Mammoth quarry to be giant
reservoir in epic engineering milestone

by ROB McMANAMY | Aug 11, 2015

Gap seal: Using 30,000 cu yds of RCC, crews sealed the quarry pass in 2012 with a 'gap' dam, 115-ft tall by 240-ft wide. — **Gap seal:** Using 30,000 cu yds of RCC, crews sealed the quarry pass in 2012 with a 'gap' dam, 115-ft tall by 240-ft wide.

Apologies to Donald Trump, but the subject of this story actually is, well... HUGE.

This month, one of the world's largest and longest-running civil engineering projects will achieve a truly epic milestone, more than 40 years and $3 billion after it all began. And it will do so with surprisingly little fanfare, even though it abuts one of the busiest interstates in the nation.

When Chicago's Metropolitan Water Reclamation District (MWRD) floods the north lobe of the Thornton Composite Reservoir in the coming weeks, the once-thriving limestone quarry will finally become what it was always intended to be: a massive reserve for the relief of area flooding and the benefit of nearby Lake Michigan and connecting waterways. The giant stone tub -- a half-mile long, a quarter-mile wide, and more than 30 stories deep -- will hold up to 7.9 billion gallons of combined sewer and stormwater overflow (CSO) from 14 communities throughout the city's south side and southern suburbs. One of the largest such reservoirs in the world, its completion represents a significant achievement in the decades-long realization of MWRD's Tunnel and Reservoir Plan (TARP), aka the "Deep Tunnel."

The 109-mile-long tunnel system broke ground in 1975 and has been a work in progress ever since, drawing on funds and talents from the city, Cook County, the State of Illinois, the U.S. Environmental Protection Agency, the U.S. Army Corps of Engineers, and a who's who of local, national and international engineering firms and contractors. Even after this summer's conversion of the Thornton basin, TARP projects will still continue across the system until at least 2029.

The latest batch of work has involved $400 million in contracts let from 2009 for construction of a roller-compacted concrete "gap" dam -- directly beneath Interstates 80/294 -- as well as a double-row grout curtain wall along the south side of the reservoir, and two reinforced concrete quarry "plugs" in the haul tunnels. Additional work has involved creation of a ground water protection system extending the entire perimeter of the reservoir; and installation of water quality instrumentation. Meanwhile, the walls of the quarry also have had to be sealed to be made watertight for its transformation into a reservoir. That effort has involved drilling hundreds of deep holes into the porous limestone walls, and then injecting them with grout.

The two Chicago contractors leading the ongoing efforts are F.H. Paschen and Walsh Construction, supported by concrete supplier Ozinga Bros. On the engineering side, participating firms include Black & Veatch and MWH. Meanwhile, except for the north lobe, the mining operation continues under the direction of Hanson Material Service Corp., and longtime subcontractor Gallagher Asphalt.

Bathtub plugs: Crews from Walsh Construction this month placed the last of four vertical wheel gates --54 tons each-- into the deep tunnel to control storm & wastewater flow in and out of the reservoi… — **Bathtub plugs:** Crews from Walsh Construction this month placed the last of four vertical wheel gates --54 tons each-- into the deep tunnel to control storm & wastewater flow in and out of the reservoir. To see the above video, click **here**.
**Below, some perspective:** Amazing (silent) drone footage, shot flying over, into, and through the quarry, just west of the Indiana border, straddling Interstates 80/294 in Thornton IL. To see what the drone saw, click **here**.

Still, some have argued that the new reservoir will not be enough to handle the higher volume of overflows caused by greater than expected levels of annual rainfall in recent years. With that in mind, MWRD announced this spring that it had been granted an extension to its easement with Hanson, amounting to a $750,000 annual investment. It allows MWRD to continue to use the adjacent 4.5-billion-gallon Thornton Transitional Reservoir for storage of floodwater through 2020.

So MWRD now has five years to figure out where to find all that excess capacity again. A daunting task, perhaps, but given the historic feat the agency is about to pull off, no one is betting against it.