The Big Walnut Augmentation/Rickenbacker Interceptor was constructed by the City of Columbus, Division of Sewer & Drainage, to increase its sewer capacity. Construction of 20,800 feet of 14-ft finished diameter sewer and 13,300 feet of 12-ft finished diameter sewer was performed under two separate contracts. Schnabel was retained by the contractor, URS Corporation, to provide services during design and build phases.

High groundwater levels were present along the alignment, along with challenging glacial terrain that consisted of very dense tills layered with highly permeable sands and gravels containing cobbles and boulders. One-pass tunneling using a pressurized face tunnel boring machine (TBM), either slurry or earth pressure balance (EPB), was specified. For each contract, the contractor opted to utilize an EPB TBM to deal with the difficult ground conditions.

During design, our responsibilities included the development and coordination of the geotechnical investigation, preparation of interpretative geologic profiles, input to alignment selection, and the development of tunnel and shaft design loadings. We conducted an optimization study for the selection of shaft construction methods. We provided a full structural design for all shafts (slurry wall and cased auger) and the bolted and gasketed precast concrete segmental tunnel lining, and designed associated ground improvement and grouting methods. We also developed the geotechnical instrumentation and monitoring program and prepared all tunnel- and shaft-related drawings and specifications as well as the project Geotechnical Baseline Report.

During construction, we provided construction management services including onsite inspection and monitoring, contract compliance verification, and collection and analysis of data related to tunnel and shaft construction.