The ABCs of a Bridge Slide

I-295 in Portland, Maine, is a 60-year-old auxiliary Interstate Highway connecting the city with I-95 just to its south and West Gardiner to the north. At U.S. Route 1/Veranda Street along the Portland waterfront, I-295 traffic was carried over the local roadway by a three-span bridge built in 1960, supported on vertical and battered 12BP53 steel H-pile foundations driven through deposits of compressible Presumpscot Formation marine silt-clay. The bridge had been classified as structurally deficient, and safety improvements were deemed necessary for the adjacent ramps and the Veranda Street roadway below. The physical constraints at the bridge site and the volume of daily traffic did not permit a conventional replacement. To overcome these challenges, accelerated bridge construction was employed. This is the story of the geotechnics behind that ABC plan, its implementation, and the outcome.