Project Update by Jake Keegan, PE
The Lower Olentangy Tunnel (LOT) is a key part of the City of Columbus’ capital-improvement program. The tunnel will provide adequate conveyance capacity and in-line storage to minimize sewage overflows to the Olentangy River, resulting in improved water quality and a reduction in pollutants in the water.
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 Ohio State University Campus, with the tunnel running south to the Arena District in downtown Columbus. McMillen Jacobs 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; geotechnical instrumentation drawings; and associated specifications. We are currently providing engineering services during construction.
LOT will connect into the upstream end of the previously constructed OSIS Augmentation Relief Sewer (OARS). There are three main shafts for the tunnel: Vine Street (consisting of an upper and lower shaft), Gowdy Field, and Tuttle Park. The tunnel 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 main TBM shaft site is located at Gowdy Field (middle, launch shaft), a former landfill, west of the Olentangy River and State Route 315. The Gowdy Field Shaft is a “Figure 8” slurry wall shaft. The TBM will first mine south to the Vine Street Shaft, then remobilize to Gowdy Field to mine north to the Tuttle Park Shaft (north).
The 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 (MTBM). There are two shafts along 2nd Avenue where relief structures are designed to connect to existing sewers. The Kinnear Subtrunk (KST) Relief Sewer is also being 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 will mine 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.
Notice To Proceed was issued to Granite Construction on March 2, 2021. 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. Slurry wall construction at the Gowdy Field Shaft and secant pile construction at the Vine Street Shaft are complete and excavation is underway. Pre-excavation grouting has been completed at both shafts. The remaining shafts will be constructed in 2023. The Herrenknechkt TBM has arrived on-site, and Granite is preparing the machine for its first launch to the south. Construction is planned to be complete in 2026.