A quarter inch of rain doesn’t seem like much, unless you live in parts of Indianapolis. Aging sewer systems in the densest part of the city can fill to capacity with only a quarter to a half inch of rainfall, flooding raw sewage into local rivers and streams. To address this problem and to comply with EPA mandates, Indianapolis has begun construction of the Deep Rock Tunnel system, a $1.6 billion, 5-tunnel network that will keep billions of gallons of waste water from contaminating Fall Creek, the White River and local streams.

​Tunnel contractor Shea-Kiewit (JV) has begun work on the first phase, a 54-million-gallon, 8-mile-long Deep Rock Tunnel Connector that will extend between the city’s two wastewater treatment plants.

Drawing on its long experience with similar jobs, Prairie has partnered with Shea-Kiewit, its concrete contractor Platt Construction, and Bencor, the drilling contractor, to deliver the unique mixes needed for this multi-year, $180 million project, including high-strength concrete for the entry shafts and tunnel lining and shotcrete coating for tunnel entrances in the bedrock.

Entry underway

Bencor is currently completing a 285-ft deep entrance shaft at the Southport Advanced Wastewater Treatment plant where the tunnel will start. The 44-ft. circumference shaft is reinforced with concrete slurry walls for the first 100 feet to stabilize the softer overburden and connect to the bedrock.

Shotcrete for safety

Below the slurry walls, shotcrete is used to stabilize the bedrock sections of the tunnel shafts, an additional 185 feet down. The shotcrete will keep the rock in place, preventing cave-ins and providing crew safety.

“This is the first time shotcrete with steel fibers has been used in Indianapolis,” reports Kerry Webb, Prairie Indiana Quality Control Manager. The mix is meeting all its specs. “Tests show that we are reaching the correct strengths and uniformity and it’s ready for production.”

Shotcrete crews are using a robotic nozzle arm to precisely layer thin coats of steel fiber reinforced, 6,000 psi concrete directly onto the rock surface of the shaft, eventually reaching 4” thick.

Partnership is key​​

Prairie Sales Manager Todd Schaller has worked with JF Shea for more than 20 years in the Milwaukee area on various tunneling projects. “We got involved early in the Indianapolis project because of our long term relationship,” Schaller says. Shea knows that we can provide the quality, the capacity and the flexibility to meet their needs over a long term project, he adds. “Prairie’s QC team has been on site for every pour,” says Mike Stolkin, project engineer for JF Shea. “We are impressed with their dedication.”

Long term construction​

The tunnel boring machine will be craned into the shaft in pieces and assembled below the surface. Mining of the 18-ft, diameter tunnel is expected to begin in late August. Tunnel lining will start in early 2013 and is anticipated to move at a rate of 100 feet per day on a 24-hour, five days per week schedule. Crews will pump high slump, high-early strength concrete up to 1 mile depending on the construction requirements. The first phase is scheduled to wrap up by 2017 with a final completion date of 2025 for all 5 tunnels.