Abstract: The employment of large-scale mechanized tunnel construction represents an effective method to ensure the safety, efficiency, and environmental sustainability of tunnel excavation. Typically, this method involves full-face tunneling, characterized by large excavation areas and rapid excavation rates, often precipitating face instability. The stability of the tunnel face, consequently, holds paramount importance for the overall safety of tunnel construction. Accordingly, the authors introduce the material point method to the investigation of the various stability modes and failure characteristics of the tunnel face. Initially, the numerical model utilizing the material point method is validated through a comparative analysis with experimental results, revealing a strong concordance in the magnitude, extent, and evolution of shear strain zones within the surrounding rock across both models. Subsequently, considering factors such as the unsupported length (L), the cohesion (c) of the surrounding rock, and the friction angle (φ), a total of 120 distinct working conditions are simulated using the material point method model. The results indicate that: (1) The stability of tunnel face presents three modes, stable face (Mode Ⅰ), temporarily stable face (Mode Ⅱ), and unstable face (Mode Ⅲ). The failure mode for Mode Ⅱ is characterized by a combination of parabolic or elliptical shapes with rectangular forms, while Mode Ⅲ exhibits an elliptical shape accompanied by a logarithmic spiral. (2) A critical boundary exists between c/D/γ and φ (where D represents the tunnel diameter and γ represents the bulk density). When c/D/γ≤0. 09 or φ≤11°, the stability mode of the tunnel face under varying L consistently aligns with Mode Ⅲ stability. Moreover, an increase in L correlates with a reduced likelihood of encountering Mode Ⅱ stability.

Key words: sandy stratum, tunnel face; unsupported length, material point method, stability mode, deformation characteristics