In an effort to improve the water quality for the City of Indianapolis, Indiana, Citizens Energy Group looked to build a combined sewer overflow (CSO) collection system at multiple locations. The project, in conjunction with the U.S. Environmental Protection Agency and the Indiana Department of Environmental Management, will run approximately 25 miles (39 meters) long and remove combined sewage from eight local rivers and streams.

The first phase of the project is the Deep Rock Tunnel Connector (DRTC), a 7.6-mile (12.2-kilometer), 18-foot (5.5-meter) diameter tunnel located 280 feet below the surface in a rock cavern. Three CSOs will capture sewage from the city’s largest overflow points and then transport it to a below ground Deep Rock Tunnel Connector Pump Station (DRTC PS), from where it will then be pumped to a wastewater treatment plant. The project is estimated to be completed in 2017 and will initially deliver 54 million gallons of the planned 270 million gallons of CSO storage for treatment, as well as provide future connections to other tunnels in the overall system.

AECOM provided design for the main tunnel of DRTC and DRTC PS. The tunnel design was completed using a tunnel boring machine. Multiple world records were broken during the course of the tunneling phase, including “Most Feet Mined in One Day,” “Most Feet Mined in One Week” and “Most Feet Mined in One Month.” The next phase will involve the construction of an 18-foot (5.5-meter) diameter cast-in-place concrete liner.

Estimated to be completed in 2025, the overall project will significantly reduce sewage overflows into Indiana waterways and improve water quality for the area.