Use of Ultra-Lightweight Geofoam to Reduce Stresses in Highway Culvert Extensions

Culvert extension under highway embankment construction is a regular and important practice when roadway widening occurs. At some existing sites, concrete thickness and reinforcing steel of culvert tops and walls were stepped-down in sections of the culvert under the embankment slopes. The part of the culvert positioned under the embankment slopes was constructed weaker because the stresses under the portions of the slopes are much less than the stresses acting on the culvert section located under the main portion of the embankment. When additional fill is placed over the culvert due to roadway widening, much greater stresses are imposed on the weaker portions of the culvert. To accommodate the increased stresses on the weaker portions of the culvert, lightweight material will be placed above the weaker portions of the culvert in the field. Before construction begins, numerical analysis using Fast Lagrangian Analysis of Continua (FLAC) 4.0 was performed to predict stresses on the culvert. Results of the analysis show that geofoam has a great effect in reducing vertical stresses above and below the culvert. There are areas of high stress concentrations at the top and bottom of the concrete culvert if no geofoam is placed above the culvert. Placing geofoam above the culvert reduces the concentrated stresses at the top and bottom significantly. The stress reduction is a function of the size of geofoam and the distance between top of culvert and geofoam. To obtain an optimal practical situation, a numerical model was created to thoroughly analyze these factors. By considering these factors, effectual curves are obtained from the numerical analysis. When geofoam is placed directly on top of the culvert, the results indicate that the concentrated stresses at the top and bottom will be minimized, but it will require excavating the fill and replacing it with geofoam. The optimal situation for each culvert should be analyzed case by case. On the other hand, foam concrete can reduce load on the culvert if it is placed correctly. Valuable results using geofoam and foam concrete to reduce loads on a culvert are discussed in this report.