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Deep Foundations: The Wharf Brings New Life to Southwest D.C.
December 02, 2019
by Brandon M. Robinson, P.E., and Joseph K. Cavey, P.E., Hayward Baker, and George A. King, Moretrench
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.
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 lay over 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 0 to 3 ft (0 to 0.9 m) below ground level. Given this profile, construction of the garages meant planning for a significant amount of diverse subsurface work.
Multiple Techniques – One Underground Contractor
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 multitier rakers. Ground improvement using rigid inclusions (RIs) was required for the support of the spread footings within the two-story garage footprint. More than 1,000 RIs are planned. Underpinning of an active 108 in (2,743 mm) diameter DC Water outfall pipe running on a north/south alignment through the center of the site between the two garage structures was also specified using large diameter 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.
Encompassing many geotechnical disciplines under one umbrella, Keller could provide a full foundation package for the subsurface parking structures. Keller was able to draw on expertise and resources from its various business units to work as one entity under a single turnkey contract to the GC. While GEI Consultants and Mueser Rutledge Consulting Engineers designed the support of excavation (SOE) elements, the rigid inclusions and dewatering system were designed by Keller. The project is unique in that the basements for the new buildings will be constructed approximately 10 ft (3.0 m) from the existing subway tunnels. The excavation for the full depth basement required that the SOE be placed adjacent to the tunnel; excavation for the shallow basement required that tie downs be constructed above to balance uplift forces caused by groundwater (after the dewatering program ended). In this very critical zone of the project, Keller installed drilled displacement piles (DDPs), continuous flight auger (CFA) piles, jet grouting, dewatering wells, tie down anchors, wales, rakers and tieback anchors.
Dealing with one subcontractor for the multifaceted geotechnical work offered logistic and economic advantages to the owner and general contractor. Because the crews undertaking various aspects of the contract were with the same company and accustomed to working together, the production effort was streamlined since stages of the work dovetailed more easily, avoiding the downtime that can occur while waiting for areas of the site to be released by other trades.
Subway Tunnel Challenges
The site is located north of the Washington Channel and south of Maine Avenue SW, less than 1.0 mi (1.6 km) from the National Mall. The support of excavation for the underground construction was made more difficult than most projects due to the Washington Metropolitan Area Transit Authority (WMATA) Green Line Tunnels that sweep through the northern third of the site. The WMATA Green Line alignment is on a curve between L’Enfant Plaza and Waterfront Station and the tunnels are constructed from steel liner plate.
The scope of the SOE work around the tunnel system involves creation of a deep foundation support wall using DDP/CFA piles to support the lateral movement during construction until the garage concrete is placed. Between the closely spaced piles, jet grouting was performed to provide a continuous wall and groundwater cutoff. Because the tunnel is directly behind the SOE wall, an externally braced (anchored or tiedback) support system could not be constructed. Therefore, wales with heel blocks and rakers were installed. The placement of the internal bracing was coordinated with the construction of the rigid inclusions — the rigid inclusions could not be installed around the rakers due to the size of the drill rig and, therefore, had to be installed from a higher elevation prior to the installation of the rakers.
The area above the tunnels consists of a 4 ft (1.2 m) thick mat slab anchored into the Potomac clay formation using hollow bar drilling methods and rigid tension/compression anchors. These anchors are designed to support the building loads above the tunnels as well as prevent uplift conditions. In the final condition, nearly 1,200 pile elements will be installed within the tunnel zone.
During excavation and subsequent construction, staged lowering of the groundwater was required to a depth of 40 ft (12.2 m) below the ground surface within the limits of the WMATA tunnels to overcome the tendency of the tunnels to heave as excavation above proceeded. To help achieve this, the dewatering design called for nearly 30, 6 in (152 mm) diameter deep wells fitted with variable speed submersible pumps to be installed within protective surface casings along the excavation perimeter to allow for individual well drawdowns to be adjusted according to the proposed staged drawdown. Eight observation wells with web-based data logger monitoring were also incorporated in the overall dewatering design.
The presence of low levels of environmental contaminates required that the pumped water be brought up to surface water quality standards before discharging into the stormwater system. A 1,000 gpm (3,786 l/min or 3.8 cu/m) temporary customized treatment system was mobilized and consisted of weir sedimentation tanks, sand filters, bag filters, reactivated carbon filtration, activated alumina filtration and pH adjustment.
