Performance of multi-faced tunnelling - A 3D numerical investigation

This paper presents the results of a 3D numerical investigation on the performance of multi-faced NATM tunnelling in soft ground. A series of 3D finite element analyses were conducted on a number of tunnelling cases with different face advancing sequences such as the ring-cut and the sidewall drift methods. The variables considered in the analyses include bench length, core volume, and lagged distance. The results of the analyses indicate that the sidewall drift excavation method is the most effective in limiting ground surface settlements as well as tunnel deformations. Also shown is that a decreased lagged distance between the right and left drifts in the sidewall drift method results in larger tunnel deformations and ground surface settlements, and that the best tunnelling performance can be achieved when keeping the lagged distance greater than one tunnel diameter. In the ring-cut method, the bench length, or ring closure time, has more pronounced effect on the tunnel crown and the surface settlements than the core volume at the face. Practical implications of the findings are discussed.