In the “Streeterville” neighborhood of downtown Chicago, bounded on two sides by Lake Michigan and the Chicago River, is the 2 million square foot River East Center mixed use project. A 58-story Residential Tower is on the east end of the block-long site, while on the opposite end, stands a 450-suite Hotel. In between these 2 structures, residents and guests enjoy a Theatre, restaurants and shops in a 3-story Podium.

For a project of this size, Chicago zoning ordinances mandate that developers create parking spaces within the property for a significant proportion of the building’s projected population. Underground parking is the best solution, but use of conventional shoring for deep basement construction in Chicago’s soft clays is expensive and time-consuming.

Case Foundation Company teamed with the Construction Manager and the Design Consultants to devise a top/down construction program for the 4-level, 1150-space underground parking garage. A major component was the 30” thick Case-designed slurry wall which provided the 1,600 lf of basement walls. Reinforced concrete panels were constructed between previously-installed perimeter load bearing caissons, limiting wall depth to just the 70 ft. required for lateral stability during site excavation. A total of 143 belled caissons (drilled shafts) 98 ft. deep support the entire superstructure. STS Consultants established a designed an end-bearing pressure of 50 ksf, one of the highest used in Chicago, based on their extensive pressuremeter testing experience in Chicago hardpan. High column loads near the core of the 58-story concrete-framed residential tower required 9’-6” diameter shafts with 22’-6” diameter bells and 8,000 p.s.i. concrete.

Because the core with its massive shear walls could not be built top/down, it was isolated by a Case-designed 90 ft. diameter circular sheetpile cofferdam and then constructed from the bottom up. All of the caissons were installed from street level; the upper 50 ft. of each interior caisson was formed with double-wall CMP with a smooth inner liner. Once the slurry wall and drilled shafts were in place, the site was excavated to the first parking level at -15 ft. and the floor and framing elements were formed and poured on grade, followed by the ground level slab on shored forms. From this point, top/down construction began. Using “long reach” excavators, the soil from beneath parking levels 1, 2, and 3 were successively excavated from grade level through slot-shaped openings in each floor slab. Small loaders and dozers moved the dirt from the cut face to the slot openings. At each level, the CMP lining was stripped from the caisson/column shafts and doweled connections were drilled and grouted into the shafts and the slurry walls to connect the floor beams and slab to these vertical elements. Each floor was poured monolithically with its framing members. The forms were then “undermined” and dropped down to the next structural level. Due to the effective water cutoff achieved by the slurry wall, extending through the lake sand and keyed into the underlying clay, site dewatering was easily controlled. When excavation reached basement level 4 at -45 ft., the Tower and Hotel Superstructures had been topped out high above. In total, 275,000 cy of soil were, “mined” from beneath River East for the underground parking structure.

While not an “architectural finish”, the exposed slurry wall and caisson shaft surfaces required very little cosmetic work, beyond cleaning, and are in full view today – one of very few Case projects where our finished work is readily visible! This is the third and by far the largest Case top/down project in Chicago. The method saved 6 months from the construction schedule compared to a conventional “bottom up” sequence using a temporary support system.