How a section of the Thornton Quarry became a key reservoir

It’s no accident that the recently completed north lobe of the Thornton Quarry is now a key part of the flood plan for the Metropolitan Water Reclamation District (MWRD) of Greater Chicago.

The giant excavated area now known as the Thornton Composite Reservoir was identified by the MWRD in 1975 as part of its Tunnel and Reservoir Plan (TARP). This “deep tunnel” system provides a series of large-diameter tunnels and reservoirs designed to reduce flooding and improve water quality in area rivers and streams.

The MWRD has been managing flood and storm water issues since 1900 when it reversed the flow of the Chicago River to divert storm runoff away from Lake Michigan. Prior to that, storm water events were often followed by cholera epidemics and other water pollution troubles. Past floodwaters have overtaxed sewer systems, caused beach closings and resulted in significant property damage for property owners.

“By the late 1960s, so much land in Chicago’s suburbs was paved that there was nowhere for rainwater to go except to run off,” says Kevin Fitzpatrick, MWRD managing civil engineer. “Rainwater from residences and business districts all goes into the same combined sewer system when big storms hit. It’s too much water for that pipe to carry, which causes it to back up into homes and basements.”

In the late 1960s, studies were conducted to evaluate the existing storm water system and determine what could be done to improve and update the infrastructure. Part of the ideas coming from the studies included plans to capture and hold storm water runoff until it could be treated and placed back into the waterway in a controlled manner.

“More than 50 alternative projects were reviewed for development of the TARP,” Fitzpatrick says. “The MWRD identified the most economical and beneficial alternatives and adopted the overall TARP plan. It has been the largest public works project in the Chicagoland area.”

In all, the MWRD’s TARP plan involves more than 100 miles of deep tunnels that have been constructed about 300 ft. below ground. The tunnels generally follow local waterways, capturing combined sewage runoff at 400 locations. Tunnels transport the water to reservoirs like Thornton Reservoir, which is one of three major TARP reservoirs.

“Once the mix of rainwater and sewage reaches the Thornton Reservoir, it’s stored there until the storm is over and treatment plants have capacity to clean the water,” Fitzpatrick says. “Overall, the Thornton Reservoir can hold about 7.9 billion gallons of floodwater.”

Hanson Material Service

MWRD’s relationship with Hanson Material Service, the company mining the Thornton Quarry, began in 1998. The two entities reached an agreement that, over a period of 15 years (1998-2013), Hanson Material Service would complete limestone-mining activities in the northern part of the quarry and turn it over to MWRD for use in the TARP system.

The last blast in the quarry’s north half, intended to loosen the final 36,000 tons of limestone that has been mined from the quarry since the 1830s, was completed in September 2013.

“Once mining activities were completed at the quarry, we had a lot of work to do to prepare it for use in the TARP system,” Fitzpatrick says.

The first step in prepping the 320-ft.-deep quarry was to make it water tight so no seepage occurred. While most rock quarries are naturally water tight, rock-fracturing actions can lead to seepage. A cement grout curtain placed around the quarry’s perimeter ensured that none of the water would escape.

“Grout was injected into fractures in the rock to reduce permeability,” Fitzpatrick says. “The quarry perimeter is more than a mile long. Along that perimeter, about every 5 to 10 ft., holes were drilled down 500 ft. from the surface. The holes were injected with grout under pressure to seal any rock fractures. Just beneath the quarry floor is a layer of shale, which has very low permeability, so we didn’t need to seal the quarry floor.”

A great deal of engineering work was necessary to address each aspect of the quarry preparation. Among the plans was sealing two tunnels in the quarry, which allowed trucks to move rock from the fracture site to the quarry plant. The tunnels were plugged with reinforced concrete to avoid flooding the still-active section of the quarry.

A 1,300-ft.-long, 30-ft.-diameter tunnel was constructed to connect the quarry to the existing TARP system. Stainless steel 18-ft.-wide wheel gates weighing 54 tons were lowered more than 300 ft. down into the quarry where they will be raised and lowered to regulate the flow of storm and wastewater into the quarry and hold it there.

“The gates are 29 ft. high and 4 ft. wide,” Fitzpatrick says. “They not only regulate water flow but can also be used to isolate the quarry for any necessary maintenance.”

The final phase of prepping the quarry for use was completing landscaping work around the quarry perimeter. The work included installation of evergreen berms to help block wind and reduce odor issues. Solar-powered aerators float across the quarry when there’s water in it, reducing development of odors.

