Lower Olentangy Tunnel
Summary
The Lower Olentangy Tunnel (LOT) is part of the City of Columbus’ capital improvement program and is a key element of the DPU’s achieving the requirements of their consent decree. The tunnel will provide adequate conveyance capacity and in-line storage to minimize sewage overflows to the Olentangy River. The designed alignment for LOT consists of approximately 17,000 feet (5,182 m) of 12-foot-diameter (3.7 m) tunnel, at depths ranging from about 43 (13 m) to 93 feet (28 m). The upstream shaft is located north of the Ohio State University Campus near Tuttle Park, with the tunnel running south to the Arena District in downtown Columbus.
- A 17,000-linear-foot (LF) (5,182 m), 12-foot-diameter (3.7 m) soft-ground tunnel
- A 1,100 LF (335 m), 90-inch-diameter (2,285 mm) curved microtunnel
- A 330 LF (100 m), 12-foot-diameter (3.7 m) rock tunnel
- Tunnels range from 43 to 93 feet (13 to 28 m) in depth
- Five main shafts 16 to 50 feet (4.9 to 15.2 m) diameter, 60 to 176 feet (18.3 to 53.6 m) deep
Scope of Work
Delve Underground served as a subconsultant and was lead designer during final design for all tunneling aspects, including tunnels, shafts, and geotechnical instrumentation, and preparation of the Geotechnical Baseline Report and associated specifications. The firm is currently providing engineering services during construction, including review of tunnel boring machine (TBM) tunneling progress data, inspection and shift reports, and geotechnical instrumentation data. Construction is planned to be complete in 2026.
Challenges & Innovations
The mining for the main 12-foot (3.7 m) LOT crosses below highways in three locations and is below the Olentangy River Road for nearly 10,000 LF (3,048 m). Any significant settlement in these areas could cause major roadway damage, potentially leading to loss of life.
Therefore, multiple risk mitigation measures were included such as pressurized TBM specifications with strict requirements to control the excavation; monitoring of excavated soils quantities; comprehensive settlement monitoring plans developed with real-time monitoring requirements; safe havens with pre-excavation ground improvement to provide dry and stable access to the front of the TBM for maintenance; and ground improvement zones below critical water mains.