As the largest combined sewer overflow (CSO) facility in the world and touted as one of the engineering marvels of the 21st century, the Thornton Reservoir (previously part of Thornton Quarry) has been voted the 2016 Water Project of the Year by American Infrastructure magazine readers
By Genevieve Smith
t all began with TARP. The Tunnel and Reservoir Plan (TARP) was adopted in 1972 as the Metropolitan Water Reclamation District of Greater Chicago’s (MWRD) cost-effective plan for complying with federal and state water quality standards. TARP’s main goals are to protect Lake Michigan’s drinking water supply from raw sewage, improve the water quality of area waterways, and provide an outlet for area floodwaters to reduce street and basement sewage backup. The Thornton Composite Reservoir, part of Phase II of TARP, is a major step towards achieving these goals.
Phase I of TARP included 110 miles of eight-foot to 33-foot diameter rock tunnels, 150-foot to 350-foot below the ground, providing 2.3 billion gallons (BG) of combined sewer overflow (CSO) storage. Phase I, partially operational since 1985 and completed in 2006, captures and stores CSOs that previously discharged to the waterways at hundreds of outfall locations. After the tunnels receive the first flush from combined sewers during significant storm events, the captured flow is pumped to water reclamation plants for full secondary treatment of the combined sewage.
Phase II of TARP consists of three large reservoirs that connect to the Phase I deep tunnel system and ultimately increase the TARP system storage volume to 20.5 BG. One of those three reservoirs is the Thornton Composite Reservoir. Completed in 2015, the reservoir has a storage capacity of 7.9 BG and is the largest CSO facility in the world. The reservoir, with a surface area of approximately 83 acres, serves a 90-square-mile area in Cook County. The reservoir provides an estimated $40 million per year in flood control benefits to 556,000 people in 14 communities, including the South Side of Chicago and 13 suburban communities.
The $420 million project is located in the expanded north lobe of the Thornton Quarry, one of the largest active aggregate quarries in the world. In existence since the 1860s and owned by Hansen Material Services (HMS), the quarry was chosen as an ideal site for the reservoir due to its highly impermeable dolomite bedrock and vicinity to the MWRD’s Calumet TARP deep tunnel system.
Black and Veatch Corporation and MWH Global, now part of Stantec, were utilized in the design of the project. Design for the reservoir focused on four major elements: sealing the perimeter of the reservoir to protect adjacent groundwater from possible pollutant seepage; sealing the reservoir to protect ongoing mining operations in the adjacent quarry; connecting the reservoir to the existing MWRD Calumet TARP deep tunnel system; and connecting the reservoir to the Thorn Creek overflow structure and diversion tunnel.
Awarding of three contracts and commencement of construction occurred in 2009/2010, resulting in an approximate five-year construction schedule. Project management and collaboration was achieved with the use of an internet/web-based project management software to exchange information in an electronic environment.