Thornton Composite Reservoir to Receive Project of the Year Award from APWA

The Metropolitan Water Reclamation District of Greater Chicago (MWRD's Thornton Composite Reservoir will receive the "Project of the Year" award from the American Public Works Assn. (APWA) on March 18 at the Medinah Banquet Hall in Addison, Ill.

The reservoir is part of the MWRD's Tunnel and Reservoir Plan (TARP) connected to the MWRD's extensive network of deep tunnels. It benefits 556,000 people in 14 communities throughout the South Side of Chicago and south suburbs. It protects 182,000 structures, such as homes, businesses and other facilities, and improves 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 via the 30-ft tunnel to the Calumet Water Reclamation Plant to be treated.

Major construction features of the project included: a double-row vertical grout curtain around the two-mile perimeter of the reservoir; a 112-ft tall roller-compacted concrete dam; a 1,300-ft long, 30-ft diameter connecting tunnel from the reservoir to the existing Calumet Deep Tunnel; and a 1,100-ft long, 20-ft diameter connecting tunnel from the reservoir to the Thorn Creek.

In its first months of service, the Thornton Reservoir has already made an impact. The reservoir took in its first water on the evening of Nov. 26, 2015. By the time the rain stopped the following day, the reservoir was filled to a depth of 17 ft and held approximately 400 million gal of water. There were no combined sewer overflows in the reservoir's service area during the rain event, pointing directly to the effectiveness of the reservoir.

"This honor is a credit to our engineers who saw a barren land of limestone and through hard work and ingenuity turned it into the largest of its kind reservoir in the world," said MWRD President Mariyana Spyropoulos. "We now have the ability to save local communities in the South Side of Chicago and south suburbs from flooding and protect our vital waterways from pollution."

Through an agreement reached in 1998, the MWRD asked HMS to create the rough hole needed for the reservoir. The deal allowed HMS 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.

To properly seal the reservoir to contain water, a dam, made of 32,000 cu 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 ft 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 ft 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 every 5 ft around the nearly two-mile perimeter of the reservoir. A second row was then constructed about 20 ft away, angled in the opposite direction in an attempt to intercept and seal as many cracks as possible.

The Thornton Composite Reservoir is part of Phase II of the MWRD's TARP. The reservoir, with a volume of 7.9 billion gal, is the largest combined sewer overflow (CSO) facility in the world. Serving a 90-sq-mile area in Cook County, the reservoir provides flood relief benefits to 556,000 people in 14 communities. The project converted a limestone quarry into a CSO facility.

The APWA serves professionals in all aspects of public works-a fact that sets it apart from other organizations and makes it an effective voice of public works throughout North America. With a worldwide membership over 28,500 strong, APWA includes not only personnel from local, county, state and province and federal agencies, but also private sector personnel who supply products and services to those professionals.

Source: Metropolitan Water Reclamation District of Greater Chicago