<p>FOSs under different geometric dimensions.</p>

Recently, the application of the strength reduction method (SRM) to stability analysis of tunnel portals has become a trend. The key to employing the SRM lies in selecting an appropriate failure criterion. It is analyzed that the application characteristics of traditional criteria. Additionally, it is proposed that a new failure criterion—the variational criterion. Based on the numerical models, the effectiveness of the aforementioned work is validated. The results show that the displacement mutation at characteristic points (Criterion Ⅰ) is cumbersome to apply and involves a substantial workload. The plastic zone penetration (Criterion Ⅱ) lacks quantitative and clear standards. The calculation program non-convergence (Criterion Ⅲ) lacks a mechanical explanation. The energy mutation (Criterion Ⅳ) can effectively reflect the failure mechanism of the model. But it requires considerable computational effort. The variational criterion addresses these shortcomings while providing results with a relative error of no more than 1.6% compared to other criteria. Moreover, this applicability and accuracy are largely unaffected by mesh densities, geometric dimensions, strength reduction factor intervals, mechanical parameters, and convergence criteria. The variational criterion offers a comprehensive indicator—the variational value, and employs a clear discrimination method—judging the sign of the variational value. This criterion can provide a new reference for failure discrimination in tunnel portals.