Assessment of MCES Sanitary Interceptor Tunnels in Minneapolis and St. Paul-Paper

Paper

Pdf Summary

Metropolitan Council Environmental Services (MCES) operates 640 miles of wastewater interceptors serving the Minneapolis–St. Paul region. About 34 miles run through deep tunnels excavated in soft St. Peter sandstone. Some date to the 1880s and contain stone, brick, or unreinforced concrete construction. Voids between the tunnel rock and interceptor liner were sometimes left after construction.<br /><br />A 2016 inspection beneath downtown Minneapolis found that sandstone had sloughed from tunnel walls onto the interceptor crown, while groundwater carried loose material away and enlarged voids. MCES stabilized the affected areas with grout in 2020. Because much of the system remained unevaluated, MCES developed a risk-based inspection program using historical records, GIS mapping, and likelihood-and-consequence analyses to prioritize work. Inspection results will guide rehabilitation, capital spending, and resilience planning.<br /><br />Key concerns include cracks, corroded or thin concrete, missing bricks, debris, and voids around the tunnel—especially above the crown. These defects can allow groundwater infiltration or sewage leakage, erode the sandstone, and lead to progressive rockfalls, tunnel deterioration, or sinkholes. Decayed timber supports may also contribute to void formation.<br /><br />Inspectors will use personnel entry where tunnel size, flow, and safety conditions permit. Hands-on inspections allow visual assessment, hammer sounding, and drilling through the liner to check its thickness and identify voids. Where entry is impractical, remote platforms equipped with video, LiDAR, and sonar will survey visible and submerged areas. Remote methods cannot verify liner thickness or voids directly, so targeted surface drilling is also planned.<br /><br />Early inspections identified corroded concrete, groundwater leaks, sediment accumulation, defects at tunnel connections, and sediment deposits as deep as 18 inches. The program is intended to identify deterioration before it threatens interceptor capacity, public safety, or the environment.

Keywords

Metropolitan Council Environmental Services

wastewater interceptors

Minneapolis St. Paul

St Peter sandstone

deep tunnels

risk based inspection

grout stabilization

voids and defects

LiDAR and sonar

tunnel deterioration