The U.S. Environmental Protection Agency (EPA) identified 746 communities using combined sewer systems (CSS) in 2004—the majority located in the Great Lake and Northeastern regions. Home to the nation’s older communities, these areas were quickly developing in the late 19th century when cities began building municipal sewer systems.

Within the Great Lakes states, the EPA identified Ohio as having the highest number of communities using a CSS, and number two, following Upstate New York, when including the Northeastern region. Here in the 21st century with water quality in question, CSS are on a path to extinction just like the cesspools they replaced 100 years ago. Columbus is just one of many cities in Ohio and the Great Lakes alike, finding tunnelling to be the best solution to relieve their combined and sanitary sewer overflows (CSOs and SSOs).

With a TBM arriving later this year, ground modification and shaft construction are underway for the 24ft (7.3m) diameter, 23,000ft (7km) tunnel that will run through downtown Columbus. This relief sewer for the existing combined Olentangy Scioto Interceptor Sewer (OSIS) will reduce the number of overflows to the Scioto River and meet a consent decree the City of Columbus has with the State of Ohio’s EPA.

Known as OARS, for OSIS Augmentation Relief Sewer, the tunnel is the backbone of the city’s Wet Weather Management Plan, which spans a 40-year timeline starting in 2005, with an estimated USD 5.44bn price tag. Two more deep tunnels with diameters ranging from 14ft to 18ft (4 to 5.5m) to relieve the city’s sanity sewers are on the books, which will meet a second, separate consent decree (Figure 1).

Construction on OARS is scheduled to complete in 2014, the same year construction should start on the approximately 70,000ft (21.3km) long Alum Creek Relief Tunnel (ART) on the east side of Columbus. In 2018 construction should start on the Olentangy Relief Tunnel (ORT), of approximately 58,000ft (17.7km), on the west side of town. For the city to meet its SSO consent decree both ART and ORT must be finished by 2045.

Deep tunnel solution
Starting in 2006 DLZ of Ohio led the OARS design team with Jenny Engineering Corporation for tunnel/shaft design, CH2MHill for the pump station design and EMH&T for hydraulic modeling. Construction management is being provided by Black & Veatch and HR Gray and Associates.

Originally the project called for a large, open trench relief sewer constructed in three phases. David Day, vice president of the public works division at DLZ and project principal for the OARS tunnel, says, looking at just the first phase, several issues with the open trench sewer became apparent.

“The city asked us to look at all three phases and there were other issues, mostly just with disturbances in the downtown area of Columbus, and the cost of trying to do an open trench project through the downtown. We proposed to look at a tunnel alternative,” Day explains.

The city also brought in a technical advisory committee to look at the options, which recommended the project move forward with the tunnel instead. This proved to be advantageous in other ways than just meeting the requirements of the consent decrees.

“Because the city is also going through a program to really rebuild their riverfront area they wanted to minimize overflows even more than what the EPA required [no more than four overflow events per year]. The tunnel project really helped with that,” says Day. “We were able to go with a larger diameter tunnel than we would have been able to go with an open trench sewer.”

The tunnel has been divided into two phases with separate contracts. The city awarded the first contract to construct the tunnel to a joint venture of Kenny and Obayashi in early September 2010, worth USD 264.5M. This first contract includes shafts 1 and 2 located at the wastewater treatment plant for launching the TBM; they’ll later be used for the pump station and the screening facility, as well. Shaft 6, the TBM retrieval shaft located in downtown Columbus, will also be constructed as part of the first contract (Figure 2) Shafts 3, 4 and 5 are unrelated to tunnelling and will come as part of the second contract.

Ground conditions
With the new system, rather than emptying into the Scioto River, flow will be taken to two wastewater treatment plants located south of Columbus: Jackson Pike, where the pump station will be located and the TBM will be launched, and Southerly, via an existing interconnect with the Jackson Pike plant. The tunnel’s alignment needed to stay close to the OSIS to catch all of the overflow locations, which comprised an area north of the plants, including right through downtown Columbus.

The tunnel’s depth ranges from approximately 186ft (56.7m) to 125ft (38.1m), where it will be constructed through limestone bedrock with groundwater pressure anticipated to vary. The design does expect to encounter sluice and karst features. DLZ had considered several alignment options, including a profile through mostly soft ground.

“We felt that was a more risky tunnel,” Day says. “In Ohio we have a lot of glacial till… and also the fact that it was going to be very difficult to get completely out of the rock and still be deep enough for the tunnel to serve its purpose. Everybody felt it was better to do it deeper and get into confident rock.”

