Project Title: OARS Deep Sewer Tunnel
Contractor: Kenny / Obayashi
Tunneling through karst limestone, the discs became dull under the Ohio River. The limestone allowed water to flow through in freely. This prevented the contractor from dewatering and initiating interventions. A grouting campaign was unsuccessful, as the ground was too porous to stop it long enough to set up. Ballard was called to assist with a solution.
Ballard was called on to set up for “wet” diving as part of the needed intervention. Challenges included understanding 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 potential for a diver to become pinned by a falling cutter was present as well. This meant that a finite volume of breathing gas would not suffice should a long-term rescue become necessary. A compressor was necessary as well. 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 manlock 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.
The TBM had manlocks which provided access to a working chamber. Through this area, the excavation chamber was entered through either a door at the 12:00 position or the 9:00. 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 9:00 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 manlocks where they were compressed to the same pressure as the working chamber. Upon entering the space, they would don diving gear and enter the water. The 9:00 door was opened and the divers swam into the excavation chamber to perform work. Discs were inspected and removed as necessary with hydraulic impacts. Discs were brought out and new discs in through the use of lift bags and chainfalls.