This first stage of the McCook Reservoir, an old limestone quarry used to hold stormwater, will flood this summer. When complete in 2029, the site be able to hold 10 billion gallons of water. | Neil Steinberg/Sun-Times
No “small pipes” here. Rather, an enormous, 33-foot-diameter outlet of the Deep Tunnel emptying into the reservoir, against blocks of concrete 9 feet tall to disperse the force of the water and keep it from chewing up the bottom of the reservoir, which the district plans to flood by the end of the summer.
“Stage one is going online this year,” said Carmen Scalise, a MWRD engineer, noting the first section will hold 3.5 billion gallons, roughly half the size of the Thornton Reservoir, which opened in 2015. When stage two, holding 6.5 billion gallons, opens in 2029, McCook will be even bigger than Thornton.
Why will completion take a dozen years?
“The reason it’s taking so long is we have to physically mine this rock out, 285 feet deep, and we can only get rid of it as fast as people are buying it,” said Scalise. “With any giant infrastructure projects, it’s not a quick process.”
After the limestone is dug out, a barrier, or “slurry wall,” had to be formed.
“The slurry wall is basically a 3-foot-wide strip of bentonite clay,” said Scalise. “We dug out this trench around the entire perimeter and filled it with clay and it forms an impervious layer. Groundwater from the outside can’t get in. We did the same thing in the rock: two rows of grout curtains: 4-inch-diameter holes drilled . . . down below the bottom of the reservoir and pumped full of grout to keep the ground water out and, when the reservoir is full, keep our CSO in.”
No, not the Chicago Symphony Orchestra. In this world, “CSO” is “combined sewer outflow.”