The MLK Jr. Memorial was planned and funded by the MLK Jr. Memorial Foundation and will be maintained by the National Park Service. The project was constructed under a design-build contract between the Foundation and a joint venture of McKissack & McKissack; Turner Const.; Tompkins builders; and Gilford Corp. Thornton Tomasetti, Inc. was the structural consultant to the joint venture.
The site is positioned at the northwest side of the Tidal Basin at the National Mall in a triangulated site bounded by Independence Ave. to the north and a re-aligned West Basin Drive to the west.
The Memorial comprises three major elements with an abundance of landscaping to integrate it with the historic cherry tree lined perimeter of the Tidal Basin. Central to the theme of the space are three massive stone sculptures symbolizing the words from Dr. King’s “I have a dream” speech – “…hew out of the mountain of despair a stone of hope.” Two opposed pieces of the “mountain” were seemingly cleaved from the central entrance to the plaza of the memorial, with the separated “stone of hope” in which King’s image is carved in relief set within the plaza facing the tidal basin and the Jefferson Memorial beyond. Flanking the “mountain” is a crescent of walls that bear inscriptions of excerpts from Dr. King’s speeches and writings and also serve as retaining walls for the landscaped earth berms that rise from street level to about a third of the height of the 9-meter “mountains.” The plaza falls off toward the tidal basin to the southeast, with the “stone of hope” and two kidney-shaped landscaping areas breaking the expanse. Where the inscription walls meet the “mountains,” there are features with a fall of water across the battered face of the walls. The sculptures were created by linked rings of shaped granite blocks stacked in tiers approximately one meter thick and filled with a cast-in-place concrete core as each tier was constructed.
Thornton Tomasetti built upon a structural scheme that was developed by the San Francisco office of GFDS Engineers in the scope documents from which the design/build team began. The structure underlying the features and finishes of the Memorial consist of 30-inch reinforced concrete foundation mats for the sculptures, a 12-inch reinforced concrete two-way flat slab cast-on-grade with grade beams edging the plaza, and cantilevered concrete L-shaped retaining walls of variable height that separate the landscaped berm from the plaza. There is also a bookstore/restroom building west of West Basin Drive and an underground pump-room/access vault for the water features. All these structures are supported by a pile foundation system. The primary challenges to implementing the design were the site geology and the incorporation of the underground service vaults for the water features.
Schnabel Engineering assisted the design/build team and Thornton Tomasetti with geotechnical input, addressing the initial challenge of the deep foundation system and retaining the landscaping berm. The site is underlain by recently deposited soft- and under-consolidated alluvial silt and sand. The alluvial deposits extend 35 feet to 50 feet below the plaza level. Residual soils, derived from in-place weathering of bedrock, lie below the alluvial deposits. Depth to bedrock varies across the site, from 40 to 55 feet below the plaza level. Because of the soft and unconsolidated deposits, and the potential for significant settlement, a pile foundation system (14-inch precast pre-stressed square concrete piles) driven to bedrock was recommended to support the memorial structures and plaza. The piles were designed for an allowable capacity of 100 tons each. Uplift and lateral loading were considered in the pile design as well. Settlement of the soft alluvial soils was also a concern for the soil berms abutting the curved memorial walk and “mountain.”
The requirements of the National Park Service for maintaining the water features brought about the second most challenging part of the design; positioning the vaults and a connecting tunnel beneath the pile-supported mat underlying the “mountain” sculptures.
The location of the service vaults was chosen after the mat foundation and piles supporting the “mountain” sculptures were designed. Therefore, to accommodate the vaults and the connecting tunnel between vaults, the piles that supported the “mountain” sculptures needed to be rearranged and reevaluated for both their new positions and the additional load resulting from the tunnel being supported by the mat foundation. Access to one of the vault rooms was provided by an access tunnel in the berm behind the inscription wall. The very high water table at the site gave cause for concern that hydrostatic uplift acting on the access tunnel would cause the access tunnel to “float” out of the ground. There was no concern where there was enough earth on top of the access tunnel to resist the uplift force, but where the stairway comes down to the access tunnel, additional concrete was added to resist the uplift force.
Thomas B. McElwain, P.E., is a vice president with Thornton Tomasetti in Washington, D.C.