To create Seattle’s new Olympic Sculpture Park, lead designers Weiss/Manfredi have fittingly sculpted the earth. At the northwest corner of downtown, overlooking Elliott Bay, on land previously used as a fuel storage and transfer facility, the New York City–based firm developed a Z-shaped path that crosses a busy street and active railroad tracks and negotiates a 40-foot grade change. The serpentine route takes visitors past carefully placed works from the Seattle Art Museum collection, including pieces by Alexander Calder, Richard Serra, and Louise Nevelson, and provides stunning views of the city and the surrounding landscape (see the July issue of RECORD, page 110).
Not surprisingly, an extensive system of retaining walls was needed to make this new topography and hold back the 8.5-acre site’s 200,000 cubic yards of fill. The only visible part of the system is its overlapping and sloping concrete panels, each 12 feet wide and as tall as 30 feet. The repetitive nature of these precast elements creates “a module and a measure, allowing the site to feel bigger,” and provides an appropriate backdrop to the sculpture, says Marion Weiss, AIA.
The real “work” of retaining the earth is actually occurring behind the precast panels. There, mechanically stabilized earth (MSE) holds back tons of fill. “Decoupling” the earth-retaining function from the architecture was a more cost-effective solution than a traditional poured-in-place concrete retaining wall backfilled from behind, says Drew Gangnes, director of civil engineering for Magnusson Klemencic Associates (MKA). The firm was both civil and structural consultant for the project.
The MSE stretches horizontally under the park surface for a distance equal to about 80 percent of the height of the wall, and is made up of 18-inch-deep layers of soil separated by geotextile fabric. Near the face of the MSE, the fabric transitions to wire mesh, L-shaped in section, and containing rock rather than soil.
The panels are attached to the MSE system with slotted connections to a continuous tie-back block just below grade level and to the footing. To prevent transfer of loads from one panel to another due to settlement or seismic activity, adjacent panels are not connected, explains Jay Taylor, MKA principal. A gap of a few inches between the panels and the MSE face was sized so that the two components do not slam into each other during a temblor, and to prevent the panels from toppling.
The project also included reinforcement of the sculpture park’s deteriorating 800-foot-long timber-and-steel seawall. City and state agencies had been studying the wall’s reconstruction as part of a larger proposal to rebuild about 8,000 feet of seawall protecting the downtown waterfront. Preliminary design documents showed the agencies planned a costly behind-the-wall intervention that would avoid disrupting the piers that line much of the Elliott Bay shore.
Since there were no piers along the park’s section of waterfront, and because its seawall was not adjacent to a seismically and politically sensitive elevated highway, designers had some freedom to explore alternatives. Their solution was to reinforce the existing seawall with an in-the-water buttress. But in order to obtain permitting, the team had to develop a system that would create a hospitable environment for migrating salmon in addition to providing the necessary reinforcement.
The team satisfied both the habitat and structural requirements using layers of riprap and smaller diameter rock carefully sized to create an uninviting environment for salmon’s predators. Incorporated into the buttress, at the level of the intertidal zone, is a so-called “habitat bench.” A hollow in this bench is filled with aggregate and other material that supports the growth of organisms that salmon like to eat. Gangnes estimates that this $5 million buttress, which he jokingly refers to as a “pile of rocks,” cost about a tenth of what the proposed city/state solution would have.
Just beyond the end of the seawall, at the northern end of the park, the team designed a crescent-shaped beach protected by two peninsulas. Environmental consultants used computer simulation to model the motion of the waves and wind to make certain that the sand would not be carried away. This beach is the culmination of a journey from the city to the water’s edge, from the most urbane part of the park to that most connected to nature. “It brings in both extremes,” says Weiss.