Infrastructure Technology Institute
In late 2008, construction began on a new high-rise condominium building in the fast-growing South Loop area of Chicago. The site of this planned 53-story reinforced concrete tower exemplifies the infrastructure congestion typically found in urban centers: the embankment of the East Roosevelt Road bridge is a mere six feet north of the site, and South Indiana Avenue, an abandoned water tunnel, and several underground public utilities lie immediately west of the tower. The City of Chicago’s Board of Underground laid out strict monitoring guidelines to ensure the safety of these nearby critical facilities.
The construction of the tower employs “top-down” methods in which the concrete slabs of the basement levels are constructed as the excavation progresses rather than after the excavation is complete. These slabs then act as bracing that prevents movement of the surrounding ground as excavation continues. The slabs are thought to be stiffer than the temporary bracing used in traditional “bottom-up” excavations, so infrastructure-damaging ground movements are minimized. This behavior, however, has never been proven because traditional monitoring techniques have not provided enough data for a complete analysis.
To investigate the effectiveness of “top-down” construction for reducing soil movement, ITI researcher Professor Richard Finno and his team have deployed a network of over 70 vibrating wire strain gages to monitor the loading of the different sections of each basement slab. Just before the concrete is poured, ITI engineers attach these gages to the reinforcing steel and immediately begin reading them. When the slab is poured, the sensors become a permanent part of the structure and provide critical information about how the slabs bear load as the soil beneath them is excavated.
Finno’s team will use the data collected from these embedded strain gages, as well as from inclinometers and survey points around the construction site and from four previous construction projects around Chicago, with the aim of developing design recommendations for protecting nearby infrastructure from excavation activities.