Evaluation of DOTD semi-integral bridge and abutment system.
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2005-03-01
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Abstract:The Louisiana Department of Transportation and Development (LADOTD) designed and constructed its first prototype semi-integral abutment bridge in 1989. In this design, large longitudinal movements due to expansion and contraction, creep, shrinkage, and settlement are mitigated with an annular space, or gap, constructed between the backwall and the roadway embankment. This annular space is created using a geosyntheticreinforced embankment constructed underneath the approach slab on the roadway side. To date, DOTD has constructed six prototype semi-integral bridges. These bridges are located in the north, central, and western parts of the state. All six-prototype bridges were replacement projects in areas of the state where soil conditions are relatively good and, therefore, settlement was not a concern. The approach slab in the DOTD prototype design is cast integral with the bridge making it one continuous structure. Construction of a geosynthetic-reinforced embankment would eliminate the lateral pressure transfer to the backwall of the semi-integral bridge. The prototype design addresses the problem of the loss of soil support under the approach slab due to settlement or lateral movement. A gap created between the backwall and the reinforced embankment would eliminate the passive pressure from developing on the backwall due to bridge movement into the backfill. Permitting free backwall movement would also eliminate the potential for abutment rotation.
In view of the review of existing records, field inspections, conventional structural and geotechnical analyses, and the finite element parametric study, the researches concluded that the present DOTD design for semi-integral bridges is structurally sound. Based on the results of a cost/benefit analysis, the researchers concluded that the present design is also cost effective. Therefore, the present design could be continued by DOTD in areas with fair to good subsoil conditions. Future designs should consider the effect of settlement and the potential for deep-seated slope stability at sites with thick, soft cohesive and/or highly compressible subsoils, specifically when the grades require relatively high embankments. At these sites, preload surcharge programs, possibly with wick drains, could be considered. Alternatively, lightweight aggregate or EPS geofoam could be used for the construction of the embankment to minimize detrimental effects of settlements.
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