The Olentangy Scioto Interceptor Sewer (OSIS) Augmentation and Relief Sewer (OARS) is a 20-ft-dia, 23,300-ft-long tunnel that provides wet weather relief for Columbus, Ohio’s main sewer, the OSIS. The $370-million project was tunneled 180 ft below the city and extends from the Arena District to the Jackson Pike Wastewater Treatment Plant.

Tunneling the relief sewer allowed the system to be operational eight years earlier than the original plan, which called for a four-phase, open-cut project. The OARS tunnel provides conveyance capacity for all storms, handling peak flows of up to 1,700 million gallons per day. The design incorporates deep surge control storage systems into the tunnel’s deep drop shafts, where flow enters the tunnel during wet weather. After karstic limestone rock was found while tunneling, DLZ developed a technique to successfully tunnel through it by using high pressure groundwater inflows exceeding 5,000 gallons per minute. A reverse-flow grout procedure also had to be developed to replace rock cutters during drilling.