Case Study of the Observational Method: Northwestern University Simpson-Querrey Biomedical Research Center

Northwestern University Feinberg School of Medicine Simpson-Querrey Biomedical Research Center (SQBRC) is a new facility in the Streeterville neighborhood of Chicago. The earth retention system (ERS) for the facility supports as much as 18.0 m (59 feet) of excavation and provided support of surrounding areas including active city streets with active public utilities on the north and south, and existing buildings on the west and east. Marginal subsurface conditions including loose sands, soft clays, and a groundwater table with a ready supply of water from Lake Michigan steps away, further challenged the design of the ERS. Designers used an observational method in conjunction with a real-time monitoring system of automated total monitoring stations (ATMS) and shape accel arrays (SAA) to determine if contingency measures were required to stabilize the ERS during excavation. The design required addressing three (3) main factors: (1) maintain active public ways and utilities by controlling deflections below tolerable levels, (2) provide stabilizing lateral support for the existing structures during mass excavation due to unbalanced earth pressures, and (3) accommodate the excavation and substructure construction with minimal impact. Using a system of steel sheet piling along the perimeter along with a combination of internal bracing and grouted soil anchors, designers were able to support the required excavation depths and stay within allowable tolerances for deflection. The observational method allowed designers to eliminate potential additional levels of bracing required in the marginal subsurface conditions, thus saving money and time, as well as to continuously monitor the impacts of the excavation on the surrounding area. This paper discusses the ERS as designed and installed, the contingency measures anticipated, the monitoring system used, the results of the monitoring, and the benefits to the project of using an observational approach.