Three-Dimensional Heading Stability of Twin Circular Tunnels

This paper investigates the three-dimensional (3D) heading stability of twin circular tunnels horizontally aligned in cohesive undrained clayey soil. The stability is described by Broms and Bennermarks’ undrained stability number, which combine all stresses into a single dimensionless number. A recently developed 3D finite element limit analysis technique is used to obtain rigorous upper bound and lower bound critical stability numbers at collapse and blowout. The interaction effects of the distance between the tunnels on the stability are determined for a series of tunnel cover-to diameter ratios. As an additional verification of the rigorous bounding solutions, the obtained stability results are compared with other published solutions available in the literature. Results from this study are summarized in stability charts and tables for use by designers and practising engineers.