“About 500,000 people will benefit from the use of the quarry as a dirty water reservoir,” Fitzpatrick says. “The project took a lot of coordination with Hanson Material Service because in the final mining stages some of the prep work was going on simultaneously. One other unique aspect of this project is the fact that MWRD funded the project because the timeline for obtaining federal funds wasn’t workable for us.”

To Fitzpatrick’s knowledge, the Thornton Quarry is the first mined quarry in the United States to hold sewage and rainwater. It is also believed to be the largest dirty water reservoir in the world.

“In the past there have been quarries used to hold drinking and even storm water, but I don’t believe one has been used to hold combined sewage and storm water,” he says. “Thornton Quarry is one of the largest quarries in the Chicagoland area. The fact that the mining process created an excavation the size that we needed in a relatively short time made this a very good fit for MWRD.”

The mining design

Scott Jorns was a young engineer with Hanson Material Service when MWRD began negotiations regarding use of the north lobe of Thornton Quarry as part of its floodwater reclamation plan.

The project involved mining the quarry to a specific depth and completing some prep work to make it usable as a reservoir. Since 1998, Hanson Material Service removed 76 million tons of dolomite, taking the last of the rock out in 2013.

“Before we started working with MWRD, the north lobe of Thornton Quarry was basically inactive,” Jorns says. “We were mining in the west lobe at that time. In order to remove enough rock to create a 7.9 billion-gallon reservoir on MWRD’s timeline, we had to mine a million tons of rock each year in the north lobe.”

The quarry had to be mined to a depth of 340 ft. in order to comply with MWRD’s plan. Because MWRD needed a larger footprint than the quarry’s north lobe provided, MWRD purchased additional property surrounding the lobe.

“That area had to be mined, too,” Jorns says. “But approximately 1 million tons of that rock didn’t meet quality requirements and had to be mined and stockpiled because we couldn’t use it in our rock products.”

While a significant portion of the Thornton Quarry’s north lobe was mined out, Hanson Material Service’s original plan was to mine the north lobe to a depth of 400 ft. Since the reservoir depth is 340 ft., about 60 ft. of rock below the reservoir is still suitable for dolomite production.

“Part of the initial negotiation process involved defining a fair price for the reserves that Hanson Material Service had to abandon in the north lobe,” Jorns says.

Because the haul from the north lobe was the longest haul at the quarry, they added a seventh truck to their fleet of haul trucks. The deeper they mined, the longer the truck cycle time became.

In order to keep up with the north lobe’s aggressive mining schedule, Hanson Material Service mined the area with three separate benches: 75, 115 and 150 ft. Blasting on the south wall came within 90 ft. of the always-busy toll road. Three tunnels totaling 1,300 ft. in length were constructed to access the benches.
Hanson Material Service also had to create a ramp that allows MWRD vehicles to drive down into the reservoir for maintenance activities when it’s empty.

“The east and north wall of the reservoir had to be smooth in order to prevent falling rock, which could be a hazard for MWRD workers,” Jorns says. “To accomplish that, pre-split blasting and a grinder attached to an excavator were used to remove all loose rock.”

Once Hanson Material Service finished its prep work on the north wall, a special team was called in to drape wire fabric over the entire north wall to further protect employees from falling rock. Helicopters and workers rappelling down the high wall were used in that process.

The original north wall had more than 40 ft. of burden at its base due to previous bench mining, making it a challenge to create a vertical wall. Through the use of laser profiling, angle drilling and bore tracking, a vertical wall was created using a multiple-step drilling and blasting process.

“First, 10-degree holes were drilled and blasted,” Jorns says. “Then 7- to 5-degree holes and finally 3-degree holes. Each hole was custom loaded based on the laser profile and bore track data.”

The west lobe of Thornton Transitional Reservoir has been used by MWRD for many years as a runoff reservoir from nearby Thorn Creek during construction of the north quarry reservoir. The quarry capacity is 4 billion gallons. It could potentially be added to the MWRD’s floodwater plan in the future.

“It’s been over 20 years since Hanson Material Service first negotiated with MWRD to prepare the north lobe for water storage,” Jorns adds. “It’s been a once-in-a-lifetime project during my career.”

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Loretta Sorensen is a freelance writer in Yankton, South Dakota. She produces material on a variety of topics, serves as a ghostwriter, and has authored her own books.