August 21, 2015
“[The current] system works well in dry weather, but in heavy rains the intercepting sewers and water reclamation plants can reach capacity and result in combined sewer overflows into the river, impairing water quality and contributing to flooding,” said Allison Fore, the public and intergovernmental affairs officer for the MWRD. “TARP is being built to capture 20.5 billion gallons of that combined sewage and store it so that it doesn’t end up in the waterways or basements.”
The Thornton reservoir, one of three reservoirs in the TARP project, is 700 feet wide by 1,400 feet long with a 300 foot depth, and holds 7.9 billion gallons.
“[Right now] I believe that this is the biggest reservoir of this kind in the United States, and maybe the world,” said Adel Awar, the project’s senior civil engineer.
To put it into perspective, this massive space could hold twelve Soldier Field’s stacked on together. It will collect water from a 91 square mile area, serving about 556,000 people south of 87th St. in Chicago, as well as in 13 of Chicagoland’s southern municipalities.
“People in the affected areas will see less pollution in the Cal Sag Channel, less flooding and basement backups, and more wildlife diversity. The reservoir won’t solve all problems but the help it will provide will be substantial,” said Fore.
The reservoir will take in water through two tunnels; a smaller one, 20 feet in diameter, will bring in stormwater from area riverways, and the other, 30 feet in diameter, will connect with the existing tunnel system to bring sewage into the storage space. Water will flow naturally into the reservoir and gates will hold the water there until dry weather conditions are reached. Then the gates will slowly open and let water flow back into the tunnels and toward the treatment plants.
“Now the flow can come easily from the tunnel to our reservoir,” Awar said. But he added that each municipality will have to make sure their tunnel systems are updated and able to send their overflow to the reservoir.
“All the municipalities, they have to do their homework to improve their system and to get a good collection system,” he said. If any municipality does not have a compatible system, “we can’t help them.”
The reservoir is a converted quarry; it was mined out by Hanson Material Service (HMS) for it’s limestone, which is used to make cement used in roadways. Once HMS finished, MWRD bought the leftover cavern and began converting it into the Thornton Reservoir. This process has included installing a “grout curtain” around the walls to ensure that the porous limestone surface does not let water escape. About one gallon of grout is inserted into the ground for every cubic foot of space, leading to a combined 150 miles of grout strips. The two tunnels also had to be dug, installed, and fortified with grout and cement.
“You have a lot of different aspects of construction, all of them under civil engineering,” Awar said of taking on the massive project. “I get sick of small buildings here and there… I told my bosses I wanted to end my career with a big civil project.”
The reservoir is the second piece of the larger three-part TARP project, which began construction in 1975 and is projected to be completely finished in 2029. The first, the Majewski Reservoir near O’Hare airport, holds 350 million gallons of water. The final piece, the McCook reservoir, will hold 10 billion gallons and become the largest reservoir in the country. At 8 billion gallons, Thornton is an impressive and impactful addition to the project and is predicted to save $40 million per year in flood damages.
The complete TARP project has been called the “largest civil engineering project on Earth,” and according to the MWRD’s website, “TARP was named by the U. S. Environmental Protection Agency as one of the nation’s top Clean Water Act success stories and is serving as a model urban water management tool worldwide.”