The new Ogden Elementary School is being constructed on the site of an existing elementary school by contractors for the Public Building Commission of Chicago (PBC) near downtown. Geosyntec was retained by the PBC to assist with the design of groundwater control measures and the earth retention structure (ERS) during the site preparation phase of work. Shallow groundwater and silty soil, which make dewatering efforts difficult, were documented as typical of the site. The scheduled opening of the new school was viewed by the client as a firm deadline that would not allow for any significant delay, so any necessary geotechnical design and construction had non-negotiable deadlines associated with them.
Geosyntec's Scope of Services
Geosyntec designed a sheet pile wall to act as both a groundwater cutoff wall and the ERS. This cutoff wall would act as a temporary groundwater control measure and ERS during site development, and as a permanent cutoff during the lifetime of the new school.
Site conditions were further complicated by the presence of a subway tunnel and subway vent structure and a large diameter water main immediately adjacent to the alignment for the sheet pile ERS. Geosyntec documented the allowable vibration acceleration during the driving of the sheet pile wall for the safety of these adjacent structures. Additionally, we were requested to address methods for improving unsuitable soil conditions encountered in the course of excavating for the underground parking structure associated with the new school. Options presented included removal and replacement with engineered fill, geotextile and aggregate to bridge the unsuitable soils, substantial thickening of the slab-on-grade, and lime and fly ash soil stabilization, which was eventually selected as least cost and least impact on schedule.
Geosyntec worked closely with the PBC and the City of Chicago to facilitate an expedited review and approval of the design. We provided on-call field engineering during construction to keep the installation of the ERS on schedule. Our analysis of the effects of sheet pile installation vibrations on nearby structures was accepted without comment allowing the support system installation to commence without schedule interruption or installation equipment modifications.