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Crews are making finishing touches to the Thornton 
Composite Reservoir, which is expected to be ready to 
collect water this summer.

(Photo by Tom Houlihan/HF Chronicle)

Tony Gaudry is a miner who works 300 feet underground – and a mile from Homewood. Since 2011, he has shored up a 30-foot-high tunnel with steel, rebar and concrete, preparing four slots for steel doors that will slide open one morning in August, letting billions of gallons of stormwater and sewage flood the north lobe of the Thornton Quarry.

What was under water 350 million years ago will be under water again this summer when the Thornton Composite Reservoir opens. A 17-year project by the Metropolitan Water Reclamation District of Greater Chicago, it will be big enough to prevent flooding in an area ranging from the south side of Chicago to Homewood and Flossmoor.

Thornton Quarry, one of the 
oldest limestone mining sites 
in the country, provides a 
glimpse into the area’s 
geological history.
(Photo by 
Tom Houlihan/HF Chronicle)

As a miner — that’s his title in the construction industry — Gaudry works with concrete, steel and carpentry and prides himself on “being able to solve problems and build things” in a below-ground world of ever-expanding tunnels.

Gaudry’s below-ground world, soon to be filled with water, presents a dramatic sight to the casual visitor. However, viewing the Thornton Quarry is equally dramatic.

All around the quarry’s north end, workers are putting the finishing touches on the new reservoir, which will act as a storage basin for up to 7.9 billion gallons of combined stormwater and sewage.

From above, the heavy machinery looks like a collection of toys so tiny, you could sweep them all into one hand. But 350 feet below, on the quarry floor, the cranes, trucks, bulldozers and other earthmoving equipment appear gigantic; they are machines being used to build one of the world’s biggest reservoirs. Looking up the limestone walls of the eons-old quarry, you have a good idea of your own positively miniature size.

John Lemon, principal civil engineer for the MWRD, pointed to the quarry’s north lobe and offered statistics. The reservoir area is one-half mile long by one-quarter mile wide by 350 feet deep.

“There is no place like it in the world,” he said.

John Lemon, principal civil
engineer for the MWRD, helps 
area residents understand 
the history and characteristics 
of Thornton Quarry.
(Photo by 
Tom Houlihan/HF Chronicle)

In part, that’s because the reservoir will be so enormous. But the rest of the story is hidden beneath the ground. The quarry reservoir will be linked to a system of 30-foot tunnels — also carved into the rock and more than 300 feet beneath the surface — that will be connected to an MWRD treatment plant at 130th Street in Chicago.

The Thornton Quarry is literally in the H-F area’s backyard. It is located just east of big box stores along Halsted Street in Homewood. A second water storage facility on quarry property — the Thornton Transitional Reservoir, completed in 2003 — is only a few blocks away from the Homewood border. Homewood and Flossmoor are both served by MWRD.

Lemon said the Thornton Quarry is one of the oldest limestone mining operations in the country, with quarrying starting in the 1830s. But the quarry site is, of course, much older than that. Many millions of years ago, it was probably a reef in an ancient tropical ocean. Fossils found in the quarry show the long-ago existence of marine animals with shells, sponges and some trilobites. Lemon said geologists have been studying the quarry area for generations.

Area residents may feel closest to the quarry when they drive over the Interstate 80/294 bridge that separates the north lobe from the main pit area.

At the bottom of the quarry, you can see cars on the interstate. But you also notice that, just below the bridge construction crews built a concrete dam separating the two sections of the quarry and designed to prevent reservoir water from flooding into the main pit. The dam is 120 feet high and was built on rock 200 feet above the quarry floor. It is, Lemon says, one of the biggest dams in Illinois.

The new Thornton reservoir is part of MWRD’s Tunnel and Reservoir Plan, or TARP, which was first initiated in 1975. TARP, well known as the “Deep Tunnel” system, provides a series of large diameter tunnels and large reservoirs designed to reduce flooding and improve water quality in rivers and streams.

Lemon said the new reservoir is expected to be operational this August but that MWRD engineers won’t know how well it works until the first major rainstorm hits the area. He speculated that a rainfall of three or four inches in a short period of time may nearly fill the quarry reservoir to the top.

On the quarry floor, workers are building a huge concrete splash pad at the tunnel entrance. Lemon pointed to a 33-foot- high opening on the quarry’s east wall. That’s where water will enter the reservoir from the tunnel system. Underground, and less than a quarter-mile away, two huge gates will regulate the flow of water from the tunnels to the reservoir. The iron gates are nearly 25 feet high and two feet thick.

During heavy rains, stormwater and sewage will be pumped to the new reservoir. After the storm, water will be sent back through the tunnel to the 130th Street treatment plant, where it will be processed and returned to waterways flowing toward the Mississippi River.

Lemon said local residents were concerned that the water flowing into the reservoir would produce unpleasant odors, so MWRD has installed seven solar-powered aerators to freshen water in the quarry.

Lemon said MWRD started working on engineering plans for the Thornton tunnel system in the 1980s. Work on the Thornton Composite Reservoir began in earnest in 1998, when MWRD reached an agreement with Hanson Material Service, the quarry operator, to use the north lobe for a reservoir.

Construction began after Hanson Material Service completed mining in the north lobe; he said about 58 million tons of stone were removed after the company signed its agreement with MWRD. Lemon said Hanson Material Service still reserves the right to mine for limestone underneath the reservoir.

Contact Tom Houlihan at [email protected]

Trucks can be seen traveling on Interstate 80/294 above 
the dam that will keep reservoir waters from spilling 
into the rest of the quarry.

(Photo by Tom Houlihan/HF Chronicle)


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