A 28 ft tall granite sculpture of Martin Luther King, Jr., titled the Stone of Hope stands in the center of the Martin Luther King, Jr., Memorial plaza. Courtesy of Thornton Tomasetti
Soft soils and late changes in underground structures provided challenges for the engineers who designed the Martin Luther King, Jr., Memorial in Washington, D.C.
President Barack Obama spoke at the October 16 dedication of the long-awaited Martin Luther King, Jr., Memorial, in Washington, D.C., telling the thousands gathered for the event that King’s work has not yet been completed. Obama said, “With our eyes on the horizon and our faith squarely placed in one another, let us keep striving, let us keep struggling, let us keep climbing toward the promised land of a nation and a world that is more just and more equal for every child of God.”
The $120-million memorial, the most recent to grace the National Mall, opened in August and includes a crescent-shaped plaza, a 28 ft tall granite sculpture of Martin Luther King, Jr., a stone gateway flanked by two waterfalls, a curving stone-clad retaining wall bearing inscriptions, a 175 ft long access tunnel, below-grade pump rooms, and a bookstore, according to written information provided by Thornton Tomasetti, the engineering firm that served as the structural engineer. The New York City-based firm joined the project in 2007 as part of a design/build team.
The memorial occupies a 4 acre site along the Tidal Basin adjacent to the Franklin D. Roosevelt Memorial and is situated on a direct line between the Lincoln and Jefferson memorials. Thornton Tomasetti’s primary structural challenge was overcoming the soft soil conditions of the National Mall, which is located on a reclaimed landfill. The memorial rests on 340 structural piles that have been driven to bedrock.
At grade, a curving, green granite retaining wall, on which are carved quotes from King’s speeches, ranges in height from a few feet along the perimeter up to19 ft at the center. The wall joins a sculpture entitled Mountain of Despair, which is flanked by two waterfalls, known as the fountains.
A stone sculpture entitled Mountain of Despair is a gateway
to the plaza and is joined by a curving stone-clad retaining
wall inscribed with significant quotes from King’s speeches.
Courtesy of Thornton Tomasetti
Tom McElwain, P.E., a vice president of Thornton Tomasetti, and Calvin D. Austin, P.E., a project engineer for the firm, led the engineering team. McElwain explains that the addition by the National Park Service (NPS) of pumps beneath each of the waterfalls changed the configuration of the underground foundation systems. “The fountains were one of the biggest challenges, primarily because over the course of the several years that we worked on it, the equipment room and how they would service those fountains kept changing,” says McElwain. Thornton Tomasetti worked with the NPS to determine the best location for the underground utility rooms. “They had a lot of issues with confined spaces and where they would occur,” McElwain continues. “In the final arrangement the underground utility rooms went directly below the fountains and are joined by a tunnel directly under a mat that supports the mountain sculpture.”
McElwain says that from the beginning the project was locked into using deep foundations. “We were faced with a project that involves as structural components a plaza that was on grade but that had to be supported by deep foundations,” says McElwain. “We ended up with a two-way concrete slab supported on pile foundations.”
The foundation system uses precast concrete piles measuring 14 in. square in cross section; the shortest is 30 ft, the longest is 65 ft. The concrete piles were sunk roughly 20 ft into the soft earth before they could be driven to bedrock. The test capacity of the piles was 100 tons apiece, but the firm limited the design load to 75 tons apiece.
To facilitate the excavation of the tunnel beneath the sculptural mountain the engineers interspersed steel piles with precast piles. Two lines totaling 24 concrete piles on either side of the tunnel were changed in the design to steel piles, and these served as excavation support. The tunnel was then hung from the pile-supported mat.
The very high water table at the site needed to be carefully considered when determining where to locate the tunnel access structures. “In flood situations we were worried that the tunnel would float out of the ground,” says McElwain. “There was enough concrete on top that we didn’t have to worry, except where the stairway comes down,” says McElwain. “We had some concrete there to make it heavy enough so it wouldn’t float in a flood condition.”
Expansion joints were also changed in the design to account for the underground rooms. “We had expansion joints in the wall and the plaza that worked nicely when we didn’t have the rooms, but we had to reconsider how we dealt with them,” says McElwain. “We moved the joints at the edge of the inscription wall and the beginning of the fountain wall, at the mountain sculpture. It was tricky because the stone had to make a change in depth, a slight return. The expansion joints had to end up between the stone that turned the corner and the stone that turned at the inscription wall.”
An inscription from one of King's speeches is carved into the side of the Stone of Hope, which overlooks the Tidal Basin and faces the Jefferson Memorial in Washington, D.C. Courtesy of National Park Service
Austin adds that the stone pieces used to create the mountain also presented difficulties. “We had to design for a crane to assemble the mountain,” he explains. A section of the plaza was thickened so that a crane could be positioned to lift the stones off a truck. “It came as an assembly of stones done in tiers,” adds McElwain. “The stones form a perimeter set of rings that diminish in circumference as they go up and are clamped together. Then the interior was filled with concrete.”
The stone sculpture of Dr. King is titled the Stone of Hope. (Both the Stone of Hope and the Mountain of Despair derived their names from parts of King’s “I Have a Dream” speech: “With this faith we will be able to hew out of the mountain of despair a stone of hope.”)
Thornton Tomasetti also designed the memorial’s bookstore—a steel structure with bearing walls—that rests on its own pile-supported platform. Austin points out that an added challenge was placing the concrete for the curved site. “Nothing in the plaza is straight,” he says.
The memorial team also included McKissack & McKissack of Washington, D.C. (the architect of record); New York City-based Turner Construction Company; Gilford Corporation of Beltsville, Maryland.; and Tompkins Builders, Inc., of Washington, D.C.