Effect of Trench Geometry on the Behavior of Negative Projecting Box Culverts

Marston’s theory is typically used to predict earth loads acting on culverts installed in negative projecting trenches. This theory assumes that trench walls are vertical; however, many excavations are done with sloping walls for constructability and safety. In this study, the effect of trench geometry on the loading conditions of negative projecting box culverts was evaluated. Centrifuge testing and numerical modeling was performed to evaluate the influence of trench wall angle, trench depth, and trench width on the vertical and horizontal earth pressures exerted on the box culvert. Sixteen model geometries were evaluated, and the experimental and numerical results are shown to be in reasonably good agreement. It was found that Marston’s theory is appropriate for negative projecting culverts constructed in trenches with vertical sidewalls; such an approach is conservative and overestimated the vertical loading conditions by approximately 15%. However, for configurations with inclined trench walls, it was found that the culvert loading was increased considerably and Marston’s theory was not appropriate. Very little positive arching and load reduction was realized when such culverts were installed in trenches with sloped walls. It is recommended that numerical modeling be performed during design of negative projecting culverts constructed with sloped sidewalls, or that positive projecting conditions be assumed to ensure that design loads are conservative.