Numerical evaluation of soil arching effects on lateral earth pressure in soldier pile retaining wall systems

In soldier pile retaining wall systems, the surface facing, commonly a shotcrete layer, is typically designed as a one-way slab subjected to a uniform lateral earth pressure. However, this conventional assumption neglects the soil arching phenomenon, which can substantially modify the actual pressure distribution and lead to conservative and uneconomical designs. Previous studies have primarily relied on simplified analytical formulations or limited physical models, and the combined influence of soil micro-mechanical properties and geometric configuration has not been comprehensively evaluated. To address this gap, this study numerically investigates the influence of soil arching on the lateral earth pressure distribution behind the wall facing using two numerical approaches: the finite difference method (FDM) and a coupled discrete-continuum approach (Combined DEM-FDM). Analyses were performed using the FLAC and PFC software packages, considering variations in inter-particle friction coefficient, particle rolling resistance, pile spacing, shotcrete thickness, and applied stress level. The results reveal that soil arching induces a non-uniform pressure distribution between adjacent piles, where the lateral earth pressure decreases linearly from the pile location to about one-fourth of the pile spacing and then remains nearly constant toward the midspan. Increasing the pile spacing, rolling resistance, and inter-particle friction significantly intensifies the arching effect and reduces the maximum bending moment in the wall facing, while increasing the shotcrete thickness leads to a more uniform pressure distribution and a reduced arching effect. The influence of stress level on the pressure distribution was found to be negligible. The outcomes of both FDM and DEM-FDM analyses show excellent agreement, confirming the reliability of the proposed numerical approach. Overall, considering the soil arching effect substantially reduces the predicted bending moment compared to the conventional uniform pressure assumption, suggesting a potential for more economical retaining wall designs.