Numerical analysis of twin road tunnels using two- and three- dimensional modelling techniques

The design of near -su rface civil infrastructure requires detailed consideration of the geological/geotechnical conditions, excavation geometry and sequence, and ground support measures to ensure stability of the underground openings and minimal impact on overlying or adjacent infrastructure. Twin parallel road tunnels have been designed and built with connecting cross passages and a piggyback busway tunnel as part of a large infrastructure project in Aus- tralia. The tunnels were excavated in poor to fair quality rocks using road headers. Temporary ground support includes shotcrete, rock bolts and canopy tubes. Extensive numerical analyses have been performed to analyse the expected behaviour of the tunnels during excavation, in- cluding two-dimensional analyses using UDEC and three-dimensional analyses using FLAC 3D. Structural defects in the rock are explicitly included in the 2D analyses to examine the potential for localised ground instability. The stress redistribution is captured by the 3D modelling with an accurate representation of the complex excavation geometry and the detailed excavation and support sequence. This paper presents the modelling methodology, numerical results, and dis- cussions on integrating both 2D and 3D analyses to achieve reasonable design.