The study of earth pressure distribution on buried structures has a great practical importance in constructing highway embankments above pipes and culverts. Based on Spangler’s research, the supporting strength of a conduit depends primarily on three factors: 1. The inherent strength of the conduit; 2. The distribution of the vertical load and bottom reaction; and 3. The magnitude and distribution of lateral earth pressures that act against the sides of the structure. Rigid culverts are frequently used in Kentucky for routing streams beneath highway embankments because of rolling and mountainous terrain, numerous streams, shallow depths to bedrock, which creates unyielding foundations, and the necessity of using high fills which create large vertical stresses acting on culverts. As a means of exploring ways of reducing large vertical earth pressures acting on a buried structure, ultra-lightweight geofoam was placed in a trench above a reinforced rigid box culvert at Russell County, KY. This study provides strong evidences from both numerical model analysis and in-situ test data to indicate that geofoam is an ideal elasto-plastic material to reduce vertical load on top of rigid culvert resting on a rigid foundation. The load on the top of culvert can be reduced to 20 percent of traditional design load after two (2) feet thick of geofoam is placed on top of a culvert. Results from numerical model are more conservative when compared to actual test data. As much as 57 percent of settlement from geofoam has been recorded. Stresses on the top of culvert where geofoam was placed have reached a relatively stable level which is expected at the yield point of the geofoam. This technology can be used in applications which require controlled pressure on rigid underground structure. Whether geofoam is used or not used, the model analysis and test data show that the earth pressure acting on the sidewall does not change significantly. Although the pressure acting on the sidewall is slightly higher when geofoam is used on top of culvert only, the value is still below the design value used by the Kentucky Transportation Cabinet. Use of geofoam placed in an imperfect trench significantly reduces the vertical stresses acting on the top of the culvert.

Report Date


Report Number


Digital Object Identifier



The contents of this report reflect the views of the authors, who are responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the University of Kentucky, the Kentucky Transportation Cabinet, nor the Federal Highway Administration. This report does not constitute a standard, specification, or regulation.