of the sections defined above. (C) 2001 Elsevier Science Ltd. All rights reserved.