Abstract: Vehicles frequently encounter immobilization problems during coastal operations and amphibious landings, which is fundamentally rooted in the time-varying characteristics of the mechanical properties of sandy beaches driven by tidal forcing. Due to the phase-lagged response of groundwater seepage to external tides, shallow sand bodies are highly susceptible to transient states of high pore water pressure and low effective stress during the mid-to-late stages of the rising tide, leading to a sharp decline in shear strength and ground bearing capacity. To quantify the bearing weakening effect induced by hydro-mechanical coupling, this paper constructs a vehicle immobilization risk assessment framework based on a "time-varying Bekker sinkage model" and Monte Carlo simulations. This framework maps the temporal evolution of effective stress onto the non-linear degradation of ground bearing parameters, incorporating the uncertainties of hydrodynamic parameters such as hydraulic diffusivity and pore pressure lag. Computational results demonstrate that the immobilization probability increases significantly with the rising tide; however, the risk peak typically lags behind the tidal peak by approximately 0.5 to 1.0 hours. This reveals that beach surface instability is governed by a delayed transition mechanism characterized by "pore pressure accumulation-bearing degradation-sinkage surge". Furthermore, sensitivity analysis indicates that the minimum ground bearing parameter and the critical sinkage criterion determine the overall level of immobilization risk. Under the synergistic action of multiple physical parameters between bearing capacity reduction and liquefaction softening, the amplification effect on the immobilization probability can reach 1.65 times that of a single-factor independent evolution. This assessment model mechanistically explains the hysteresis phenomenon of vehicle instability in the intertidal zone, providing a quantitative reference for the early safety warning of tidal-flat vehicle traffic, the selection of amphibious landing windows, and the risk assessment of coastal operations.

Key words: Tide-groundwater interaction, Sandy beach, Vehicle immobilization risk, Bekker pressure-sinkage model, Monte Carlo simulation