Deep Below Indianapolis, Tunnelers Race to Control Waste
The tunnel boring machine looks fairly modern, but it is actually more than 35 years old. Rebuilt shortly after churning its way through the Second Avenue Subway in New York City, the 450-ft-long Robbins main-beam TBM is now taking a spin through subterranean Indianapolis as part of the Circle City's $1.6-billion, 20-year plan to control its combined sewer overflows.
"It's very unusual for a TBM to finish up and go right to another job," explains Stuart Lipofsky, project manager for S-K JV—a joint venture of Walnut, Calif.-based J.F. Shea Construction Co. Inc. and Omaha, Neb.-based Kiewit Corp.—and general contractor on the city's Deep Rock Tunnel Connector. "Usually, it sits in the yard and looks like junk."
The 1.2-million-lb TBM is a key item on the critical path to completing the connector, an eight-mile, 20.2-ft-dia tube that workers are drilling through limestone and dolomite 250 ft below the city. Working around the clock under a $179.3-million contract for Citizens Energy Group, S-K is off to a solid start: So far this year, its tunnelers have bored out more than two miles, including a world-record-breaking daily drive of 409 ft. When completed—the schedule says fall 2017—the DRTC, costing $234 million in total, will connect a 25-mile-long system of rock tunnels and drop shafts. They will channel waste to treatment plants, which are being upgraded to handle the increased flow.
"The biggest challenge is coming to do a deep-rock tunnel where no deep-rock tunnel has been done before," says Lipofsky. "Not knowing what's really down there—even though we've got our borings—from design to construction management, it's all a first."
Like many cities across the country, much of Indianapolis is built on top of combined wastewater and storm sewers. Under heavy rain, the system is often overloaded, dumping about 7.8 billion gallons of sewage per year into local rivers and streams. Situated between two treatment plants, the DRTC, coupled with four other planned tunnels, will be able to store 250 million gallons. Together, they will help reduce annual discharge to less than 414 million gallons of sewage, a capture rate of about 95%. For designers and contractors, the work also sets the stage for more than a decade of heavy construction throughout Indianapolis.
The DRTC is the biggest wastewater project ever let in the city, according to John Morgan, manager of special projects for Citizens Water, a unit of CEG. Standing on the 15-acre launch site near the Southport Advanced Wastewater Treatment Plant, on the city's south side, he explains that this tunnel is the easy part.
"This is the biggest site we'll ever have for a tunnel," Morgan explains. "Our retrieval shaft is less than one-quarter the size of this, and that will be our next launch shaft. As we get more metropolitan, and as we go more upstream in the system, it just keeps getting tighter."
Work around the Southport launch shaft is busy but organized. Dump trucks are continuously driving into the site empty and leaving full of rock, muck and other material, which a conveyor heaps onto a massive pile next to the 251-ft-deep launch shaft. The trucks proved a bigger nuisance than the tunneling, whose early work required explosive emulsions. (Project engineers prefer to describe the work as "drill and shoot," a softer-sounding expression.) "We were getting complaints about the tailgates slapping, which we took care of over time," says Morgan. "But never did we get a complaint about the blasts."
S-K clinched the job from a highly competitive pool of nine bidders and began work in December 2011. "Like every tunnel job, the first thing we do is set up our utilities," says Lipofsky, adding, "it was a grass field when we got here." The owner gave the contractor the option of hooking up the TBM's 13.2-kV electrical lifeline to the nearby treatment plant. For an extra $1 million, it also had the option of running a line from a nearby substation. Fortunately for both parties, the plant's own power was sufficient.
While S-K was preparing the site, crews at J.F. Shea's yard in Mount Pleasant, Pa., were busy rebuilding the TBM, which came off duty from the Second Avenue Subway in November 2011. The cutterhead was built new at Robbins' facility in Akron, Ohio. The machine, originally built in 1976, was upgraded with variable-frequency drive motors, a back-loading cutterhead with 19-in. discs and other modern equipment.
