The Deep Tunnel Plan constructs a series of large tunnels underneath Chicago. The system is designed so that when the surface is flooded raw sewerage is not discharged to rivers, streets, and basements, but instead is sent to the deep tunnel and reservoirs. After the flood is over the sewerage is pumped up from the tunnels, treated in the sewerage treatment plants, and discharged. The project is one of the largest public works projects in the United States.
The Deep Tunnel "is the largest undertaking of its kind ever attempted by mankind," declared Raymond Rimkus, general superintendent of the Metropolitan Sanitary District of Greater Chicago, the agency that planned the project. For once, many of the superlatives were deserved.The Deep Tunnel, or the Tunnel and Reservoir Plan, was the largest public works project in Chicago's history and one of the biggest in the U.S.
Phase 1 involved the creation of 109 miles of tunnels ranging from 9-33 feet in diameter up to 350 feet underground. It was adopted in 1972, commenced in 1975, and completed and operational by 2006. In Phase 2 reservoirs primarily intended for flood control remains underway with an expected completion date of 2029. Currently, up to 2.3 billion gallons of sewage can be stored and held in the tunnels themselves while awaiting processing at sewage treatment plants, which release treated water into the Calumet and Des Plaines Rivers.
Additional sewage is stored at the 3.1 billion gallon Thornton Transitional Reservoir, which will return to use as a quarry when the nearby Thornton Composite Reservoir is completed, and the 350 million gallon Gloria Alitto Majewski Reservoir near O'Hare International Airport. The 10 billion gallon McCook Reservoir is scheduled for completion in 2017 (3.5 billion gallons) and 2029 (6.5 billion gallons) and the 7.9 billion gallon Thornton Composite reservoir is scheduled to be completed in 2015. As the reservoirs are decommissioned quarries, construction has been subject to demand for the quarried gravel. Upon completion, the TARP system will have a capacity of 17.5 billion gallons of storage.