Design and Construction of Freestanding Expanded Polystyrene Roadway Embankment in Downtown St. Louis, Missouri

摘要

This paper discusses the design issues and construction features for the replacement of the North Tucker Bridge in downtown St. Louis, Missouri. The existing bridge was originally constructed in 1931 as part of the realignment of the passenger railway between McKinley Bridge and Union Station. A 30-feet (9.1 meter) deep excavation was completed for the passenger railway and the bridge was constructed at ground level to carry vehicular and pedestrian traffic. The steel bridge structure has undergone numerous repairs within the past 20 years and has experienced significant deterioration from corrosion of both its super- and substructure which poses safety concerns. The railway has since been abandoned. The purpose of this project was to eliminate further costly repairs and to restore the proper and safe use of the roadway and sidewalks. The designers were also presented with the challenge of providing a 100-year maintenance-free structure. This was accomplished by removing the existing bridge, backfilling the excavation to ground level and supporting the roadway on grade. The construction material selected to backfill the excavation was a combination lightweight Expanded Polystyrene (EPS, or Geofoam) and soil fill. Several historic buildings run parallel to North Tucker Boulevard and the elimination of any lateral load induced onto these buildings by the placement of soil backfill to support the new roadway was a significant design consideration. Several retaining wall alternatives were evaluated, but ultimately it was decided that a series of free-standing 30-foot (9 meter) high EPS adjacent to the buildings and soil fill farther away from the buildings would the most cost effective and practical solution to minimize lateral loads on the existing buildings. A 6-foot (1.8 meter) thick soil planting layer was placed on top of the EPS fill to accommodate buried utilities and to support tree growth. Since the site is situated within the New Madrid Fault Zone, the response of the EPS/soil system to strong seismic ground motions was a significant element of the design. The seismic design approach for this project followed the principles and guidance discussed in papers by Drs. Stephen Bartlett and Evert Lawton for similar lightweight fill applications. Specific design challenges included: (1) analyses and predictions of stresses and strains throughout the free-standing EPS structure during traffic loading and earthquake shaking using finite element methods; (2) the design of an underdrain system to collect groundwater and prevent uplift on the EPS from the buildup of groundwater , water line breaks, utility leaks, or surface water infiltration; (3) conventional MSE wall design and associated ground improvement; and (4) design of downdrag-reducing elements on existing foundations to receive earthen fill. The 6-foot (1.8 meter) of cover soil on top the EPS produced a top-heavy structure that presented significant design challenges, particularly in regard to seismic soil-structure interaction and prediction of deformation.