Mechanical behavior of a twin-tunnel in multi-layered formations

Model tests of twin circular tunnels in homogenous material, two-layered formations, and three-layered formations are performed. Induced strains and displacements are measured around the tunnel openings during excavation. A two-dimensional numerical simulation based on the fictitious stress method (FSM) is also developed to study the mechanical behavior of a twin-tunnel of circular cross section in multi-layered formations by considering the continuity equation for stress and displacement at the interface. Different values of initial stress, modulus ratio, and coefficient of earth pressure (K) are considered. The distributions of displacement around the unsupported tunnels are analyzed. The fractured zones around the tunnels are determined based on the Hoek and Brown's failure criterion under various configurations. The numerical results are found in a good agreement with the experimental results within an error of 2-4%, indicating the FSM model could be regarded as a suitable mean to solve the mechanical behavior of a tunnel excavated in the multi-layered formations. The result also shows that the fractured zones are found to develop along the minor principal stress direction, whereas the major displacement is found to occur in the major principal stress direction. (c) 2006 Elsevier Ltd. All rights reserved.