O COVER STORY Rendering of The Wharf (courtesy of Hoffman & Associates) SPECIAL ISSUE : The Wharf Brings New Life to Southwest D.C. First mapped by Captain John Smith in 1608, the southwest neighborhood of Washington, D.C. has a long and storied history dating back to the early Native American fishing and farming communities that settled along the banks of the Potomac and Anacostia Rivers. With the establishment of Washington, D.C. as the nation’s capital in 1791, city planner Pierre L’Enfant designated Southwest D.C. to house a major inland seaport. During the Civil War, the port served as a staging point and supply center for the troops. After the war, affordable housing and the promise of jobs attracted thousands to the area and tight-knit working-class communities sprang up. Over the years, the area has seen many changes, including commercial construction that limited the public’s access to the waterfront. In 2003, the District of Columbia called for the redevelopment of the waterfront to increase access and commerce to the area. The city formed a public-private partnership (P3) with developer Hoffman-Madison Waterfront (a joint partnership between Hoffman & Associates and Madison Marquette) to develop the $2.5 billion, 3.2 million sq ft (297,300 sq m) waterfront neighborhood — one of the largest private developments in the city. In 2017, following a decade of planning, development and construction, the first phase of The Wharf opened and reestablished Washington, D.C. as a true waterfront city and destination. Construction of the second phase of The Wharf began in 2019. Balfour Beatty was selected as the general contractor (GC) for the construction of the new below-grade parking, horizontal site, parks, and office buildings in Parcels 6 and 7. The Wharf’s Phase 2 development includes approximately 1.2 million sq ft (111,485 sq m) of above-grade construction including five multistory, mixed-use buildings built over up to three levels of underground parking. The underground parking structure is split into two separate garages accommodating a total of more than 1,200 vehicles. Together, the two garages cover the entire approximately 7 acre (28,330 sq m) construction footprint, with excavation depths up to 30 ft (9.1 m), and are to be constructed concurrently. Subsurface Conditions The site subsurface profile is typical for the Washington, D.C. area, with fill consisting of sand and gravel with varying density underlain by alluvial deposits (clayey sand and sandy clay) which in turn are underlain by granular terrace deposits. The terrace deposits overlays the Potomac Formation, which consists of an upper layer of medium-stiff clay, with the lower portion consisting of very dense sand. Groundwater fluctuates with the tide and is generally encountered at 12 to 15 ft (3.6 to 4.6 m) below ground level. Given this profile, construction of the garages meant planning for a significant amount of diverse subsurface work. 14 • DEEP FOUNDATIONS • NOV/DEC 2019 AUTHORS Brandon M. Robinson, P.E., and Joseph K. Cavey, P.E., Hayward Baker, and George A. King, Moretrench J R E P C T A S G E & M