Reusing a TBM is cost effective but not always possible due to project parameters, such as the local geology, says Lipofsky, who estimates that a new machine costs about $20 million. (A used machine costs about half that.) "It helps us get work, and it's a cost savings to the owner," Lipofsky says. Because the heavy equipment is specialized and costly, design engineers can help projects find savings by tailoring tunnels to available machines. "When we were sizing the tunnel system, we were looking for the sweet spot in the industry," Morgan adds. "There seemed to be more machines in the 20-ft-dia range."
Another task on S-K's plate was to extend an existing 15,000-ft-long levee by 11,000 ft to wrap around the Southport treatment plant. Designed by Rosemont, Ill.-based Christopher B. Burke Engineering, the new levee, which S-K raised up to 5 ft, aims to protect the plant against a 500-year storm and take it off the flood-insurance map. The tunnel design, prepared by AECOM, was completed under a hectic, 16-month schedule. The owner also hired AECOM, with subcontractor Black & Veatch, for construction inspections.
S-K excavated the 44-ft-dia launch shaft using the slurry-wall method for the first 100 ft—to the top of the bedrock. Blasted out to 39 ft in dia, the rock section then tapers to a concrete-finished tube 35 ft wide. Down at the floor, crews blasted a horseshoe-shaped starter tunnel 478 ft long and 23 ft tall, finished with 4 in. of shotcrete. The main tunnel, drilled out to 20.2 ft and rock-bolted, will have an unreinforced concrete liner with an internal diameter of 18 ft.
The DRTC is part of a landmark settlement between Indianapolis, the state of Indiana, the U.S. Environmental Protection Agency and the Justice Dept. In 2006, federal regulators directed Indianapolis to clean up its act by building 12-ft-dia soft-ground tunnels 60 ft below the surface, upgrading plants and performing other measures. The plan was estimated to cost $1.73 billion, but early work encountered delays and overruns, sending the budget closer to the $2-billion mark.
Mayor Greg Ballard in 2008 brought in industry veteran David Sherman to value-engineer the decree. The city came back with a counterproposal to build wider, deeper rock tunnels, upgrade treatment plants and install green measures that help capture, treat and absorb runoff. Financed through user fees, the revised plan is estimated to save $740 million and treat an additional 3.5 billion gallons a year.
ENR lauded the effort, handing a 2011 Newsmaker Award to Sherman, then director of the city's Dept. of Public Works. Ballard ushered in other reforms that streamlined procurement. At the time the DRTC bid was opened, in August 2011, DPW moved under Citizens as a charitable public trust. "One agency bid the job and a different agency awarded it," notes Morgan.
The new model gives Citizens the flexibility to procure future work privately, and there is a lot more on the way. The upgraded CSO system will include some 40 new drop structures designed with tangential vortices to help channel wastewater into the deep tunnels. The DRTC connects three such structures as well as a 90-mgd pumping station feeding into the Southport plant. The Fall Creek and White River tunnels, now under design by Black & Veatch Corp., are estimated to total 8.6 miles together and cost $389 million. Construction is expected to begin in 2016. Lower Pogue's Run tunnel, designed by Parsons Brinckerhoff and expected to start in 2016, will snake 10,000 ft through downtown Indianapolis and cost about $200 million. The 40,000-ft Pleasant Run tunnel, expected to start in 2020, awaits a designer. All will follow the basic profile of DRTC.
Citizens has tried to anticipate problems through advanced planning and extra soil tests. As lab analysis predicted, the TBM is chewing through ideal rock. "It ranked high on cutter wearability," says Alexander Varas, AECOM vice president. Change orders have amounted to 1.34% of the DRTC project value, mainly due to near-ground structures failing under construction loads. The changes were minimal—the job is months ahead of schedule—but the lesson was learned.
"We did not go out of our way to replace everything prior to having all this heavy equipment on top of piping," Morgan explains. "I'm pushing pretty hard internally for future designs to look at the footprint and replace everything within that. It is just better to replace it while we are there."
