Completion 17: 55 E. Erie St.
Cost: $126 million

The 55 E. Erie St. condominium in Chicago's River North neighborhood is reportedly the second-largest all-residential building in the United States.

Before the building could rise 59 stories, a large number of underground obstacles were encountered.

Two levels of basement walls and slabs and 46 caissons for the support of the 10-story American College of Surgeons building previously on the site were discovered. A 9-ft.-dia. brick tunnel, which was constructed 80 ft. below grade in the 1870s as a water-supply channel, cut across the location. A storage tank, sheeting and ballast slab were detected in a corner of the site.

Containing all this was a clay deposit that, in sections, was unstable and prone to cave-in.

Sealing the tunnel was done before the new caissons were drilled.

The city of Chicago requires the closing off of antiquated passageways because of a famed mishap from the decade before.

In 1992, a breach occurred in a century-old tunnel under the Chicago River during a construction project, flooding Loop basements with 250 million gal. of water and resulting in cleanup costs of $40 million.

At 55 E. Erie St., the grade-level slab of the previous building was removed, permitting access to the 140-ft.-long, 70-ft.-wide, 27-ft.-deep basement. However, the walls and 2-ft.-thick basement slab were saved because removing them would have required extensive sheeting and bracing.

Twenty-seven openings were cut in the slab at locations where caissons were to be drilled. Steel shoring towers were built atop the slab to support 360,000 lbs. of drilling equipment.

Grout was injected under pressure to seal the tunnel. Steel sleeves were installed 10 ft. above the tunnel, 10 ft. below it and anywhere penetrations had been made to provide further stiffening.

Looking Up

Issues arose above ground, too.

Transfer beams on four floors allow for shifting of the building's load to the caissons.

The 13th floor, for instance, absorbs the weight of the floors above because of the massive size of the 14 poured-in-place transfer beams: 16 ft. high and 10 ft. wide.

The 11 floors of parking were post-tensioned partly for efficient use of space. Because post-tensioning produces harder floors, the slab thickness are less compared with those reinforced with rebar.

Two sections make up the project, the main tower and a 12-story annex where parking is located.

A 3-ft.-wide pour strip, or unformed space, was incorporated into plans to account for the uneven settling between the two sections.

The structure will have 214 residences and feature numerous amenities and stylish architecture.

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