Reduction of Stresses on Buried Rigid Highway Structures Using the Imperfect Ditch Method and Expanded Polystyrene (Geofoam)

OBJECTIVES: The objective of this study is to examine the use of expanded polystyrene (geofoam) and the imperfect ditch method for reducing the vertical stresses on rigid buried highway structures, such as pipes and culverts. BACKGROUND: Construction of highway embankments above highway pipes and culverts has great practical significance because of the stresses imposed by the fill on the buried structure. Relative stiffness of the culvert and soil control the magnitude and distribution of earth pressure on the buried structure. The vertical earth pressure on a flexible culvert is less than the weight of the soil about the culvert due to positive arching. However, the vertical earth pressure on a rigid culvert, or pipe, is greater than the weight of the soil above the structure because of negative arching. Experiments have shown that the loads on rigid embankment culverts were some 90 to 95 percent greater than the weight of the soil directly above the structure. In model tests performed by Hoeg (1968), the crown pressure was about 1.5 times the applied surcharge. Penman et al. (1975) measured the earth pressure on a rigid reinforced concrete earth pressure below 174 feet of rock fill and found that the vertical earth pressure on the culvert crown was about 2 times the overburden stress due to the fill above the top of the culvert. To avoid high stresses on rigid buried structures, the imperfect trench, or induced, ditch method of construction was invented. The imperfect trench method involves installing a compressible layer above the culvert within the backfill. Traditionally, such compressible materials as baled straw, leaves, old tires (used in France), or compressible soil, have been used. As the embankment is constructed, the soft zone compresses more than the surrounding fill. This process induces positive arching which reduces the stresses on the buried structure. FY 2006 ACCOMPLISHMENTS: Construction of an embankment located above a reinforced concrete box culvert on Ky Route 127 (Jamestown Bypass) was almost completed. The contractor is scheduled to complete the pavement before July 2006. Installation of all instrumentation of the culvert was completed. Twelve stress cells, 2 inverted settlement platforms, and 3 strain gages were installed on and in culvert. This site was selected to demonstrate the use of the "imperfect ditch" (backfilled with compressible geofoam) technique to reduce stresses acting on a culvert. Stress measurements on the top slab and sidewall of the culvert, strain in the top slab, and settlement of the geofoam were obtained once a week as the fill was constructed. Analysis of measured versus predicted stresses were performed. Personnel in the Division of Bridges have been contacted about other culverts to analyze. An interim report was issued documenting the numerical analysis (using FLAC 4.0) to predict stresses and to fmd the optimum location of the trench and geofoam to reduce stresses. Field measurements show a great reduction in vertical stresses acting on the culvert when the imperfect trench with geofoam is used. FY 2007 PROPOSED WORK: Measurements of stresses and strains of the culvert at the Jamestown Bypass site will continue periodically after construction of the embankment Measurements will be made periodically after construction. The Research Study Advisory Committee is working to locate another site to instrument. Numerical analysis has been performed at two other sites and a potential site has been evaluated. This site will be instrumented depending on the timing of the contract letting. Stress results obtained from field measurements will be compared to those predicted using finite differences analyses (FLAC 4.0). A fmal report will be issued.