Chicago’s Thornton Composite Reservoir – Flood Protection for 182,000 Homes and Businesses


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Yesterday marked the official opening of the Thornton Composite Reservoir, a new 7.9 billion gallon capacity reservoir connected to Chicago’s “Deep Tunnel” network that  will offer flood protection for over half a million people throughout the South Side of Chicago and south suburbs.

“Thornton Composite Reservoir is the world’s largest. It has drawn intrigue from across the world, and given the success of TARP, more cities are emulating our strategy,” said Mariyana Spyropoulos, President of Metropolitan Water Reclamation District of Greater Chicago (MWRD).

Photo credit: MWRD
Photo credit: MWRD

Chicago’s Tunnel and Reservoir Plan (TARP)

The reservoir forms part of the MWRD’s Tunnel and Reservoir Plan (TARP), commonly referred to as the “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 total cost of TARP is $3.8 billion, about half of which came from federal money.

TARP Map. Image credit: MWRD
TARP Map. Image credit: MWRD

Grand Canyon of the South Suburbs

Dubbed the “Grand Canyon of the South Suburbs” the Thornton Composite Reservoir has been connected to a tunnel system and prepared to hold up to 7.9 billion gallons of water. The reservoir will reduce flooding in the area while preventing pollution in Chicago area waterways.

The total cost of Thornton is projected at $429 million. It will protect approximately 182,000 structures, such as homes, businesses and other facilities; and improve water quality in the Calumet River and Calumet-Sag Channel by collecting combined sewer overflows before entering waterways. The new reservoir’s capacity holds these overflows before pumping the water to the Calumet Water Reclamation Plant for treatment.

Attending the opening ceremony, U.S. Rep. Robin Kelly said “With the reservoir and tunnel system, we have a new and gigantic tool that will help us face an increasing amount of intense rainstorm events flooding Thorn Creek and our streets, homes and businesses. The reservoir will provide an outlet for these waters and give us a chance to treat water before it enters the Little Calumet River.”

Building the Reservoir

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 their existing Thornton Quarry, which dates back to the 1860s. That aggregate is used in several area road and building construction projects.

Photo credit: MWRD
Thornton Composite Reservoir – Photo credit: MWRD

Beyond mining the large reservoir, the other challenge was properly sealing the reservoir 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 the sides of the reservoir, 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.

Photo credit: MWRD
Photo credit: MWRD

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 odours.

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.

Photo credit: MWRD
TARP Flood Tunnels, Chicago. Photo credit: MWRD
Gloria Alitto Majewski CUP Reservoir. Photo credit: MWRD
Gloria Alitto Majewski Reservoir – 350 million gallon capacity reservoir in the north-west of Chicago and part of the TARP network of tunnels and reservoirs. Photo credit: MWRD