Stress Reduction of Buried Thermoplastic Pipes Using EPS Geofoam

Field and laboratory tests and numerical analyses of rigid culverts have shown that induced trench installation (ITI) can reduce the vertical stress acting on culverts installed under high embankments. In this research, the ITI technique was adapted by incorporating expanded polystyrene (EPS) geofoam as a compressible inclusion to reduce stresses on large-diameter thermoplastic pipes beneath high embankments, and the system was examined through numerical modelling. A numerical model representing the field configuration was calibrated using available field measurements and subsequently employed to evaluate the mechanical response of large-diameter thermoplastic pipes buried under a high embankment. The effects of EPS thickness and width on pipe stresses and deflections, together with the comparative stress responses obtained using different EPS material representations, were analysed. Creating a soft compressible zone above the thermoplastic pipe promoted positive arching and reduced the stresses acting on the pipe, while the deflection response varied with pipe type and EPS geometry. In the EPS15 thickness series, increasing the thickness beyond 1.4h (h = D/10) did not produce a consistent additional reduction in vertical crown stress. Under the fixed 1.4h-thick EPS15 condition, increasing the width beyond D did not consistently improve the stress and deflection responses. Within the adopted material representations, EPS15 produced the largest stress reductions, reaching approximately 60–85% in vertical stress and 35–52% in horizontal stress. Because different constitutive formulations were used for EPS15 and EPS20–EPS40, the observed differences reflect the combined influence of nominal EPS density, material properties, and constitutive representation.