Visitors who marvel at the beauty of Millennium Park cannot see what is below.

But contractors who were part of the Herculean effort to construct the park's below-grade infrastructure certainly know what is there.

Rising from track level below and supported by many of the more than 900 caissons beneath the park are concrete piers resembling highway bridge piers. Resting on these piers are specially designed, double-T precast concrete beams that fit together like an enormous jigsaw puzzle. Each beam is 6 ft. wide and 3 ft. deep.

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"They're similar to beams found in many parking structures, but these are much more heavy duty," said Karl Hanson, assistant chief structural engineer at Chicago's McDonough Associates, the designer of the below-park structure.

A parking structure, for example, is designed for 50-lb.-per-sq.-ft. loads, but these beams can carry approximately 600 lbs. per sq. ft., he added.

"They can hold up about 4 ft. of earth, a truck and trees," Hanson said. "So that's a lot to ask. That's a heavy-duty loading."

The genius of using double-T beams was in their ability to eliminate concrete form work at the park level, he added.

"The beams are the working surface for the park itself," Hanson said. "Once they were down, workers could drive the cranes on top, walk around and handle material."

It took a great deal of preliminary planning before the early construction could begin, said Mike Shymanski, architect and urban planner with McDonough Associates. The firm worked out the interface between the city, Metra rail line and the Metropolitan Pier and Exposition Authority, as well as CDOT and the Art Institute, Shymanski said.

The busway - a dedicated lane beneath Millennium Park to be used by buses ferrying convention-goers from riverside hotels to McCormick Place - was an integral feature of the interface. McDonough Associates plotted the alignment of the busway from Randolph Street to 26th Street and between Metra and city-controlled properties.

"The other thing was to provide future expansion for Metra," Shymanski said. "In the project, we provided two new tracks used by the South Shore. And there's capacity for three additional tracks and two additional platforms. We also had to engineer the track work and the overhead traction power for the electric trains, and we had to develop a current and future track plan."

A Supportive Garage

To build Millennium Park, the old Grant Park North Garage, which was originally built in 1953, was demolished down to the piles and rebuilt, said Ed Uhlir, project design director for Millennium Park.

"They created a foundation that transfers loads more uniformly to the pile structures," Uhlir said. "We added an additional structure to hold up the peristyle, the ice rink and the Park Grill. We added micropiles in the lowest levels of the garage to support the Anish Kapoor sculpture.

"We also added to the floor-to-floor height on the top level of the garage, so we had to raise the elevation on the east side of Michigan Avenue by about 1.5 ft. If you're walking west to east across Michigan Avenue, you're actually walking uphill."

The garage also figures in the drainage of the park, as well as the support of the Kapoor. A below-grade retaining wall about one-third of the way into the park from the west separates the garage from the Metra tracks to the east. It also travels directly below the middle of the Kapoor, helping to support its weight, said Mike Hanneman, vice president of McDonough Associates.

Water draining from the western one-third of the park west of this retaining wall/dividing line flows into the Michigan Avenue drainage system. The eastern two-thirds of the park, or everything east of the Kapoor, drains through the Metra track area and the Millennium Park garage into a sewer built in the middle of the busway.

"The piping was fairly large," Hanneman said. "It was tricky because it's only a few feet above the level of Lake Michigan. It's tricky in the sense you're discharging into the river at a certain elevation, which is close to the actual elevation of the lake. So the whole drainage system had to be designed with that in mind."

Above-Grade Park Topography

Hanson said a waterproofing system was placed on the "working surface: deck formed by the double-T beams. On boulevard and sidewalk areas, cast-in-place concrete surfaces were laid in.

Most of the remainder of the park was then blanketed with 2.7 billion lbs. of soil, but because of loading limits on the structure below, no area of the park could feature elevations of more than 4 ft. of soil.

That problem was partially addressed beneath the park.

"The interesting thing was the concept of the park itself had to be carried down into the structure," Hanneman said. "If there were vertical curves or contours in the park, that contributed to the irregular design features in that the structure below had to match the surface with the support mechanism."

He said that meant below-grade columns were longer below higher levels of the park and shorter below lower levels, which created higher and lower elevations in the park surface.

But in some areas, such as the Lurie Garden, changes in the elevation of the concrete surface of the park weren't enough. The solution came in the form of lightweight poly-foam fill.

"The designer wanted to have all these contours, but we were restricted by the amount of soil we could use," Uhlir said. "The deck underneath had a slight slope for drainage but not the major elevation changes desired for the garden. We used big white blocks of poly fill."

He said the fill provided a lightweight means of achieving varying elevations without using heavier materials. Sand or soil would have been too weighty to meet specified load limits.

The park boasts almost 700 trees and eight to 10 species. "These are spectacular trees, some with 10-in. trunks," Uhlir said. "They came from all over. A lot came from the Midwest, but some from as far away as New Jersey and Maryland. We planted hybrid elm trees along Michigan Avenue that are resistant to Dutch elm disease."

These were intended to mimic the historic "double row" of American elms along Michigan Avenue planted in 1917. Some of the elms remain in Grant Park, most near Buckingham Fountain.

As he looks back on the project, Hanson said the major hurdle was the construction timetable.

"We were under the gun to do this really quick," he added. "It was like building a Wild West town. Just throw it up. We were under a lot of pressure."