Semi-Empirical Prediction of Lateral Earth Pressure on Pile–Panel Retaining Walls with EPS Geofoam Inclusions: Coupling Soil Arching with Geofoam Compressibility

To improve the design efficiency and structural safety of pile–panel retaining walls in high-slope engineering, this study investigates the load-reduction effect of expanded polystyrene (EPS) geofoam placed between the retaining panels and the backfill material. A semi-empirical method is developed to estimate the lateral earth pressure by coupling the compressibility of EPS geofoam with the soil arching effect mobilized between adjacent stabilizing piles. The proposed method requires only conventional soil parameters and EPS material properties, enabling lateral earth pressure estimation without a priori knowledge of wall displacement, an advantage over existing displacement-dependent approaches. A total of 37 model test conditions were conducted to validate the proposed approach. Parametric analyses further evaluate the influence of the pile spacing-to-width ratio (D/W), EPS elastic modulus, and EPS thickness on load-reduction performance. The results indicate that EPS geofoam substantially reduces lateral earth pressure; lower elastic modulus and greater thickness produce more pronounced reductions. An optimal pile spacing-to-width ratio of D/W = 4 is identified, at which the soil arching effect is most effectively mobilized and the load-reduction performance is maximized. The proposed method is validated for dry cohesionless backfill under model-scale conditions; field validation under varied soil types and loading conditions is recommended before general application.