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

PPT Presentation

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The Metropolitan Council Environmental Services (MCES) assessed sanitary interceptor tunnels serving Minneapolis and St. Paul to prioritize risks and guide future inspections and capital work. The systems include tunnels dating to the 1880s, many built in St. Peter Sandstone, where voids and caves can form.<br /><br />MCES organized available as-built and inspection records into a document library and GIS database, mapping tunnel features and risk priorities by segment. Risk evaluation considers likelihood of failure, based on condition, and consequences, based on service impacts.<br /><br />The Minneapolis system is largely in sandstone, has more inspection documentation, and includes some redundancy. Its assessment used a quantitative “reverse risk register” that combines likelihood, consequence, and a weighted segment impact factor. St. Paul’s system has less documentation, more unknown caves, shallow and unlined sections, some tunnels in soil, and no system redundancy. Quantitative scoring produced inflated risks across its segments, so MCES used case-by-case qualitative evaluations addressing data gaps, geotechnical and structural conditions, and system-level vulnerability.<br /><br />Recommended follow-up includes detailed inspections; geotechnical, hydrologic, and structural assessments; hydraulic and operations studies; emergency planning for catastrophic failures; and evaluation of interplant redundancy. MCES also recommends maintaining a data-management program and planning additional inspections, urgent repairs, and major capital projects through a multidisciplinary process.<br /><br />Inspection options include surface drilling to locate voids, personnel entry for close examination and material testing where tunnel conditions allow, and remote sonar, LiDAR, and CCTV. Remote methods can reach smaller spaces and support repeated review, but may not reveal fine defects or conditions behind the liner. Future planning should also address route alternatives, environmental concerns, and public outreach.

Keywords

sanitary interceptor tunnels

risk assessment

Metropolitan Council Environmental Services

Minneapolis

St. Paul

St. Peter Sandstone

GIS database

inspection methods

structural vulnerability

capital planning