A New Criterion for Judging Tunnel Failure by the Strength Reduction Method

Recently, the application of the strength reduction method (SRM) to tunnel stability research has become a trend. The failure criteria for the SRM in tunnels often draw from those used in slopes. The applicability of the newly proposed energy mutation criterion from slopes to the tunnels is explored. Additionally, a new criterion—the variational criterion—is suggested for tunnels. The effectiveness of the aforementioned work is validated based on unlined tunnels, lined tunnels, and constructed tunnels. The results indicate that the energy or variational value can serve as a new indicator for tunnel stability. For the variational criterion, the applicability is not coincidental with respect to mesh densities, strength reduction factor intervals, geometric parameters, and mechanical parameters. In specific tunnels, the variational criterion exhibits several relatively distinct characteristics of simplicity, comprehensiveness, clarity, reliability, and uniformity. This criterion provides a theoretical basis and mechanical explanation for tunnel failure.