MnDOT's I35W Stormwater Storage Facility - Flooding Resolution in Challenging Ground using a Series of Six Diaphragm Wall Cells-Paper

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MnDOT’s I-35W Stormwater Storage Facility was built to address recurring flooding near 42nd Street in Minneapolis, caused by watershed runoff and backflow in the stormwater tunnel system. The selected solution comprises six underground storage cells beside the highway, constructed beneath an existing embankment that also supported a soundwall.<br /><br />A staged soil nail wall replaced the embankment while maintaining support for the soundwall and nearby utilities. Vertical nails installed before excavation helped stabilize loose granular soils and limit movement. The storage cells were formed with circular slurry diaphragm walls, whose ring-compression design required little reinforcement. Each cell was excavated to about 93 feet in variable glacial soils, with groundwater controlled by external depressurization wells. Testing showed that 15 active wells per shaft—not the initially estimated eight—were needed to meet drawdown requirements and prevent piping.<br /><br />Construction encountered boulders that slowed panel excavation, threatened wall verticality, and caused bulges in the diaphragm walls. Real-time monitoring helped control panel alignment, while the bulges were removed before the final concrete lining was placed. Jet-grouted blocks supported excavation of connecting passages between cells.<br /><br />The final tanks were designed as environmental structures with a 100-year service-life requirement. Their base slabs resist uplift, and the diaphragm walls carry permanent earth pressure, allowing the thinner inner lining to resist primarily hydrostatic pressure. Roof slabs account for earth, traffic, snow, and other project-specific loads.<br /><br />The project demonstrates the importance of identifying buried utilities and accounting for all loads early in design, providing temporary ground support in granular soils, and using instrumentation and testing to refine groundwater-control plans. Treating the diaphragm walls and inner lining as a composite system also provided an economical structural solution.

Keywords

MnDOT

I-35W

stormwater storage

Minneapolis flooding

diaphragm walls

soil nail wall

groundwater control

depressurization wells

jet grouting

composite structure