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Drainage tunnel

At the base of the tunnel, crews wait on the train trolley to take them further inside to do work.
Department of Transportation

Drainage tunnel

What is a drainage tunnel? Video

What is a drainage tunnel?

Rebuilding freeways in large metropolitan areas presents unique challenges. Proper drainage, especially on freeways that sit below street level, is one of biggest tests for designers. Proper drainage of the freeway not only ensures safety and mobility, but it also provides protection for neighborhoods adjacent to the freeway. A drainage tunnel was recently built along I-75 in Oakland County, with one proposed for a section of I-94 in Detroit. This modern drainage system is being used to address climate resiliency challenges.

Flooded I-75 at I-696 in August 2014.

Why is a drainage tunnel an option?

Tunnels provide the ability to store stormwater after major rain events. This differs from traditional stormwater drainpipes and allows for more efficient flow of municipal systems that serve residences, businesses and local streets.

An inside look of the I-75 tunnel in Detroit.

What are the benefits of a drainage tunnel?

A drainage tunnel allows for all freeway and service drive stormwater to be separated out from community combined sanitary and stormwater systems. During heavy rain events, the stormwater flows from the freeway and service drive basins directly into drop shafts and then into the tunnel.

Large capacity tunnels store millions of gallons of water, allowing municipal systems to drain first into treatment facilities before stormwater from the tunnel is discharged. This prevents local systems from being overburdened.

Drainage tunnels can discharge at reduced rates not overwhelming treatment facilities protecting the local environment.

A front view of the massive tunnel boring machine (TBM).

How is a tunnel built?

Tunnels are built using a tunnel boring machine (TBM) that mines below the surface from 50 to 100 feet. A TBM is an automated machine with a rotating cutting wheel with teeth at the front that digs away at the soil while it turns. Miners in the tunnel simultaneously install precast concrete wall segments within a shield to protect them. Hydraulic jacks at the back of the TBM push against the ends of the wall segments after they were installed to force the cutting wheel into the soil and move the tunnel forward. Behind the shield is an assembly of support equipment, including electrical, mechanical and ventilation equipment. A small locomotive on rail tracks travels with boxes to haul out excavated soil and flatbed cars to bring in the concrete wall segments.

An aerial view of the tunnel shaft.

How does the tunnel work?

Stormwater is collected from the freeway and service drives in a network of pipes that direct flow into “drop shafts” (large pipes). These shafts connect to the tunnel below, where the stormwater then flows to a pump station. The pump station lifts the water and discharges it into a treatment facility.