Effect of EPS geofoam inclusion parameters on the structural performance of rigid box culverts under induced trench installation

Buried rigid box culverts are widely used in transportation and water structures and are often constructed beneath high embankments. Due to the stiffness contrast between the rigid structure and surrounding backfill soil, the soil column directly above the culvert experiences smaller settlement than adjacent soil columns, which may result in stress concentration and increased vertical pressure on the culvert. The induced trench installation (ITI) method was introduced to mitigate this problem by adopting a compressible inclusion above the structure to initiate positive soil arching and redistribute the loads away from the culvert. This study investigates the influence of expanded polystyrene (EPS) geofoam inclusion parameters on the structural behavior of buried rigid box culverts through a series of experimental tests. Eleven reduced-scale laboratory model tests were conducted, including one reference test without EPS and ten tests incorporating EPS inclusions with varying densities, thicknesses, widths, and installation locations. Static surface loading ranging from 20 to 140 kPa was applied using a rigid footing system, while vertical pressures within the backfill were monitored using miniature pressure sensors installed above and beside the culvert. The results show that EPS inclusion significantly alters the load transfer mechanism within the backfill and promotes the development of positive soil arching. Among the investigated parameters, EPS thickness had the most pronounced influence on stress reduction, increasing the pressure reduction efficiency from approximately 50% to about 70% as the thickness increased from 2.5 cm to 10 cm. A lower EPS density also slightly enhanced stress reduction due to its higher compressibility. The findings demonstrate that properly configured EPS geofoam inclusions can effectively reduce vertical stresses acting on buried rigid culverts and improve their structural performance under embankment loading conditions.