Stress paths around a 3-D numerically simulated NATM tunnel in stiff clay

The use of New Austrian Tunnelling Method (NATM) in soft ground has been increased worldwide in recent years because of its great flexibility. Tunnel excavation adopting the NATM is usually divided into sub-sections which are to be excavated under sequential steps. The excavation of each subsection is then followed by the application of temporary reinforcements. Different unsupported span lengths in the longitudinal direction at each step has considerable effects on stress redistribution in the ground, soil deformations as well as stress induced in tunnel supporting systems. In this study, Type 2 construction sequence in the Heathrow Express Trial Tunnel is simulated by 3D elasto-plastic coupled-consolidation finite element analyses. Ground deformations, stress changes and excess pore water pressure around the tunnel opening are predicted by the numerical analyses. Results are compared with those measured on site. Stress paths of those representative positions around the tunnel opening during the excavation are obtained and discussed.