Abstract:
Construction of the second phase of The Wharf began in 2019 and geotechnical construction was completed by December 2020. The overall project consisted of 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 square feet of above-grade construction including five multi-story, mixed-use buildings built over up to three levels of underground parking. The site is located between the WMATA Green Line Tunnels to the north and the Washington Channel to the south; it is bisected by a 108-inch diameter DC Water outfall pipe. This arrangement, along with very difficult ground conditions, created challenges in the geotechnical design and construction.
The support of excavation for this complex project required sheet piles, soldier piles, displacement piles, tiebacks, deep foundation anchors, jet grouting, and internal bracing with multi-tier rakers. Ground improvement using Rigid Inclusions (RI’ s) was required for support of spread footings in soft soils over the garage footprint. Underpinning of an active 108-inch diameter DC Water outfall pipe running on a north/south alignment through the center of the site between the two garage structures was also performed using jet grouting. Since portions of the support of excavation are below the groundwater table, an extensive pre-excavation dewatering program was necessary and was separate from the foundation package.
On the north side of the project, support-of-excavation (SOE) was required on a curved alignment, offset approximately 10 feet from the outbound Green Line tunnel. Due to the proximity of the tunnel, a stiff, near zero-displacement wall was required. The wall consisted of a combination of drilled displacement piles (DDP) interlaced with auger-cast piles in a pattern designed to minimize tunnel movement.
A portion of the buildings extends over the tunnels, imparting load to the tunnels to approximately restore original overburden loads excavated for the basement space. The building foundation is a 4 foot thick mat slab anchored into the Potomac clay formation. The anchors were designed either purely for tension or to work in both compression and tension, and were installed by hollow bar drilling and grouting. The anchors were installed between and on both sides of the inbound and outbound tunnels, requiring low-disturbance drilling. Continuous movement monitoring of the tunnels was performed by an automated system.
This paper describes the design and construction of the SOE and foundations for this huge project in Washington, DC, which required collaboration between the Owner, Engineer, CM and Specialty Geotechnical Contractor to solve many challenging problems.