MPM analysis for the stability mode and failure characteristics of tunnel face considering unsupported length

The stability of the tunnel face assumes a pivotal significance in tunnel construction safety. This paper introduces the material point method (MPM) to scrutinize the stability mode and failure characteristics of the tunnel face. Considering the impact of the unsupported length in the numerical model significantly enhances the applicability of the model in practical engineering. Firstly, the MPM model underwent validation through a comparison with model test results, demonstrating a harmonious accord in both magnitude, range, and evolution process. Secondly, 180 distinct simulation cases are conducted based on the MPM model, incorporating variables such as unsupported length (L), soil cohesion (c), and soil friction angle (φ). The results suggest that the stability of the tunnel face manifests in three modes: tunnel face stability (Mode Ⅰ), temporary stability of the tunnel face (Mode Ⅱ), and tunnel face instability (Mode Ⅲ). The failure pattern of Mode II is parabolic or elliptical + rectangular shape, whereas Mode III is elliptical + logarithmic spiral shape. For L less than 1.5 m, the failure pattern of the tunnel face assumes a parabolic shape in Mode II. Conversely, when L is 1.5 m or more, the failure pattern transitions to an elliptical + rectangular shape. For Mode III, variations in L do not influence the failure pattern observed on the tunnel face. The reduction in run-out distance fluctuates between 16.85 % and 30.77 % with each incremental 10 kPa augmentation in c, and 25.08 % and 45.03 % for each gradual 5° augmentation in the φ. For cases where c/D/γ ≤ 0.09 or φ ≤ 11°, the stability mode for tunnel faces of varying L uniformly aligns with Mode III.