This study quantifies vertical earth pressure on the roofs of box culverts under high fills in valley terrain using centrifuge model tests with expanded polystyrene (EPS) geofoam for load mitigation. We compare buried-type culverts with valley-terrain high-fill culverts and isolate the effects of the EPS installation height and panel thickness on the roof pressure and the associated concentration factor. The analysis of fill settlement elucidates the terrain-dependent load reduction mechanism and the efficacy of EPS panels. The results show that the roof pressure increases with EPS installation height but decreases and then plateaus once the panel thickness exceeds 75 cm; the load reduction benefit weakens when the installation height exceeds 2 m. Optimal performance is achieved with panels installed at 2 m and with a 75 cm thickness, which lowers applied loads while maintaining structural stability. These findings clarify soil–structure interactions in complex topography and provide practical guidance for deploying EPS in high-fill valley projects.