Imagine excavating a 58’ deep basement on a tight urban site in Chicago’s Streeterville neighborhood, just two blocks from Lake Michigan. To do so means removing 98,815 cubic yards of dirt in 11,800 truckloads. That’s more than 30 miles of trucks lined up end to end.
Northwestern University’s new Simpson Querrey Biomedical Research Center is being built in 3 phases on the site of the former Prentice Women’s Hospital. Phase 1 is the entire below grade structure. Phase 2 is a 12-story, 600,000 SF structure that will provide state-of-the-art research and support space for approximately 23 research groups per floor. The 3rd phase will be a future vertical expansion that adds another 15 floors.
In this tight urban site, the team faces unusual complexity because they are working around existing caissons that supported the old Prentice Hospital. Using the same caissons for the new building was not possible because they weren’t situated where the load of the new structure must be supported. In addition, the old hospital had only one basement level to about 14’ below grade. Because the new building is so much deeper, most of the concrete and reinforcing of the old caissons will be torn out.
In addition, the team is installing perimeter sheeting in 75’ long sheets, supported by diagonal bracing. There are 4 different layers of tiebacks – over 200 in total. Soldier beams and lagging support the lowest/deepest foundation elements. Once all the sheeting is installed, 17 wells will pump out all the water from within the basement’s footprint.
To properly support the new building, more than 100 caissons will be installed in a pattern that weaves between the old Prentice supports. They are approximately 5’ in diameter with the longest going 102’ below street level. The largest of the caissons includes a bell at the base measuring 22’ 6” in diameter – to distribute the weight of the building across a larger footprint.
For more information about the Research Center, visit Northwestern's web site.