...link to the city’s five-mile, underground pedestrian walkway. The pedway is lined with shops, eateries and services. The shopping center contains about 280,000 sq ft of retail and restaurant space within 500,000 total sq ft. Among the new tenants are Bigsby & Kruthers, Puma, Anthropologie and Muvico Theaters. The office tower, measuring 440,000 sq ft, is about 90% leased. Anchor tenants are CBS Broadcasting Inc. and Morningstar Inc.

Uhlir points out that the mall’s glass curtain wall reflects nearby architectural icons, such as the Chicago Theatre marquee and Marshall Field’s clock. Horizontally woven steel panels reference the Millennium Park amphitheater, which architect Frank Gehry crowned with metal curls.

More Ups and Downs

Structural engineering was intensely complicated, says Joseph Burns, managing principal of Thornton Tomasetti. The office building has one subterranean level, and the retail center has four. In addition, the train tunnel bisects the block diagonally. The office building followed the traditional construction method, with a concrete core rising several stories ahead of the steel-floor framing. But the retail building took 50 ft of excavation.

One big time-saver was an “up-down” construction method, which allowed for the simultaneous building of substructure and superstructure. In this method, the deep-foundation elements go in first and steel then rises at the same time the excavation goes down—one level at a time.

“It’s like deep mining,” says Uhlir. Thornton Tomasetti’s Burns adds that all excavation called for serious reinforcement to keep the holes from caving in. Mostly, that meant slurry walls, but a more costly, secant-pile wall was placed between the office tower and the deep basement of the retail center. “That’s where the subway is, so we needed something much stiffer there,” Burns says.

Working on one of the world’s busiest streets left little space for staging. Building materials needed to come in, and four stories of excavated earth had to go out. To quickly get rid of it, the construction team devised a maneuver that coordinated with city traffic signals. Every 3.5 minutes—exactly how often the lights change—a dump truck pulled into place, loaded and took off, and another one then pulled in. The entire job came to an estimated $450 million, but the developers have declined to discuss costs.