An approximately four-mile-long, 18-ft-diameter and 200-ft-deep tunnel to mitigate storm water and combined sewer overflow into the Connecticut River and Long Island Sound is slated to begin construction this month.

Granite Construction Inc. subsidiary Kenny Construction Co. of Northbrook, Ill., and Japan’s Obayashi Corp. recently won the $279-million contract for the South Hartford Conveyance and Storage Tunnel. The joint-venture team will construct a launch shaft, a retrieval shaft, a pump-station excavation shaft, seven drop shafts, seven adits with deaeration chambers and an odor-control building at the retrieval shaft. Running from West Hartford, Conn., to the Hartford Water Pollution Control Facility, the 41.5-million-gallon-capacity tunnel is critical to the metropolitan district’s 20-year clean-water initiative to mitigate combined sewer overflows in the greater Hartford region.

The nonprofit municipal corporation’s CEO, Scott Jellison, said this is the first project of its type in Connecticut. “We’re excited to get into the ground so that we can start to see if conditions are what we expect,” Jellison says. “The challenges come from uncertainty in ground conditions. We had a robust geotechnical investigation program and risk management process in the design phase to identify and mitigate underground risks and to fairly allocate remaining risk between ourselves and the contractor.”

AECOM’s Rocky Hill, Conn., team designed the estimated $500-million design-bid-build project, which has yet to award pump station and consolidation conduit contracts. Tunnel-boring machines are scheduled to arrive in the fall of 2017, and the tunnel is slated to begin its full operation by 2023.

Jellison says the project has several significant benefits, including for water quality and public health. “The project helps us achieve the bottom line economically by being the least-cost solution,” he says, “socially by avoiding significant disruption to the local community as compared to other alternatives, and environmentally by significantly improving the quality of local waterways.”