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Thorton Tomasetti Inc.

Michael Goodman / ENR

Shelbourne Development Group Inc. Walton, Baker recommended foundation for Spire and surrounding garage.

Chicago foundations in the downtown Loop, where most of the skyscrapers are sited, use two primary systems: Drilled, concrete piers that bear on “hardpan” clay, located about 85 ft to 90 ft below the surface, and piers that bear on dolomite bedrock, located more than 110 ft below grade. Hardpan caissons typically have a flared, “belled” bottom to increase bearing capacity, while the rock caissons are socketed several feet deep into the limestone. Before the 1950s, deep foundations were dug by hand. Mechanized cranes, Kelly-bar tools and clamshell diggers are now the norm.

Belled caissons were long held to an end-bearing capacity of 12 kips per sq ft until Baker drew from a little-known 1919 load test, soil mechanics and triaxial lab testing at Lake Point Tower in 1963 to raise bearings to 30 ksf. Since then, Baker has used newer, in-situ methods, such as the “pressuremeter,” a bladder device that goes into a borehole and radially expands to determine strength. That method has, bit by bit, brought up bells to 60 ksf, an overall increase of 400%.

Rock pressures have risen dramatically, as well. After Hancock, Baker took careful settlement data and tried new test methods as they arrived. He pushed the Chicago Building Code’s maximum bearing for rock caissons from 200 tons per sq ft to 230 tsf in 2001 at UBS Tower using the Osterberg load cell (see sidebar, p. 42 and chart, p. 41). Rock changes “all started” at UBS tower, recalls William H. Walton, STS senior principal.

Walton, the official geotech for Spire, also worked on Chicago’s almost-completed Trump International Hotel & Tower and was inclined to push its 270-tsf design to the next level. Baker provided the “muscle” in design meetings, Walton says. Spire’s 34, 10-ft-dia. rock caissons set a new record, being designed to 300 tsf.

Shelbourne Development Group Inc. Spire’s 21 columns reduce to seven at grade. Each bears on twin piers.

Last summer, rock under the Spire, founded on 20 core and 14 supercolumn caissons, was tested to 600 tsf for a safety factor of two. The site now has an 80-ft-deep cofferdam in the center. It is the largest domestic contract that Keller Group, which owns Case Foundation Co., has ever held.

As more foundations have arrived in Chicago slurry-drilled caissons, steel piles, micropiles and auger-cast piles Baker has helped contractors prove them and push limits. He also pioneered reuse of old caissons, “another area where few geotechs tread,” says Robert E. Schock, Case vice president.

Spire is on a global sales “road show” while the foundations, a source of mild controversy from neighboring townhome owners worried about settlement, are nearly complete. The project hasn’t sunk any nearby buildings.

The site’s collection of high-capacity rock caissons, belled piles, slurry walls and secants are the culmination of one man’s gradual quest to firm up the science of soil. Walton, who climbed a mountain in Vermont with Baker last summer, says, “You can follow the footsteps, but they’re mighty big.” Baker thinks more ground work has yet to be done. But his vision comes with a warning: “Follow my advice,” he says, “take small steps.”

...speciality. As luck would have it, Baker came to the birthplace of the skyscraper just in time for Chicago’s first supertall building boom. While Baker lucked out in joining what was then a small, 15-person consulting firm called Soil Testing Services Inc., he later left little to chance in his marathon quest to understand the behavior of soils under the pressure of heavy structures. STS now employs about 600.