Multiple Support of Excavation Elements
The owner’s commitments to future tenants meant that the building construction schedule was very aggressive. The accelerated schedule required that multiple subcontractors work side-by-side within a restricted space. Therefore, excavation was started prior to the full installation of the SOE system. This required that, in many
areas, tiebacks and rakers would be installed from a 25 to 50 ft (7.6 to 14.2 m) wide bench around the perimeter, and the sides of the excavation within were sloped at 2:1 to accommodate the soil removal from the site. In addition, pipe piles from the previous building foundation were removed during this time.
Drilled Displacement Piles — Due to the concern that soil loss during the drilling for soldier pile installation adjacent to the tunnel would cause tunnel movement, 443 DDPs that were 18 in (450 mm) in diameter and reinforced with a central steel beam were used along the north side of the deep garage and extended to the Potomac Formation. Due to project restrictions, a portion of the planned DDPs were replaced by CFA piles that were also reinforced with a steel beam in the center. The DDPs and CFA piles were braced by a waler and raker system as the excavation was advanced.
Soldier Beams and Lagging — To the north of the DDP line, the shallow garage excavation is supported by a tied back soldier beam-and-lagging wall system. In this area, the excavation was only 15 ft (4.6 m) deep, as the bottom of excavation is directly above the WMATA tunnels.
Tiedowns — In the area directly above the WMATA tunnels, the building mat slab is supported by tie down anchors bonded in the Terrace and Potomac Formations to provide resistance to uplift loading and to support of the building loading. However, the dewatering will be terminated at the end of construction.
Steel Sheetpiling — The remainder of the excavation support for Garages 2 and 3 consists of approximately 1,140 lineal ft (347.5 lineal m) of steel sheetpiling embedded 5 ft (1.5 m) into the Potomac Formation and supported by tiebacks, internal bracing and tie rods. Steel sheetpiling was used on the west and east sides of
the site, as well as on each side of the 108 in (2,743 mm) diameter outfall pipe.
Jet Grouting —Jet grouting was used to create soilcrete wedges that sealed gaps and created a water cutoff between the DDPs. In addition, jet grouting was used to seal gaps in the sheetpiles within the terrace deposits and the Potomac Formation.
Outfall Pipe Underpinning
In addition to constructing the SOE around the WMATA Green Line on the north side of the site, it was also necessary to provide SOE and underpinning of a 108 in (2,743 mm) diameter DC Water outfall pipe running on a north/south alignment through the center of the site between the two garage structures. The outfall pipe was first underpinned with 6 ft (1.8 m) diameter jet grout columns installed beneath the cradles that supported the pipe. During jetting operations, the outfall pipe remained in service, which required ongoing monitoring for movement. Next, steel sheetpiling was placed on both the west and east sides of the pipe, and tie rods were installed between the sheets for lateral support. An all-thread steel bar was used for each of the tie rods, connecting the west and east sheetpiles above the pipe. These tie rods were installed by excavating trenches (and setting the bar) between the rows of sheet piles.
Rigid inclusions were installed for ground improvement within a portion of Garage 2A to support the floor slabs at level P2 as well as to support the spread footings at level P2 to an average allowable bearing pressure of 8,000 psf (383 kPa). Post-construction settlement of no more than 1 in (25 mm) was specified. Prior to production work, load tests on two individual RIs were performed to verify the design capacity. The first phase of the installation of RIs were performed from a berm at the north side of the garage prior to the installation of the wale and rakers along the DDP line. This sequence was used because the equipment to install the RIs would not be able to access the area once the wales and rakers were set. The remainder of the RIs for the slab support and foundation support were installed using two drill rigs from the subgrade elevation. A load transfer platform (LTP) that was 12 to 18 in (305 to 457 mm) in thickness was constructed beneath each foundation location after the RIs were installed.
On large, complex projects such as this, experience is key to a smooth operation. Keller’s history of successful geotechnical work throughout the Washington D.C. area, its ability to undertake multiple aspects of the foundation package under one contract, and the various crews’ familiarity with working together were key to the overall smooth running of the project. During peak operations at the jobsite, more than 50 Keller personnel were employed across the various tasks at the jobsite. Any issues that arose could be quickly resolved through close cooperation among all parties (owner, GC and Keller), which translated into schedule and economic advantages to the owner and general contractor.
To complete the significant amount of work in such a congested space on an accelerated schedule required collaboration and communication among all parties involved, including the several other subcontractors. The ongoing foundation work is expected to be completed by February 2020.
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