This combined sewer outfall (CSO) storage and conveyance tunnel runs below downtown Akron to store a minimum of 26.6 million gallons (MG) (100.7 ML) and convey flows exceeding 1.4 billion gallons per day (5.3 billion L/day) during wet weather. The project included a 27-foot finished diameter (8.2 m), 150-foot-deep (46 m), precast segmentally lined tunnel through soil, mixed face, and rock. These ground conditions necessitated the use of a multimode tunnel boring machine (TBM) capable of earth pressure balance and single-shield rock excavation.
The project also included design of several smaller rock tunnels, 6 to 13 feet (1.8 to 4 m) in diameter, to consolidate flows to the main tunnel.
Delve Underground led the development of the TBM specifications and lining design for the challenging ground conditions. The firm also performed final design of all tunnels and related structures, was the lead author of the Geotechnical Baseline Report, and led the development of construction cost estimates and schedules. The final sections of the tunnel were put into service in June 2020.
Drop shaft flows were conveyed via multiple baffle-type drop shaft structures whose flows exceed 400 MGD (1.5 billion L/day), which was greater than any known physical hydraulic scale modeling for baffle-type drop shafts at that time. They were built and analyzed for the project by the Iowa Institute of Hydraulic Research (IIHR), whose results were incorporated into the tunnel designs to minimize air entrainment into the tunnel while still providing a means for both dry and wet weather flow to be conveyed.
Because the OCIT is a dry weather sanitary flow conveyance tunnel as well as a wet weather storage tunnel, special attention had to be placed on the design life of the deep tunnel linings. Delve Underground led the evaluation to determine the durability requirements for the deep tunnels, as well as the near-surface pipelines to be used on the project, to meet a 100-year design life goal.