Overview
In an effort to reduce combined sewer overflows, the City of Columbus and the State of Ohio initiated the OSIS (Olentangy Scioto Interceptor Sewer) Augmentation Relief Sewer, known as OARS, a project involving the construction of a sewer tunnel to intercept wet weather overflows and redirect them to wastewater treatment plants instead of the Scioto River. Challenges were encountered when tunneling through karst limestone as water flowed through freely, dulling the discs, and preventing the contractor from dewatering and initiating interventions. A grouting campaign was also unsuccessful as the ground was too porous.

Scope of Work
As part of the needed interventions, Ballard set up for “wet” diving requiring an understanding of known safe working times and pressures. Calculations for breathing gas volumes were done, and emergency reserves were considered. A very large volume of high-pressure cylinders of breathing air was assembled and connected via manifolds. The Tunnel Boring Machine (TBM) had man locks which provided access to a working chamber. Through this area, the excavation chamber was entered through either a door at the twelve or nine o’clock position. The pressure in the working chamber was set to the hydrostatic pressure of the midway point between the two doors. A valve was then opened near the nine o’clock door leading into the excavation chamber. This allowed the area to flood until the water reached the midway point between the doors. Teams of commercial divers entered through the man locks where they were compressed to the same pressure as the working chamber. Upon entering the space, they donned diving gear and enter the water. The nine o’clock door was opened, and the divers swam into the excavation chamber to perform work.

Innovative Solutions
The potential for a diver to become pinned by a falling cutter was of high concern. This meant that a finite volume of breathing gas would not suffice should a long-term rescue become necessary. A commercial diving compressor was reconfigured, so it would fit under the TBM’s gantries. This compressor is typically powered by a diesel engine but because it would be on the TBM, an electric motor was selected. This compressor was connected to the dive supervisor’s station at the entrance of the man lock and served as a tertiary supply, should the worst-case scenario happen. All utilities the divers would need, such as primary and secondary air, audio, and video communications with the supervisor, were run through the bulkhead into the working chamber.

Results
The Ballard team inspected and removed cutterhead discs as necessary with hydraulic impacts. The discs were brought out and new discs were brought in using lift bags and chain falls. OARS Phase 1 consisted of a twenty-foot finished diameter, a 22,892-linear-foot-long tunnel that was 180 feet deep. The completed OARS system provides adequate storage capacity to mitigate downtown-area overflows for a ten-year level of wet weather protection and is anticipated to remove nearly two billion gallons of combined sewer overflow discharges annually.