Metropolitan Water Reclamation District unveils Thornton Composite Reservoir
The Metropolitan Water Reclamation District of Greater Chicago recently unveiled its latest engineering feat, the Thornton Composite Reservoir -- dubbed the "Grand Canyon of the South Suburbs."
Sept. 4, 2015 -- The Metropolitan Water Reclamation District (MWRD) of Greater Chicago recently unveiled its latest engineering feat, the Thornton Composite Reservoir -- dubbed the "Grand Canyon of the South Suburbs." Now connected to a tunnel system and prepared to hold up to 7.9 billion gallons of water, the reservoir will help reduce flooding in the area while preventing pollution in Chicago area waterways.
The reservoir, tantamount to 12 Soldier Fields in size, is part of the MWRD's Tunnel and Reservoir Plan (TARP), or "Deep Tunnel," consisting of more than 100 miles of tunnels deep below the surface of the Chicago region and three reservoirs designed to capture and hold stormwater and sewage for treatment at water reclamation plants. Together with the Thornton Reservoir, the Gloria Alitto Majewski Reservoir (350 million gallons) in the northwest suburbs, the yet-to-be-completed McCook Reservoir (10 billion gallons) in the west suburbs, and 109 miles of tunnels (2.3 billion gallons), TARP will accommodate for 20.55 billion gallons of water.
The Thornton Composite Reservoir will benefit 556,000 people in 14 communities throughout the south side of Chicago and south suburbs. It will protect 182,000 homes, businesses and other facilities and improve water quality in the Calumet Rivers and Calumet-Sag Channel by collecting combined sewer overflows before entering waterways. The new reservoir's capacity holds these overflows before pumping the water back to the Calumet Water Reclamation Plant to be treated. The total cost of TARP is $3.8 billion, about half of which came from federal money. The total cost of Thornton is projected at $429 million.
Through an agreement reached in 1998, the MWRD asked Hanson Material Services to create the rough hole needed for the reservoir. The deal would also allow Hanson to sell the rock through its existing Thornton Quarry, which dates back to the 1860s. That aggregate is used in several area road and building construction projects. Beyond mining the large reservoir, the other challenge was properly sealing it to contain the water much like a bathtub. A dam, made of 32,000 cubic yards of roller compacted concrete, was constructed below the Tri-State Tollway (I-80/I-294) to separate the reservoir and its contained water from reaching the main lobe of the quarry. Two mining haul tunnels at lower elevations were also plugged with concrete.
At the bottom of the reservoir is an impermeable natural layer of shale existing approximately 500 feet below ground, preventing water from leaving through the bottom of the reservoir. To keep water from escaping through its sides, a double-row grout curtain was installed around the outside perimeter of the hole and tied into the layer of shale. From the surface, holes were drilled as far down as 500 feet deep at a 15-degree angle and then filled in stages from the bottom up with grout under pressure. The grout then migrated into all of the cracks and fissures in the rock mass to reduce the permeability. The holes were drilled about every five feet around the nearly two-mile perimeter of the reservoir. A second row was then constructed about 20 feet away, angled in the opposite direction in an attempt to intercept and seal as many cracks as possible.
Solar-powered aerators were installed at the bottom of the reservoir. These will float up and down with the water elevation, keeping the surface layer of water from going septic and causing any odors. Lastly, the reservoir was connected with the tunnel system by removing a 10-foot, thick concrete mass that operates as a plug. The tunnel runs about 1,300 feet before meeting up with the existing, operational deep tunnels. A concrete apron was constructed in front of the tunnel to withstand the force of the water coming out of the tunnel, which can be at velocities of up to 30 feet per second. The apron will prevent erosion of the stone reservoir floor.