A double row of rigid anchor supports, installed by hollow bar drilling, are located between twin Metro tunnels. The team also installed foundation vibration isolation material. Photo courtesy of Thornton Tomasetti

When Phase 1 of The Wharf opened in October 2017, the massive mixed-use development instantly offered a nearly 2-million-sq-ft addition to Washington, D.C.’s Southwest Waterfront area. Delivered all at once, it created the basis of a new neighborhood, featuring a broad mix of buildings surrounded by public spaces with access to the Washington Channel. For developer Hoffman & Associates (H&A) and its design and construction partners, the project also provided valuable insight into how it would approach the development’s 1.2-million-sq-ft Phase 2. “We learned a lot, and our team did a great job of keeping track of lessons learned—from major change orders to soil conditions,” says Maria Thompson, executive vice president of development and construction at H&A. “We ended up with a 25-page Excel spreadsheet that we used extensively in the planning and design of Phase 2.” 

The support of the excavation wall consisted of tangent piles installed by displacement drilling. The wall was braced by preloaded rakers with heel blocks integrated with the permanent building footings. Photo courtesy of Hoffman & Associates

Early planning of the 3.2-million-sq-ft, $3.6-billion development began in 2006. After years spent gaining site control from the District of Columbia under a long-term ground lease, a master plan, created by Perkins Eastman, was in the works by 2010. That plan called for breaking the site—which spans nearly a half-mile of waterfront—into 10 parcels with large underground parking garages serving as podiums for new buildings.

Shawn Seaman, H&A’s president, says the team wanted to replace the existing “superblocks” on the site—some of which were as long as 1,200 ft—with a series of smaller buildings intersected by public access points. “What really needed to happen to transform the site was to look at a smaller grain development,” he says. “Our blocks are 250 feet long at most, which has a lot of permeability to allow pedestrians access through the site out to the water. Putting the parking below grade was the fundamental change that allowed us to come up with that plan.”

Targeting a 2014 construction start, Seaman says H&A had financing in place, a fixed budget and a fixed schedule, all of which drove the decision to sign a nearly $600-million design-build contract with Clark Construction. “At that point,” he says, “we were running short on time to start the project with the District and we were really intent on maintaining cost, maintaining schedule and getting the project delivered in the three-and-a-half years that we had allocated.”

Located on D.C.’s Southwest Waterfront, The Wharf includes a historic fish market, hotels, residential units, restaurants, shops, parks, piers, docks, marinas and live music venues. Rendering courtesy of Hoffman-Madison Waterfront

For Phase 2, which broke ground in 2019, Seaman says H&A preferred to use a more traditional design-bid-build delivery, contracting directly with multiple architects and contractors. While Perkins Eastman remained as the master planner of the Phase 2 garages, H&A tapped a new set of design and construction  firms to offer a fresh perspective on other elements. The team of SHoP Architects and WDG Architecture with Balfour Beatty Construction are delivering a 500,000-sq-ft, two-tower, trophy-class office building. 

Donohoe Construction is building a 380,000-sq-ft facility, designed by ODA Architects, which features 255 apartments and a 131-room hotel. Donohoe is also building the 240,000-sq-ft Amaris luxury condominium building, designed by Rafael Viñoly Architects. Last year, DPR Construction completed work on a 90,000-sq-ft trophy office building designed by Morris Adjmi as well as two retail buildings on the pier. 

“It’s a nice assemblage of architects with a little bit of a different approach than Phase 1,” Seaman adds. “Visually, it’s different than Phase 1, which is a positive.”

Although each building in Phase 2 provides a unique look, Douglas Campbell, associate principal and managing director at Perkins Eastman, says the development retains the appearance of a cohesive neighborhood. The “big idea” of the master plan, he says, was to deliver a development that looks like it was created over a number of years, rather than in large phases. “If you look at each piece individually, Phase 2 makes some bolder statements [than Phase 1], but it all holds together,” he says.

With those bold statements come more complex structures, Campbell says. Design of the parking garage, which serves as the podium of the building, also faced significant structural challenges. To help streamline the structures, a joint venture of SK&A and Thornton Tomasetti serves as structural engineer for   the various projects. “We needed the same structural team working from the head to the feet because this all had to work together,” he says.

In addition to driving piles around two Metro tunnels, the project team also built an underground parking garage next to the tunnels. Drawing courtesy of Thornton Tomasetti *Click on the image for greater detail

Before work on the main structures could get underway, H&A addressed the seawall. Thompson says the Phase 1 seawall was delivered in sections, as teams excavated portions then built new concrete walls in sequence. For Phase 2, the team decided to use an 800-linear-ft king pile system. The team installed the entire new seawall 24 in. from the existing seawall to depths of up to 85 ft into the channel. Once that work was complete, crews could demo the entire existing seawall. Thompson says the seawall, which was designed by Moffatt & Nichol and built by Cianbro, was completed two months faster than it would have been under the Phase 1 method, according to Thompson.

Installing a deep king pile system also enabled designers to add a third level to parts of the parking garage. Campbell says that strategy allowed the team to maximize available parking spaces, while also accommodating the placement of building utilities in the parking garages. With a deeper parking garage, the site was also excavated down to firmer soils, says Mark Tamaro, managing principal at Thornton Tomasetti. He says the soils near the Washington Channel contain silty alluvial material, especially close to the ground level. For the two-level parking garage, crews installed approximately 2,800 rigid inclusions at depths of 20 to 30 ft to help strengthen the soils. “That allowed us to use a higher bearing pressure but allowed us to implement spread footings—an economical approach,” he adds.

