Completion 15: Deep Tunnel Torrence Avenue Leg
Cost: $142 million

The 8.1-mi.-long Torrence Avenue Leg is the next-to-last underground channel in the Tunnel and Reservoir Plan - also called the Deep Tunnel.

The tunnel will prevent sewage and rainwater from spilling into Lake Michigan and the Calumet River, said Richard D'Ambrosia, assistant chief engineer for the Metropolitan Water Reclamation District of Greater Chicago, the owner.

The project is located between 91st and 138th streets, on Chicago's South Side and part of suburban Calumet City.

Has Tunnels, Shafts

Excess rainwater from existing and future sewers will enter soft-ground connecting structures with sluice gates that control inflow.

The overflow is directed into a drop shaft, and eight concrete-lined shafts were mined, D'Ambrosia said. Three other shafts used for access or servicing were also opened, and the shafts range between 31 ft. and 7 ft., 2 in. in diameter.

The drop shafts convey the water approximately 275 ft. directly downward into a reinforced chamber, or boot, that leads to the tunnel. Tunnel at 25 ft. diameter makes up 6.5 mi. of the system, and tunnel at 15 ft. diameter stretches for 1.6 mi. Concrete lining was done.

Flows are conveyed to a reservoir in south suburban Thornton where the effluent is carefully metered. The water is conveyed to a filtration plant where biological agents treat it.

The tunnel will capture and deliver excess combined sewage from the nearly 22-sq.-mi. service area. An estimated 2 billion gallons of combined raw sewage and 2.1 billion lbs. of organic material will no longer be discharged into waterways.

Two tunnel boring machines were used to mine the tunnel, which is surrounded by rock. Soft-ground boring machines mined the structures in the overburden above the rock.

A unique project element is that a 100-ton wheel gate was installed, and one of shafts was located to provide services to it.

Because of different ground elevations, "you have to be able to shut [the tunnel] off" with the gate, D'Ambrosia said. "It has wheels to guild [the gate] down, rather than slide down."

The Deep Tunnel system will encompass 109.4 mi. of passageways beneath Chicago and 51 Cook County suburbs. The last tunnel, the Little Calumet Leg, is under construction nearby.

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