Pressure Reduction on Wide Culverts with EPS Geofoam Backfill

Buried structures may be subjected to excessive stresses that are detrimental to their expected service life. A review of previous design methods shows that there are effective methods to reduce loadings on narrow culverts. The Induced Trench Method introduces compressible fill over a buried structure to mobilize positive arching, and transfer stresses to the side fills. However, this method has not been studied with Bridge Culverts (large spanning culverts). This study evaluates the use of EPS (Expanded Polystyrene, Geofoam) as a compressible/lightweight fill material. The displacements and stresses of the classic narrow trench are examined in detail using the finite difference computer modeling program, FLAC (Fast Lagrangian Analysis of Continua). The narrow trench is then altered to study the overall effect of thickness and width of the compressible zone upon the stresses acting on the buried structure. To extend the service life of the culvert, recommendations are given on how to design with nearly equivalent horizontal and vertical stresses within an Induced Trench. In deeply buried structures, great consideration must also be given to the stresses applied on the geofoam. When geofoam is subjected to large confining axial and confining pressures, it exhibits modulus degradation and can lead to large deformations. This is observed in detail with the case study of the failure of the geofoam within a Bridge Culvert located in Carrs Creek, NY. Proper design methods with geofoam on large spanning, deeply buried culverts are evaluated and outlined. The effects of continuous joints, mixed densities, hydrostatic pressures, overloading, confining stresses, and creep on EPS geofoam are considered. An exponential model that captures the effect of confinement on modulus degradation has been programmed using FISH (FLAC internal coding) and verified with complimentary laboratory testing. A properly designed culvert section is shown, in which all the design flaws of the culvert at Carrs Creek have been addressed. The stresses in the geofoam and underlying culvert are reduced to tolerable stress levels, while deformations have been greatly reduced. The results of this computer analysis have been presented in such a way that they can be used to aid in proper design of buried structures, using geofoam to reduce stresses and surficial displacements.