CHICAGO'S DEEP TUNNEL – HISTORY AND EVOLUTION

Before the enactment of the 1972 Federal Clean Water Act, Metro-Chicago began its search for a comprehensive solution to its severe and then-worsening combined sewer overflow (CSO) pollution and flooding problems. Many studies and extensive interagency cooperation singled out the Tunnel and Reservoir Plan (TARP) as the best means of cost-effectively achieving three anti-pollution and anti-flooding objectives: protect Chicagoland's main drinking water supply (Lake Michigan) from raw sewage backflows; clean the area's polluted inland waterways; and alleviate basement sewage backups.Since TARP was such a unique project, there was little or no design, construction or operations experience to draw upon to help implement the project. TARP consists of four independently operated systems (O'Hare, Mainstream, Des Plaines, and Calumet). Upon completion of a new tunnel segment, it was quickly placed into operation. This allowed the Metropolitan Water Reclamation District of Greater Chicago (MWRD or District) to assess design strategies, construction procedures/techniques and operational performance in order to refine and optimize strategies for successive TARP tunnel contracts. Herein is a brief discussion of TARP's history and development; and also a look into the evolution of the processes, techniques and strategies used to construct its mammoth tunnels.