The US Environmental Protection Agency (EPA) is stepping up enforcement in response to combined sewer overflow (CSO) problems, which pose a major water pollution concern, bringing states into compliance with the Clean Water Act.

In Ohio there are currently approximately 1,233 permitted CSOs in 81 communities ranging from small, rural villages to large metropolitan areas. What’s more, the EPA has identified the region as having the highest number of communities using combined sewer systems. "The EPA has caught up; east of the Mississippi is where the majority of CSO systems are," says Paul Smith, senior construction manager at Black & Veatch. "If you go west a lot are separate systems. There seems to be a big push for this, and there is going to be a lot of work in the Midwest over the next 15 to 20 years."

The OSIS Augmentation Relief Sewer (OARS) tunnel project is the backbone of Columbus’s Wet Weather Management Plan and is a huge undertaking for the city, which will augment the existing Olentangy Scioto Interception Sewer (OSIS) to further limit combined overflows into the city’s Olentangy and Scioto Rivers. When completed, the sewer tunnel will intercept wet weather overflows and carry the flows to the city’s Jackson Pike and Southerly wastewater treatment plants.

"The project is a result of two consent decrees given by the EPA," explains Smith. "They have a CSO and sanitary sewer overflow (SSO). The CSO tunnel is a key component of their CSO consent decree, which will take a billion gallons (4.5bn litres) of overflows out of the river each year."

The project is designed for the city by consultants DLZ Corp in association with Jenny Engineering and CH2M Hill, and construction management is with Black & Veatch in association with local consultants H. R. Gray. The project has been divided into two phases with separate contracts. The city awarded the first contract to construct the 23,000ft (7km) tunnel with a diameter of 24ft (7m), as well as shafts one, two and six to a joint venture of Kenny and Obayashi in early September 2010, worth USD 264.5M. Phase two, which includes shafts three, four and five is unrelated to tunneling and will come as part of the second contract.

"In 2011 the contractor was able to sink shafts one and two located at the wastewater treatment plant for launching the TBM," says Smith. "The shafts will later be used for the pump station and the screening facility, and they are only 220ft (67m) apart. Shaft two is the main working shaft for TBM assembly and all the mining. There is a connecting tunnel between the two shafts and the contractor is going to utilise both shafts, extending out the screening chamber further north to facilitate assembly of the TBM. So, right now, we’ve completed the two shafts, the tail tunnel and the starter tunnel."

However, 2011 was a difficult year and progress was slow. Shaft excavation and supports within soft ground were specified to be impermeable excavation/support systems, and the contract also required the contractor to pre-grout the bedrock before shaft excavation. "We had some difficulties sinking shafts one and two, which has caused delays," says Bob Rautenberg, project manager with Kenny Construction. "Once we got into rock we hit some highangle fractures that were clay fill, which our pre-excavation grouting did not handle. At times we had inflows of up to 3,000 gallons (13,600 litres) a minute."

The tunnel’s depth ranges from approximately 186ft (57m) to 125ft (38m), where it will be constructed through Columbus limestone with karstic and solution features, voids, cracks, seams, and hydrostatic water with potential pressures of up to 6 bar.

Launching sequence
To deal with the difficult combination of hard rock with high ground water pressures, the contract specifies a pressurized face TBM that has the capability of utilizing slurry, and a precast segmental lining. Kenny-Obayashi decided to use a hybrid machine capable of operating in open mode, in EPB mode and in slurry mode, and worked with Herrenknecht.

"A slurry machine was recommended for the job but we worked with Herrenknecht and came up with a hybrid machine that can run in open mode, will have a continuous conveyor system in the tunnel, can run in EPB mode – it has a double screw – and it can also run in closed mode," says Rautenberg. "So we’ll have a complete conveyor system and slurry system in the tunnel as we proceed on and we’ll be able to change from mode to mode. We want to stay in open mode as much as we can; with the EPB and the double screw, even with some wet ground we feel we can handle quite a bit of water before we have to switch over to slurry mode."

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. "There are also difficulties related to the very complex geology we are facing in Columbus," says Greg Fedner, the project manager for the City of Columbus. "There are potential problems relating to grouting and the consequences. It is a challenge to make sure the grout does not spread too far."

The TBM arrived in late February this year, although it was expected onsite by January 2012. It will be launched from shaft two using shaft one, about 220ft (67m) apart, to assist with the TBM installation."This year we’ll finish the TBM assembly and launch the machine on June 1," says Rautenberg. "We’ll continue on our mining operation; the rest of this year we will be mining. When the mining gets to 1000ft (305m) we’ll drop back and start the construction of the pump station."

Following excavation of a starter tunnel and machine assembly at the bottom of shaft two, Rautenberg explains the launching sequence for the TBM: "We then installed our steel launch frame so we could push out our push frame and we also installed the first half of the screw, and that was all connected to the TBM and jacked forward with hydrologic shoulders into the starter tunnel. There are 11 gantries on the TBM, we’ve assembled gantries three through eight on the surface, and we’ve lowered them in shaft two and pushed them back towards shaft one. We’ve got these precast concrete blocks that gantries one and two will ride on and then we’ll install those gantries and push them up to the machine, and pull the other gantries forward, and tie them all together. At that point we’ll install our vertical conveyor belt in the shaft and the rest of our conveyor system while we’re fine tuning the gantries; making all the hydraulic and electrical hook ups. Then we’ll be ready to launch."

Making up time
Rautenberg adds that the team built the whole launch frame on the surface, which was not the original plan. "This was instead of building them in the tunnel. All the gantries – rather than building them piece by piece inside the tunnel – were built on the surface. This has cut some time; because of delays sinking the shaft, we’ve done a lot of pre-assembly work and taken advantage of the time we have.

When the contractors start mining the 23,000ft (7km) tunnel they expect to be running two 10-hour shifts, five days a week with maintenance carried out on Saturdays. Currently, the schedule is showing advance rates of an average of 68ft (21m) per day.

Completing the project by 2014 is going to be a challenge. "We’re behind schedule at the moment," says Smith.

"When the machine is up and running we’ll see where we’re at – we’ll get a pretty good sense after a couple of months of production whether we can make up some time. The beginning is a little crazy, but then we’re expecting the ground will get a lot better. In fact, once we get in far enough the ground becomes pretty consistent for a good stretch."

Smith adds that it is a complex job with many obstacles affecting the timescale. "The schedule will be a challenge as it is a tight job to begin with; due to the consent decree we have a date and work backwards," Smith says. "So, I’d say that the schedule, completion of the tunnel excavation, grouting, and annular space between the tunnel and segments could all be a challenge. These are the basic difficulties and we have to be prepared."

Fedner adds, "The EPA consent decree requires us to have an operational tunnel by December 31, 2014. The project is very challenging, but we still remain hopeful that we are able to achieve that goal, but it is very difficult. It will depend on our production and our success in mining with the TBM."

Phase two
Notice to proceed with phase two of the project was given in September 2011. The project’s second contract will construct offline shafts three, four and five, and the adit/de-aeration chamber that connects to all three, which will require microtunnelling. The design specifies a pressurized face micro TBM.

Phase two will also include several structures adjacent to the site where shafts one and two at Jackson Pike are to be constructed, plus installation of mechanical, electrical, instrumentation and control components of the pump station. "The phase two guys have different overburden and different geology, but they have to wait for us to get finished first," says Smith.