Producing Undrained Stability Factors for Various Tunnel Shapes

Tunneling in soft clay requires accurate estimation of the inner support pressure to prevent undesired ground settlement and maintain the overall soil stability. Previous researchers have shown that the stability factor approach, which is analogous to Terzaghi’s bearing capacity factors [critical stability number (Nc), Ns, and Nγ)], is effective and efficient to determine the limit loads of many geotechnical stability problems. Therefore, this paper aims to study the stability factors of four various tunnel shapes in undrained clay: the two-dimensional (2D) tunnels of circular, square, and heading in plane strain, and the three-dimensional (3D) circular tunnel heading. Using the advanced finite-element limit analysis (FELA), rigorous upper-bound (UB) solutions of the undrained stability factors (Fc, Fs and Fγ) will be produced for various tunnel depth ratios (C/D). Numerical UB results are compared with lower bound (LB) solutions. Comprehensive design charts, tables, and equations will be presented for practitioners to estimate the critical tunnel support pressure at collapse