Several other tunnels have been built in the Columbus area in recent years, but nothing with a diameter in comparison to that of OARS (see side bar). In particular, the city has had difficulties with the amount of groundwater in the limestone and karst features while building the Upper Scioto West Interceptor tunnel, which DLZ also designed.

The OARS contract specifies a pressurized face TBM that has the capability of utilizing slurry, and a precast segmental lining. Contractor Kenny- Obayashi decided to use a hybrid machine capable of operating in open mode, in EPB mode and in slurry mode, and is working with Herrenknecht.

“The biggest driving force with the pressurized face was groundwater inflows that we knew we’re going to have,” Day explains. “And that way by putting up the precast segmental lining, and having the pressurized face we would reduce the impact of having groundwater coming in.”

There have been preliminary discussions for the precast reinforced concrete segments, but the lining has not yet been finalized. Currently, piloting testing is being done on several rings of Combisegments, both fiberglass and plastic lined reinforced concrete segments. Kenny-Obayashi will also probe ahead of the machine and grout any voids it encounters, as required by the contract.

Using shafts strategically
Shaft excavation and supports within soft-ground are specified to be impermeable excavation/support systems, and the contract also requires the contractor to pre-grout the bedrock before shaft excavation. This where the project is at now as Nicholson Construction of Pennsylvania started pre-excavation cementitious grouting in early 2011.

“They’re installing casings to go through the overburden, and they’re being socketed to the rock,” explains Bob Rautenberg, project manager with Kenny Construction. “They’ll start to drill the rock, full depth to 25ft [7.6m] below the invert of each shaft. And then they’ll do some stage grouting to seal the water.”

Nicholson will also do grouting for Shafts 1 and 2 (Table 1) and the slurry walls for each shaft. Once those are finished Kenny-Obayashi will start doing the overburden excavation, followed by the excavation down to the tunnel’s invert and development of the starter tunnels. TBM delivery is expected in October 2011 and the machine should be onsite by January 2012. It will be launched from shaft 2 using shaft 1, about 220ft (67m) apart, to assist with the TBM installation, says Rautenberg.

After tunnelling, part of phase 1 includes the installation of screening equipment at shaft 2 and construction of a screen services building. In the second contract, shaft 1 will be converted into a deep submersible pump station (Figure 3).

At the other end of the alignment, in downtown Columbus shaft 6 will include an internal surge chamber and a hydraulic drop pipe to handle any kind of surge conditions (Figure 4).

“One of the unique things about this project is the tunnel is actually being designed as conveyance tunnel and a pump station. But it also has the capability of acting as an inverted siphon, says Day.

He explains the first mode of operation—to get the flow to the plant—is the pumps. And then as the level rises in the tunnel and eventually the shafts, it will overflow by gravity to the already interconnected Southerly plant. And then the last flow would go to river as the design capacity is exceeded. “By utilizing it as an inverted siphon it would allow us to get flow to the interconnect if there is still treatment capacity available at the southerly treatment plant.”

Designing the tunnel with the Jackson Pike plant location is beneficial in two additional ways; any overflows that exceed the design—they are allowed four per year—will happen there, miles away from the downtown its recreational activities. Secondly, existing quarries near the plant can take limestone muck from the tunnelling operations, which the contractor will remove using both a conveyor and slurry lines depending on how the hybrid machine is operating.

“The downtown area is really developed, so it limited the number of shaft sites that we could choose from,” says Day. “In fact two of the connections are going to require some microtunnels. They are going to be in the range of 78in [2m] diameter for several hundred feet because it would be very difficult to do even open cut or open trench those.”

The project’s second contract will construct off-line shafts 3, 4 and 5, and the adit/dearation chamber that connects to all three, which will require microtunnelling. The design specifies a pressurized face micro tunnel boring machine. Phase 2 will also include several structures adjacent to the site where shafts 1 and 2 at Jackson Pike are to be constructed, plus installation of mechanical, electrical, instrumentation and control components of the pump station.

The estimated USD 90M second contract will be bid the first quarter of 2011 and should see construction begin in the fall. Both phases will be operational by the end of 2014 to comply with the consent decree.


Figure 1 – Citywide, Columbus has three tunnels to build totalling more than 50km. The 7km OARS has started construction this year Figure 2 – The TBM will start in the south of the city and will pass beneath downtown Columbus Nicholson Construction started pre-excavation grouting in early 2011 Figure 3 – Shafts 1 and 2, in the first contract, will be used to launched the TBM and later converter in the pump station and other service buildings Figure 4 – Shaft 6 will be used for TBM retrieval; shafts 3 and 4 are part of a separate contract Nicholson will also grout shafts 1 and 2, and construct their slury walls Table 1