The Olentangy and Scioto rivers lend scenic beauty to an urban landscape as they flow through Columbus, Ohio, but when substantial rains occur, the city’s sewer system can overflow into them. That eventuality will be eliminated when the Olentangy Scioto Interceptor Sewer (OSIS) Augmentation and Relief Sewer (OARS) project becomes operational in 2016.

Organized into two phases, the first phase will be completed in late 2016 or early 2017. That phase includes boring a tunnel along the Scioto River 20 feet in diameter and 170 feet under downtown, Scioto Audubon Metro Park and the Berliner Sports Park, along with residential neighborhoods. The $77 million Phase 2 – building many of the structures underground that will connect the tunnel to the city sewer system and prevent overflows – will be completed in late summer 2017.

The tunnel will provide relief to the existing OSIS from just north of the Arena District in downtown Columbus to the Jackson Pike wastewater treatment plant, a distance of approximately 23,300 feet. “We’re going to eliminate all the overflows in a normal year, and the way we’re doing it is by strategically locating relief structures on the Olentangy interceptor sewer,” promises Jeff Coffey, project manager for the design professional DLZ. “By allowing the flow to go into the OARS project, we’re going to keep the level of flow low enough that the overflows aren’t activated. By doing that, we’re eliminating 12 overflow points and getting all the flow down to the Jackson Pike wastewater treatment plant. We’re going to treat as much as we possibly can and get the benefit of a lot of storage in the tunnel, minimizing the flows to the river and maximizing treatment.”

Project Team

The Phase 2 project team includes design firms DLZ Corp. in association with Jenny Engineering and CH2M Hill. Construction management is being provided by Black & Veatch in association with local consultants H.R. Gray. Trumbull Corp. is the general contractor that is completing construction of three shafts 30 feet in diameter and 150 feet underground. The shafts will be connected to a tunnel 20 feet in diameter. At the surface, an inlet structure connects the existing OSIS sewer line to the new shafts and tunnel. 

Trumbull also is installing six submersible and two grit pumps in a shaft constructed during Phase 1, which will empty the tunnel after an overflow and transport the water to the Jackson Pike wastewater treatment plant. Additionally, Trumbull is constructing a river outfall structure that will allow the combined sewer to overflow to the nearby Scioto River in the event that the wastewater cannot be transported to the wastewater treatment plant. 

Hard Rock

The tunnel being bored in Phase 1 is mostly through solid rock. “The primary advantage of being in solid rock is that as you excavate, you don’t have to worry about the stability of the face of the tunnel, and you don’t have to worry about settlement occurring above you,” Coffey says. “So our first choice is always going through rock, especially in a downtown area.”

In Phase 2, the shafts being built to access the tunnel have been excavated 35 to 40 feet deep through soft ground filled with stones and rock and then through a shale layer and into limestone. Phase 2 included electrical work and installation of pumps and mechanical equipment. “We installed basically four 800-horsepower pumps, two 450-horsepower pumps and two 105-horsepower grit pumps,” Trumbull Corp. Superintendent Steve Owens explains. “We’ve also got one 100-horsepower shaft mixing pump, and about 2,200 lineal feet of ductile iron pipe 24 inches, 8 inches and some 42 inches in diameter. And that basically will all be used to pump the sewage back out of the tunnel and send it to the wastewater treatment plant.”

Keep It Flowing

A challenge was dealing with the water held in porous Karstic limestone. Another was building the OARS in an urban area – especially when blasting was required – excavating in the middle of one of the city’s major thoroughfares and keeping the riverside Scioto Audubon Metro Park open during construction. Surface construction was limited to daylight hours, and the hours for blasting and hauling were further limited.

Grit and determination were required to build the multitude of shafts and chambers underground. Additionally, the OARS had to be built without impeding the city’s regular sewer system. “We had to manage all the flow that was going through the existing sewer, which in dry weather was 35 million gallons a day – in wet weather, it was several hundred million gallons a day,” the city’s project manager for Phase 2, Raisa Pesina, explains. “We had to design a bypass system to do that.”

This required brainstorming ideas and interaction among team members. “From a contractor’s standpoint, it was very helpful to have the designer accessible and available to come and look at drawings and sketches and look at solutions,” Owens adds. “The owner and the designer are not always that accessible on projects.”

At one point, 90 feet of the main sewer was removed. “It is 17 feet tall and 10-1/2 feet wide, and it is completely full during a storm event,” Pesina says. “We had to remove 90 feet of that and bypass that water around the new portion of sewer. That was a huge task. It’s the largest sewer in the city’s whole system.”

A rigorous safety program is being employed on the project, which requires workers to be lowered into shafts by cranes and to receive sufficient ventilation air.

Monthly coordination meetings among the contractors and constant informal meetings, as well as socializing occasions, keep the channels of communication flowing among team members. Members of the project team agree that perseverance is one reason for the success of the project. “We have a pretty good relationship from a contractors’ and owner standpoint,” Owens maintains. “We may not always agree on issues, but we are able to come back to the table and work.”