Regulatory, Environmental and Geotechnical Solutions to Construction of a Roadway Embankment Over a Landfill

The new Illinois Route 143 alignment was constructed with the new Clark Bridge that crosses the Mississippi River in Alton, Illinois. The new bridge forced the Illinois Department of Transportation (IDOT) to cross the Alton Commons Landfill with a new alignment. Approximately 318 meters of embankment with fill heights ranging between 4.5 to 7.6 meters were constructed on top of the landfill. An Environmental Impairment Assessment, Preliminary Site Assessment (PSA), Phase II and Phase III investigations were completed in 1989 and 1990. Using Brownfield concepts, the Illinois Environmental Protection Agency (IEPA) approved the alignment construction without having to remediate the site. This approval was contingent upon the following three conditions: - requiring a deed restriction, - restricting groundwater use, - prohibiting intrusive activities. The landfill was preloaded with a test fill of 5.2 meters of soil and reloaded with 7.6 meters of soil when embankment construction commenced with the planned fill and an additiona13 meters surcharge. Settlement plates and inclinometers were used to monitor the earthwork performance during and after construction. As the landfill was loaded, consolidation was immediate, typically 5 to 7 days of loading. Total settlement including movement during preloading ranged between .6 and .9 meters for fill heights ranging between 5.5 and 7.6 meters. Settlement reading showed 10 to 15% of the landfill thickness consolidated beneath the embankment. Settlement two years after embankment construction showed the landfill was settling 1.8 centimeters per year. Settlement after three years showed less than 1 centimeter per year. The ongoing movements is characteristic of secondary compression and possibly undrained creep. The landfill material does not fit the category of soil types or conditions that would be susceptible to creep; however, this is the most similar plastic deformation model available to the authors and as such undrained creep was qualitatively analyzed. The initial log of strain rate versus log time plots after primary consolidation was completed have a slope that is flatter than 1:1 indicating creep to failure. The plots for the last year have steepened indicating a stable condition