The present study numerically investigates the effectiveness of expanded polystyrene (EPS) geofoam inclusions for reducing lateral earth pressure and wall displacement under static drained and undrained conditions. A two-dimensional finite-element model was developed in PLAXIS 2D for a 5 m-high concrete retaining wall supported by layered soil profiles comprising soft clay (CL), soft silt (ML), and loose silty sand (SM). Three EPS unit weights (0.22, 0.29, and 0.39 kN/m³) and four inclusion thicknesses (0.5, 1.0, 1.5, and 2.0 m) were investigated. Soil behavior was represented using the Mohr–Coulomb model, while the wall and EPS were modeled as linear-elastic materials with interface elements for soil–structure interaction. The numerical results were compared with an analytical at-rest earth-pressure reference as a consistency check.
Within the investigated range, EPS-22 (unit weight 0.22 kN/m³) with a thickness of 1.0 m was adopted as a practical reference configuration, reducing lateral earth pressure and maximum wall displacement by approximately 79% and 22%, respectively, for the reference clay–sand profile. Geofoam performance was strongly influenced by soil type, stratification, wall height, and drainage condition. Under undrained conditions, lateral earth-pressure reductions were approximately 22–28%, compared with 50–80% under drained conditions. The results indicate that EPS geofoam can be used as an effective lightweight, compressible inclusion to mitigate static lateral loads on retaining structures. EPS grade and thickness will be selected as appropriate for site conditions and design considerations.