Lower Olentangy Tunnel

The Lower Olentangy Tunnel (LOT) is part of the City of Columbus’ capital improvement program. The tunnel will provide sufficient 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 (11 m) to 93 feet (28 m). The upstream shaft is located north of Ohio State University Campus near Tuttle Park, with the tunnel running south to the Arena District in downtown Columbus. Delve Underground served as subconsultant to DLZ and was lead designer during final design for all tunneling aspects, including tunnel, shaft, and trenchless design; preparation of the Geotechnical Baseline Report; geotechnical instrumentation; protection of adjacent structures; and associated drawings and specifications. Delve Underground is currently providing engineering services during construction.

LOT will connect into the upstream end of the previously constructed OSIS Augmentation Relief Sewer (OARS) at Vine Street. There are three main shafts for the tunnel: Vine Street (consisting of an upper and lower shaft), Gowdy Field, and Tuttle Park. LOT will terminate at the Vine Street Upper Shaft. At that point, the Vine Street Lower Shaft will serve as the starting point for the connector tunnel and will end at the existing OARS Shaft 6. The Vine Street Lower Shaft and connector tunnel will be constructed mainly in Columbus Limestone. The TBM is excavating through mainly glacial deposits with cobbles and boulders; sand and gravel; silty sand and gravel; and cohesive silt and clay. The northern portion consists of a mixed face condition beneath the river with soft ground over­­lying bedrock and a weathered rock zone for the remainder.

The main TBM shaft site is located at Gowdy Field (middle, launch shaft), a former landfill, west of the Olentangy River. The Gowdy Field Shaft is a “Figure 8” slurry wall shaft. The 90-inch (2,286 mm) 2nd Avenue Tunnel will be constructed from the Gowdy Field South Shaft and mined beneath the Olentangy River to 2nd Avenue, using a microtunnel boring machine. There is a curve in the alignment and two shafts along 2nd Avenue where relief structures are designed to connect to existing sewers. The Kinnear Subtrunk Relief Sewer has been constructed by open cut and auger boring at 3rd Avenue west of the river and will be tied into the LOT at the Gowdy Field North Shaft.

The TBM holed through at the Vine Street Shaft to complete the LOT South Tunnel on November 9, 2023. Granite (the construction contractor) is preparing for the launch of the LOT North Tunnel in early 2024. Warde & Burke is the microtunnel subcontractor and is planning construction of the 2nd Avenue Tunnel for 2024. The connector tunnel construction is currently underway. Pre-excavation grouting has been completed at the three main shafts. Slurry wall construction at the Gowdy Field Shaft and secant pile construction at the other shafts are complete and excavated. Ground improvement along the alignment includes 8 break-in/break-out zones, 14 safe havens, 5 water main ground improvement zones, and the river crossing ground improvement zone. Ground Improvement for the LOT North Tunnel Safe Havens is ongoing. Construction is planned to be complete in 2026.