As transport networks continue to improve, more tunnels are being commissioned. The complex topography and geological conditions of certain mountainous regions have led to the construction of road and railway tunnels that must pass through geologically sensitive zones which are prone to landslide-debris flows at tunnel entrances during operation. This study, based on real engineering cases, combines scale-model tests and numerical simulations to examine the maximum impact force and dynamic behavior of landslide-debris flow at tunnel entrances in mountainous areas. The main findings are as follows. (1) Laboratory model tests were used to quantitatively analyze how particle gradation, flume inclination, and source volume influence impact force, and particle gradation was identified as the most significant factor. (2) A three-dimensional physical model of landslide-debris flow was developed, based on the physical and mechanical properties of landslide-debris-flow particles and their size distribution obtained from testing. This model simulated the entire process, from the initiation of landslide-debris flows to their attenuation, revealing the evolution of flow depth and velocity during impact. (3) The debris flow accumulates from east to west, reaching a maximum depth of about 12 m at the tunnel entrance, posing a considerable threat. The impact process can be divided into three stages: 0–6 s of initial acceleration, 6–13 s of slow acceleration, and 13–23 s of deceleration and cessation of accumulation. Because particle size substantially influences the impact behavior of debris flows, we recommend adopting a tiered protection system that classifies and designs countermeasures according to particle-size composition.
Li et al. (Wed,) studied this question.