Long-term performance of high-fill box culverts with geogrid-expanded polystyrene load reduction system

The Geogrid-Expandable Polystyrene System (GES) is widely employed in high embankments to alleviate backfill loads on buried structures such as box culverts, particularly in mountainous regions. However, the long-term performance of high-fill box culverts incorporating this system remains poorly understood. In this study, model tests were performed to examine the stress characteristics of a high-fill box culvert with geogrid-EPS during embankment construction. A validated finite differential model was subsequently used to investigate the post-construction long-term behavior, including EPS deformation, geogrid tensile forces, and earth pressure evolution around the culvert. Parametric analyses were conducted to assess the influence of key geogrid-EPS parameters on long-term performance. Results indicate significant long-term redistribution of earth pressure due to creep in both the geogrid and EPS geofoam. Vertical earth pressure at the culvert crown decreases progressively, while horizontal pressure on the sidewalls increases gradually. Foundation contact pressures remain relatively stable, with increases of only 5.9% and 0.6% at the monitored locations over 600 months. Long-term load reduction is highly sensitive to the elastic modulus and thickness of EPS geofoam as well as the tensile stiffness of the geogrid. Reducing the EPS modulus from 5 MPa to 1 MPa lowers crown pressure by 37.7% at the end of construction and by 47.4% after 600 months; however, it also elevates sidewall pressures, which can be effectively controlled by optimizing the vertical spacing of geogrid layers.