While replacing the existing 15-ft-long concrete slab bridge for this $7.2-million project, the team faced challenges that included compressible soils and tidal conditions. Because a sealed cofferdam was not employed, all work was conducted during 8-ft tidal cycles.

Native tidal marsh marine sediments were removed and replaced with foam glass aggregate—a first for New England—when the causeway roadway was widened approximately 10 ft to the north. 

These deep excavations beside the operating highway, along with use of the foam that is less dense than water, presented challenges, according to the team. 

When the team realized that the contractor’s original work plan was not viable, it brainstormed with structural and geotechnical engineers to develop the best possible action plan. 

Ultimately, the alternative project approach involved having the contractor employ an innovative slide-rail system comprising steel walls and driven steel piles, which included a series of adjustable trench boxes. 

When the tide receded, the trench boxes were lined with geotextile fabric that formed 20-ft by 10-ft cells into which the foam glass aggregate material was placed on the roadway slope. These were then wrapped and covered with steel plates or soil to prevent them from floating. 

The team says it completed the project at budget and 50 days ahead of schedule.