Dynamic Response Mechanism of Retaining Structures of Soft-Soil Foundation Pits Subjected to Traffic Loads

Urban soft soils exhibit high compressibility, low shear strength, and significant rheological characteristics, making roadside foundation pits highly susceptible to cyclic traffic disturbances. Repeated vehicle excitation easily induces abnormal deformation and internal force variation in retaining structures, threatening pit stability. To clarify the quantitative response mechanism of soft-soil foundation pits under multi-parameter traffic loads, this study establishes a refined three-dimensional numerical model using PLAXIS 3D Advanced CONNECT Edition V22, based on a typical coastal soft-soil excavation project in Guangdong Province. Different from previous simplified traffic-load simulations, the numerical model incorporated the soil small-strain stiffness constitutive model and realized the coupling of groundwater seepage and dynamic load fields. The influences of four key parameters, namely excavation depth, static vehicle load, dynamic load amplitude, and load frequency, on the deformation and internal force evolution of retaining walls were systematically analyzed. Quantitatively, the traffic-load contribution ratio averaged 27.86% for the additional structural responses. Parametric comparison shows that excavation depth dominates the bending moment response, while static vehicle load is the primary factor controlling wall deformation. Dynamic load amplitude and frequency produce relatively minor and comparable influences. Based on a clear decomposition method of static and dynamic load effects, static vehicle load accounts for 80–90% of the total structural response (wall displacement and bending moment), whereas the dynamic increment induced by pavement unevenness only contributes 10–20%. This study clarifies the differentiated contribution mechanism of multi-factor traffic loads and provides a scientific reference for dynamic simulation, structural optimization, and construction risk control of urban soft-soil foundation pits under vehicle cyclic loads.