Probabilistic Evaluation of Tunnel-Face Sliding Collapse in Clay-Filled Fractured Rock Masses Using the MPM

Tunnel excavation in clay-filled fractured rock masses is highly susceptible to sliding collapse due to the weak mechanical properties of clay-filled discontinuities and excavation-induced unloading. To investigate this problem, this study employed the Material Point Method (MPM) and developed an improved contact algorithm incorporating a strain-dependent dynamic friction coefficient to capture the nonlinear shear behavior of clay-filled fractures. A two-dimensional tunnel excavation model was established to investigate the possible deformation response and fracture-controlled sliding mechanism of the tunnel face, including stress release, base heaving, shear slip, and collapse. A sliding-block benchmark was used to verify the numerical implementation of the friction update, and field monitoring data provided a limited check of the small-deformation response during normal excavation. Numerical results suggest that persistent clay-filled fractures significantly alter stress redistribution and promote large-scale sliding deformation along the weak structural plane. Furthermore, a probabilistic evaluation framework based on Latin Hypercube Sampling and extensive numerical simulations was established to quantify the conditional probability distribution and exceedance characteristics of tunnel-face deformation under the adopted geological, mechanical, and numerical assumptions. The proposed framework provides a site-specific and model-conditioned method for probabilistic deformation analysis and scenario comparison in tunnels excavated through clay-filled fractured rock masses.