Response of Twin Tunnels in Soft Soil with Different Geometric Arrangements and Construction Sequence: A Numerical Study

This paper deals with the parametric study of circular twin tunnels constructed through soft soil. The interaction effects of closely spaced twin tunnels in terms of transverse settlement and internal forces using tunnel boring machine (TBM) were analysed under the influence of varying cover depth, centre-to-centre tunnel spacing, relative position of tunnels and construction sequence. The maximum surface settlement increases with increase in cover depth from 1 to 3D where D is the inner diameter of the tunnel. A decrease in the S/D ratio from 2.5 to 1.0 influences the subsurface settlement at the middle pillar width (PW) region due to the enhanced interaction. Higher cover depth and reduced spacing lead to increased hoop force and bending moment. The settlement or the lining force variation for side-to-side tunnels was not influenced by the construction sequences. Conversely, in tunnels with offset or vertical arrangements, construction sequence emerges as a critical factor affecting settlement and lining force behaviour.