Try to imagine a space large enough to hold as much water as thunders over Niagara Falls in fi ve minutes. Now imagine stashing that water beneath Indianapolis in a cavity nearly as deep underground as the Indiana Soldiers' and Sailors' Monument on Monument Circle is high.
That's the goal of city engineers and consultants as they fine-tune plans to build a colossal tunnel to temporarily store water and raw sewage that now shoots into local waterways during rain storms.
The project was big when the city filed its latest plan with the federal government three years ago. Now it's twice as long—20 miles.
At a projected cost of $3.5 billion, the sewer upgrade is easily the largest public works project in Indianapolis since the interstates were built nearly two generations ago.
The plan includes a lot more than the tunnel and related surface improvements, which will cost $1.7 billion. Also on the drawing boards are $1.8 billion in connections to extend sewage systems to homes now on septic systems, plus numerous improvements to sewers and treatment plants.
Indiana companies don't have the massive machines needed to grind a tunnel as tall as a two-story house through limestone. But local firms will have plenty of opportunity to sink access shafts, supply concrete, haul debris, and design and build interceptors or sewers to feed the tunnels.
"That's where the local contractors are going to be in their hey-day," said Steve Nielsen, deputy director and chief engineer for the Department of Public Works.
The project has been a long time coming.
For decades, Indianapolis has been plagued by the so-called combined sewer overflow. In 2006, the city reached a consent agreement with the U.S. Environmental Protection Agency and the U.S. Department of Justice to eliminate 97 percent of the overflow by 2025.
The tunnel will begin near the Indiana State Fairgrounds and, shadowing Fall Creek, proceed west around downtown. Then it will roughly follow White River south, ending at the Southport wastewater treatment plant.
The latest plans call for an additional seven miles of deep rock tunnel at the southern end, between the city's Belmont and Southport treatment plants.
The longer tunnel opens possibilities of extensions toward Pogue's Run and Pleasant Run creeks to better capture sewer overfl ows.
No matter how details shake out, one thing is clear, Nielsen said: "This is the most massive project for the city of Indianapolis in public works."
Residents will put up with untold interruptions as contractors dig up yards and streets to route drainage to the tunnel, but about all the public will see of the tunnel portion of the project will be trucks carrying away seemingly endless loads of dirt and rock, enough to fill Conseco Fieldhouse three times over.
Indeed, the tunnel will be unlike anything ever built here—mass construction in the unseen realm of bedrock that begins at about 75 feet beneath the sidewalk.
Doing the work will be a tunnel boring machine, a cylinder the length of a couple of school buses and about 25 feet wide in diameter. At the business end is a round, rotating head with disc-shaped cutting teeth. Advancing the monster through the rock will be grippers pushing against the freshly cut wall.
Rock and muck leave via conveyor belt. Getting the machine to the depths will require constructing a vertical shaft at least as wide as a semi-trailer is long, and more than 200 feet deep.
The exact route and size of the tunnel is still being worked out, however.
"The project is in its very early stages," cautioned James McKelvey, associate vice president and tunnel practice leader with Black & Veatch Corp., an Overland Park, Kan., engineering firm hired by the city.
McKelvey has worked on sewer overflow tunnels in such places as Charleston, S.C.; Las Vegas; and South Africa. One recent Black & Veatch project was a 7.1-mile tunnel for Milwaukee, completed in 2005.
Milwaukee's tunnel also went through bedrock, but at a relatively shallow depth of about 150 feet. Indianapolis' tunnel will be deeper—225 feet to 275—because the rock begins farther down.
Though the city could tunnel through clay that lies above the limestone, the risk of disrupting wells and of causing ground to settle was too great.
Good for tunneling
By contrast, hundreds of borings show the limestone bedrock under Indianapolis is relatively dense.
"We found what appears to be an excellent tunneling medium," McKelvey said.
Not so in Milwaukee. There, workers ran into coral from an ancient sea that allowed water to pour in.
Tunneling into limestone deep beneath Indianapolis is not unprecedented. Martin Marietta Materials operates a limestone mine about 160 feet below the Southside Sanitary Landfill, along Kentucky Avenue.
While Indianapolis' limestone is favorable for tunneling, getting to it poses perhaps the biggest challenge.
Along the length of the tunnel system will likely be at least 10 access shafts. The excavation could rupture groundwater tables—especially those now tapped for water wells. Also, sewage flowing down the shafts could leak through walls and into the nearby water tables.
"One of the real challenges has been our [proposed] alignment passes close to the White River well field," said McKelvey, noting that the city draws almost half its water supply from underground wells along White River and Fall Creek.
It doesn't appear that the deep tunnels will pose much risk to groundwater. Many of the wells are above the bedrock or are shallow in the limestone.
Early designs call for installing concrete liners in the tunnels and injecting grout between the rock face and the liners.
The tunnel boring machines can't do the work alone. Some blasting will be needed, particularly during construction of the shafts. Initially, that was a big concern in Milwaukee, where the tunnel ran close to a medical center that performed eye surgery. The solution was to bring in an expert who designed less disruptive blast patterns.
"Truthfully, the biggest headache is what is the [best] alignment" of the tunnel, under the city, Nielsen said.