Revisiting Circular Tunnel Stability Using Broms and Bennermarks' Original Stability Number.

The stability of a single circular tunnel in cohesive undrained soil was investigated under plain strain conditions using a two-dimensional finite-element limit analysis (FELA) program and a shear strength reduction method (SSRM). The stability for this problem was well expressed by using Broms and Bennermarks' original stability number (N = (σs + γH − σt)/Su) owing to the undrained loading conditions. The variations of the factor of safety and the critical stability number were presented for a series of tunnel cover-to-diameter ratios (C/D) in both collapse and blowout scenarios. Also, the extent of surface failure for some selected depth ratios was determined in the case of tunnel collapse. The obtained results were compared and validated by using the finite-difference method, as well as other existing solutions available in the literature. The dimensionless ratios employed in this study make the design charts suitable to cover a broad range of tunnel geometries and soil parameters. [ABSTRACT FROM AUTHOR]