For the three-level garage, however, spread footings could be used without ground improvements. “The fact that they brought the garage down to a better bearing strata … had the tangential benefit that you got a simpler foundation system,” Tamaro adds.

While the parking lot design enabled the team to better achieve its larger vision for The Wharf, that strategy came with significant challenges. In addition to the garages’ proximity to the Washington Channel, a 108-in.-dia DC Water pipe bifurcates the site between the apartment building and the Amaris condominiums. Structurally, designers decided to split the parking garage at the pipe location. Throughout construction, the pipe remained in place, monitored and protected. Jet grout columns were installed to ensure that the pipe remained isolated and protected.

During excavation, which reached depths of 35 ft, a wide variety of support systems were employed. In addition to the king pile system, crews installed approximately 1,400 linear ft of sheet pile walls, 157 drilled soldier piles, tiebacks, tie rods, cross braces that were preloaded by jacking and rakers with heel blocks—including some so large that they were combined with spread footings or crane foundations. 

The project team erected a seawall and 12-story Viñoly Tower with sloped columns directly above the Metro tunnels. Photo courtesy of Hoffman & Associates

Perhaps the project’s most challenging aspect, however, was the alignment of two decades-old Washington Metropolitan Area Transit Authority (WMATA) tunnels that curve through the site, directly below the Amaris project. The tunnels themselves are constructed of steel liner plate, rather than concrete, which is more prevalent in the Metrorail underground sections, which needed to be protected, says David Good, principal at Mueser Rutledge Consulting Engineers. The steel liner plate “is thinner than cast iron or reinforced concrete and therefore requires additional protections,” Good says.

To address concerns, a 5-ft construction exclusion zone was created around each tunnel. However, Good says the tunnels were held in place by the soil loads, so any change in those loads threatened to move or damage the steel tunnels. Because the parking garage and the Amaris are located above the tunnels, soils had to be excavated from above the tunnel alignment at depths of roughly 14 ft, and could differentially relax the load on those tunnels. 

Mueser Rutledge and Thornton Tomasetti devised a system that would preserve the tunnels and maintain loads within a threshold of 300 pounds per sq ft. Above the tunnels, crews installed a 4-ft-thick mat slab foundation. Traditional drilled piles couldn’t be used because any removal of soil would cause differential relaxation, Good says. Instead, a support of excavation wall was built using 443 drilled displacement piles and 18-in.-dia continuous flight auger piles in tangent configuration within 10 ft of the tunnel structure.

“We essentially pushed piles into the ground,” he says. “So instead of the tunnels moving toward the excavation—which is traditional—they actually moved away because of the force of those DDP piles. Then when we did the excavation it would come back a little bit.”

Installation of these elements required constant tuning of loads, as drilling piles in proximity to the tunnels could cause beyond-threshold load changes. “The creative solution that the Mueser Rutledge team came up with was a series of [16] dewatering wells, where they could adjust the water level around the tunnels,” Tamaro says. “As you lower the water table, you’re actually increasing the overburden above because instead of being material that’s buoyant, it’s now become heavier.” 

Workers pour concrete for the sloped columns on the third level of the Viñoly Tower. Photo courtesy of Hoffman & Associates

In addition, the structural design of the Amaris tower placed a stair core directly above the tunnel alignment with no transfers within the tower itself. The team designed a system of load-sharing elements. It installed in this parcel 132 grouted hollow bars with diameters of 8 in. to 12 in., at depths between 60 ft and 90 ft. By using hollow bars, the system could be installed beside and even between tunnels, functioning in both compression for load sharing and tension for uplift resistance.

With so much structure near the tunnels, concerns were raised about the potential for vibration from the WMATA Metrorail trains being felt in the condominium building. H&A committed to spending $12 million on vibration mitigation, installing specialized vibration isolation material below and beside all foundations near the WMATA tunnels. “We couldn’t do all of this and, at the end of the day, have residents feel that 7 a.m. train pass underneath them,” says Monty Hoffman, the company’s founder and chairman. “That’s a case where we erred on the side of caution to do the right thing.”

Above the tunnels, the Amaris is one of The Wharf’s signature buildings, serving as a virtual gateway from the eastern end of the development. The building features an orthogonal design at the elevation that meets the rest of the development and an expansive curved facade that faces into an open park on the opposite side. “The idea of the tower was to pull it back from the public park and effectively our tower becomes the face for the whole development,” says Jay Bargmann, vice president at Rafael Viñoly Architects. “Then you have views of the marina and downtown D.C. that follow the curving sightlines.”

At either end of the curved facade, the building slopes out at approximately 28 degrees. At the eighth floor, the structure transitions from vertical and sloping to just sloping. The sloped columns resolve back at a thicker non-post-tensioned slab at Level 3.

To create the buildings dramatic curved elements, the design team specified cold-warped glass panels. The technology allows panels to be shipped flat, then during installation, corners can be pulled into place by a mullion system.

“It was probably one of the more intimidating elements of the design for the owner to take on, but we talked them through the technology,” recalls Jac Selinsky, project manager at Rafael Viñoly Architects. “During construction it was actually one of the smoothest constructed components of the building. Those curtain walls went up in about eight weeks or so with no hiccups or delays. It all went up like clockwork.”

Thompson estimates that the cold-warped glass component was completed one month ahead of schedule. 

According to the developer, the major parcels of Phase 2 are on schedule to be delivered this year, starting with the office building in March and finishing with the Amaris in November. 

“We learned a lot in Phase 1,” recalls Hoffman. “It was all a bit of an experiment, but we were really able to dial it in on this one.” 

Bruce Buckley is a freelance writer and photographer based in the Washington, D.C., area.

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