Thornton composite Reservoir thornton, illinois

Recently named the 2016 American Public Works Association (APWA) National Project of the Year, this engineering marvel was nearly 40 years in the making.

Tunnel and Reservoir Plan

Adopted in 1972 by the Metropolitan Water Reclamation District of Greater Chicago, the Tunnel and Reservoir Plan (TARP) was established to protect Lake Michigan's drinking water, improve the quality of area waterways and to provide an outlet for area floodwaters to reduced street and basement sewage backup.

Delivering the Final Component

Walsh was hired by the Metropolitan Water Reclamation District to construct the final portion of the concrete lined Deep Tunnel, that connects the existing Calumet Indiana Avenue Leg TARP Tunnel to the Thornton Composite Reservoir.

One of the Largest Civil Engineering Projects

The Deep Tunnel is part of a 100+ mile system of 30-foot diameter tunnels that are 300 feet deep. The tunnel gathers storm and waste water from the South Side of Chicago and delivers it to the 83 acre Thornton Composite Reservoir which has a capacity of holding 7.9 billion gallons of storm and waste water.

BUILDING THE deep tunnel

After mining and sealing the reservoir, a dam, made of 32,000 cubic yards of roller compacted concrete, was constructed to separate the contained water from reaching the main lobe of the quarry. To keep water from escaping, the bottom of the reservoir has an impermeable natural layer of shale with a double-row grout curtain around the perimeter.

100-Ton Stainless Steel Wheel Gates

At the bottom of the reservoir, the Deep Tunnel leads back to four 100-ton stainless steel wheel gates that raise and lower to keep the reservoir from overflowing.

Underground Exploration

Walsh crews installed these 18-ft x 29-ft x 4-ft wheel gates 300 feet below the ground to regulate the flow of storm and waste water. The video below captures the underground environment of the site.

Withstanding the Force of Water

A concrete apron constructed in front of the tunnel is designed to withstand the force of the water coming out of the tunnel, which can be at velocities of up to 30 feet per second, and prevents erosion of the stone reservoir floor.

Solar-Powered Aerators

These aerators were installed in the reservoir and will float up and down with the water elevation, keeping the surface layer of water from going septic and causing odors.

"Chicago's grand canyon"

With nearly 40 years of construction, the Thornton Composite Reservoir put an end to the once-frequent practice of dumping the region's waste into Lake Michigan, the source of drinking water for more than seven million people in the city and suburbs.

It will benefit more than 556,000 people in 14 communities; approximately 182,000 homes, businesses and other facilities; and improve water quality by collecting combined sewer overflows before entering local waterways.

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