Long-Term Monitoring of Culvert Load Reduction Using an Imperfect Ditch Backfilled with Geofoam

Rigid culverts resting on unyielding foundations are frequently required in Kentucky for routing streams beneath highway embankments because of shallow depths to bedrock, rolling terrain, numerous streams, and the need to use high fills, which create large vertical stresses. Expensive structures are usually required. To find a way to reduce large vertical pressures, ultralightweight geofoam was placed in a 2-ft-thick trench above a reinforced, rigid concrete box culvert resting on an unyielding foundation at a roadway site in Kentucky. In situ measurements of stresses, strains, and geofoam settlements obtained over 5 years showed that geofoam was an ideal compressible material to use in the imperfect trench. At two culvert sections where geofoam was used, vertical pressures were reduced to about 10% of the normal pressures measured at the top of a culvert section where geofoam was not used. Stresses on top of two culvert sections where geofoam was used remained relatively stable after final grade elevation was reached. Earth pressures on the sidewalls of all three sections of the culvert did not change significantly after completion of the fill and were much lower than the vertical pressures measured in the culvert section without geofoam. Geofoam settlement of 60% has been recorded. Settlement behavior of the geofoam showed that movement of the soil prism above the imperfect trench was rapidly decreasing with increasing time and suggested that the reduced vertical stresses observed under the geofoam culvert sections could remain throughout the culvert design life.