Jefferson Memorial Seawall
Decades of settlement and lateral movement of the Jefferson Memorial’s north plaza and adjacent seawall prompted the National Park Service to close the north plaza in 2005 due to safety concerns. To prepare for a $13.5 million reconstruction – one of the first large projects to take place in the District under the American Recovery and Reinvestment Act – HNTB hired Schnabel to perform a conditions assessment.
Unlike the memorial itself, which opened in 1943 and is supported by a network of deep foundations and grade beams, the surrounding areas were constructed on grade. Vertical and battered timber piles supported the seawall, which separates the north plaza from the Tidal Basin. The original timber foundation piles didn’t extend to bedrock, resulting in settlement. Our site investigation was rigorous, with an instrumentation program consisting of piezometers, inclinometers, extensometers, and an optical survey. We used the resulting data to design a movement mitigation scheme that called for the demolition of the 500-foot-long seawall and construction of a new one supported by 48-inch-diameter caissons and 18-inch-diameter inclined pipe piles.
Permitting services to authorize construction in the historically and environmentally sensitive Tidal Basin required coordination between the U.S. Army Corps of Engineers, the National Park Service, and the District’s Department of the Environment. While construction was underway, a 5.8 magnitude earthquake centered 83 miles away tested vertical and lateral movement, which was minimal. During construction, we monitored the instrumentation and to this day continue to confirm the wall’s performance on a quarterly basis.