GEOSTRATA Magazine

On the second Sunday in June 2023, Philadelphians woke up to the news that a bridge on Interstate 95 was “on fire” and had collapsed. Suddenly, a critical route through southeast Pennsylvania, and a crucial connection for commerce and travelers along the east coast, was out of service. Instead of looking forward to summer weekends “down the shore,” many had horrific visions of the traffic gridlock that was sure to be part of their daily lives until the highway was fixed.

First opened in 1929, the Milton-Madison Bridge connects the historic communities of Madison, Ind., and Milton, Ky., and is one of the few Ohio River crossings between Louisville, Ky., and Cincinnati, Oh. Besides the Milton-Madison Bridge, the next closest bridge is located on the Markland Dam, some 26 miles away. In its 80 years of existence, the original steel truss bridge had far exceeded its useful life. Despite multiple rehabilitations, the bridge's 20-ft-wide road deck was too narrow to handle the 10,000+ vehicles that cross it every day. Also, a biennial inspection indicated advanced deterioration of various components, including gusset plates, forcing the Kentucky Transportation Cabinet to impose a 15-ton weight limit on the structure. In August 2008, KYTC and the Indiana Department of Transportation combined resources to initiate a project to remove and replace the 3,180-ft-long deteriorating bridge.

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.

Major developments in London have happened over the last 200 years, meaning the city has generations of archaeology, foundations, and numerous tunnels, all of which impact the construction of new foundations. Victorian buildings were initially founded on early unreinforced shallow foundations or unreinforced rafts that were founded either on dense gravels or stiff clays. As steel framed structures were introduced, they were founded on steel grillages resting over shallow pads and strip foundations, or grillages embedded within the footings.

Increasingly, in bridge widening and replacement projects, engineers are considering foundation reuse or, if retaining the same bridge alignment, the reuse of the whole substructure element, including foundations, piers, and walls. Foundation reuse projects are particularly beneficial in urban areas due to high mobility and traffic demands. A notable application includes pairing foundation reuse with accelerated bridge construction technologies for superstructure replacement — such as using prefabricated bridge elements and systems. The obvious rationale is cost savings, but other factors, such as decreased construction time, sustainability considerations like reduced carbon emissions, reduced site impacts, and historical preservation, are also important considerations. However, reuse has its challenges, not the least of which is to accurately assess the structural integrity, durability, and the capacity of existing foundations, because it's essential that reused foundation elements function at least as acceptably as the superstructure elements they will bear for the remainder of their expected service life.