In the 1970s, a team of engineers from the District, the City of Chicago, Cook County and State agencies considered various plans to solve the problem of flooding and water pollution. The hybrid plan selected as best and most cost-effective was the Tunnel and Reservoir Plan — or TARP. Under this plan, 109 miles of huge underground tunnels would be burrowed under the city to intercept combined sewer overflow and convey it to large storage reservoirs. After the storm had subsided, the overflow could then be conveyed to treatment plants for cleaning before going to a waterway.
The United States Environmental Protection Agency provided nearly 75% of the funding for this revolutionary project. The first 31 miles of tunnel extending from Wilmette on the north shore to Hodgkins just southwest of Chicago were completed and put into service in 1985. Other tunnel systems were constructed near O’Hare on the north, under the Des Plaines river to the west and under the Cal-Sag Channel and Little Calumet River to the south. (See Map)
The Mainstream tunnel is 35 feet in diameter, bored in limestone rock 240 to 350 feet below ground, and holds one billion gallons of water. TARP has received many awards, including the American Society of Civil Engineers award for most outstanding Civil Engineering Project of 1986. Mainstream is one of the largest rock tunnel bores on record. Since tunnel contractors would be working beneath homes, businesses and streets, excavation by extensive blasting was ruled out. Boring by huge tunnel boring machines (TBMs) was selected instead to cause less rock disturbance, noise and vibration. The specialized TBMs on TARP represented the largest such machines ever built, and TARP TBM technology has led to other major projects, such as the English Channel “Chunnel”.
Nearly fifteen years later the success of this project is evident by the dramatic improvements in the water quality of the Chicago River, the Calumet River and our other waterways. Game fish have returned marinas and riverside restaurants abound, river recreation and tourism are booming, and waterfront real estate values have skyrocketed as Chicago area residents see the river system as a major asset rather than an embarrassment.
Because of the success of the District’s pollution control projects, it is no longer necessary to use Lake Michigan water to dilute sewage and flush it downstream. Since the amount of lake water which can be diverted for all purposes is limited by a Supreme Court decree, the District’s conservation of Lake Michigan water frees up enough from our allocation to supply fresh drinking water to residents of Lake, DuPage and other collar counties.
The Tunnel and Reservoir Project construction continues. As tunnel segments are completed, they are immediately placed in service to provide benefits. About 93 miles of tunnels are completed, and in service, and 8 miles are under construction. All 109 miles of tunnels should be completed by 2004. The first of three huge terminal reservoirs (O’Hare CUP) was completed in 1998. These reservoirs are a joint project of the Water Reclamation District and the Army Corps of Engineers.
When completed, the reservoirs will further increase the capacity of the TARP system by 15.6 billion gallons, providing major flood relief benefits and additional pollution control improvements. The completion of this entire project, along with local municipal sewer upgrades and connections to make use of TARP’s outlet capacity, is greatly reducing pollution and flooding problems in Cook County’s 375 square mile combined sewered area serving over 3 million people. TARP relieves each local municipality of the burden of designing, building and operating its own system to capture and treat combined sewer overflow to comply with State and Federal regulations. The District provides a regional approach to urban water management that is considerably more efficient and cost-effective for the communities within its service area.
TARP was named by the USEPA as one of the nation’s top Clean Water Act success stories, and is serving as a model urban water management tool worldwide.