sustainability Article Evaluating the Role of Geofoam Properties in Reducing Lateral Loads on Retaining Walls: A Numerical Study Muhammad Imran Khan 1,2 and Mohamed A. Meguid 1, *   Citation: Khan, M.I.; Meguid, M.A. Evaluating the Role of Geofoam Properties in Reducing Lateral Loads on Retaining Walls: A Numerical Study. Sustainability 2021, 13, 4754. https://doi.org/10.3390/su13094754 Academic Editor: Hany El Naggar Received: 20 March 2021 Accepted: 22 April 2021 Published: 23 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Civil Engineering, McGill University, 817 Sherbrooke St. W., Montreal, QC H3A 0C3, Canada; muhammad.khan14@mail.mcgill.ca 2 Department of Civil Engineering, University of Engineering and Technology, Lahore 54890, Pakistan * Correspondence: mohamed.meguid@mcgill.ca Abstract: Expanded polystyrene (EPS) geofoam is a lightweight compressible material that has been widely used in various civil engineering projects. One interesting application of EPS in geotechnical engineering is to reduce the lateral earth pressure on rigid non-yielding retaining walls. The com- pressible nature of the EPS geofoam allows for the shear strength of the backfill soil to be mobilized, which leads to a reduction in lateral earth pressure acting on the wall. In this study, a finite element model is developed and used to investigate the role of geofoam inclusion between a rigid retaining wall and the backfill material on the earth pressure transferred to the wall structure. The developed model was first calibrated using experimental data. Then, a parametric study was conducted to in- vestigate the effect of EPS geofoam density, relative thickness with respect to the wall height, and the frictional angle of backfill soil on the effectiveness of this technique in reducing lateral earth pressure. Results showed that low-density EPS geofoam inclusion provides the best performance, particularly when coupled with backfill of low friction angle. The proposed modeling approach has shown to be efficient in solving this class of problems and can be used to model similar soil-geofoam-structure interaction problems. Keywords: rigid retaining walls; isolation efficiency; finite element modeling; lateral earth pressure 1. Introduction Expanded polystyrene (EPS) is a lightweight, closed cell, rigid foam material that is almost 100 times lighter than soil and 10–30 times lighter than other construction fill materials [1]. Horvath [2] first used the term “geofoam” for expanded polystyrene and recommended its addition to the geosynthetic category. Early application of EPS geofoam as engineering fill material started in the 1960s. The Norwegian geotechnical engineers, in 1965, used EPS geofoam in a road project for thermal insulation [3]. In 1972, EPS geofoam was used to construct embankments on soft soils [4]. Over the past 40 years, EPS geofoam has been successfully used in a variety of engineering projects, most of which have involved the use of molded blocks of EPS geofoam as light weight construction material for slope stabilization [5–9], subbase fill material [1,10–12], embankments on soft ground [10,13–17], earth retaining structures [5,18], bridge approaches and abutments [5,19–23], and buried pipes [5,24,25]. The compressible nature of the EPS geofoam also encourages its use as a compressible inclusion [26–29]. Retaining structures are integral components of many important structures including bridges, ports, highways, railway, and underground structures. These retaining structures are typically designed to resist lateral earth pressure exerted by the surrounding soil mass. The design of a retaining wall is strongly related to the magnitude and distribution of earth pressure acting on the wall. The higher the earth pressure, the more the cost of the wall structure. Various methods have been suggested to reduce the magnitude of lateral earth pressure on retaining walls. Pressure reduction can be achieved by allowing controlled yielding of the backfill soil by introducing compressible material between the wall and Sustainability 2021, 13, 4754. https://doi.org/10.3390/su13094754 https://www.mdpi.com/journal/sustainability