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Spanning roughly 3.9 miles from Clayton Road to Interstate 44, the upcoming tunnel is designed to capture and temporarily contain peak wastewater overflows from the Deer Creek sewer system. Historically, intense rainfall would overwhelm the existing system, resulting in basement backups and sewer overflows. Expected to be finalized by late 2022, this new tunnel aims to alleviate challenges faced by various communities, including Clayton, Richmond Heights, Brentwood, Maplewood, Webster Groves, and Shrewsbury.

In the initial phase of the project, MSD explored various construction alternatives, such as redirecting excess flow to above-ground storage tank farms or establishing a network of open-cut sanitary sewers. Considering the densely developed landscape and the involvement of major roadways, MSD concluded that a deep storage tunnel presented the least disruptive and most cost-effective solution.

Utilizing an immense Tunnel Boring Machine (TBM) named "Mrs. Nancy," contractors excavated the tunnel through deep rock layers, reaching depths between 150 and 250 feet below the ground surface. The interior will be lined with concrete, leaving a diameter of approximately 19 feet. The construction shaft, positioned at the downstream end of the tunnel near I-44 and Shrewsbury Avenue, will eventually serve as part of the pump station responsible for draining the tunnel.