The Des Plaines Inflow Tunnel is part of the Metropolitan Water Reclamation District of Greater Chicago’s Tunnel and Reservoir Plan, which consists of various projects aimed at improving drainage conditions in the greater Chicago area and reducing the amount of flood damage and pollution into Lake Michigan.

Chicago and many of the older suburbs are served by combined sewers that carry both sanitary water flow and stormwater through the same pipes. During rain events, the sewer system and treatment plants cannot accommodate the additional flow, causing combined sewage overflow to the local waterways.

The 20-foot diameter Des Plaines tunnel connects directly to the McCook Reservoir and was built to allow stormwater to drain from the city, separating it from the old CSO (Combined Sewer Overflow) system and directing it to the old rock quarry.

For the construction of the tunnel, the contractor requested single phase pouring, which means that the entire full round section of the tunnel was poured monolithically. The tunnel diameter transitioned from 20 feet to 33 feet with the crown of the tunnel staying at the same elevation. This geometry combined with the requested pour pressure resulted in six-million pounds of axial force downstream towards the rock plug. PERI designed a custom formwork system with special walers and CNC gusset forms to meet this challenge.

The tunnel formwork panels were delivered preassembled by PERI before they were lowered through a shaft by the contractor and moved to their final location with a forklift. No form ties were used. Custom spud pins held the formwork off the bottom of the excavation and kept it from floating in any direction.

McCook Reservoir Stage I provides an additional 3.5 billion gallons of storage capacity to capture flood water and CSOs and is estimated to provide $114 million annually in flood damage and CSO pollution reduction benefits.

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