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2006 ACEC Grand Award

Hiawatha Avenue (TH55) / Crosstown (TH62) Interchange
Owner: Minnesota Department of Transportation

This highly visible project converted a congested at-grade signalized intersection on Crosstown to a folded diamond interchange at the south end of a 20-year improvement to Hiawatha Avenue from Downtown Minneapolis to the Mpls/St. Paul International Airport. The interchange underwent nearly constant design revisions to accommodate the Hiawatha LRT, the Minneapolis Airport Commission’s (MAC) runway extension, water quality issues for Minnehaha Creek, the presence of a nearby historic spring and, most challenging, high groundwater.

Required modifications required depressing TH 62 approximately 15 feet below grade as it passed under the TH 55 extension. Temporary and permanent excavation or construction had the potential to negatively impact the groundwater flow and quality of historic Camp Coldwater Spring located just 1,300 feet away. Of the three groundwater solutions studied, SEH and Mn/DOT engineers chose to elevate the eastbound lanes of TH 62 and construct a multi-layer “reverse bathtub” geotechnical liner that covered approximately six acres and required 28,000 square yards of welded lining. The design included a sufficient factor of safety to keep it from floating during periods of extremely high groundwater table and to minimize groundwater flow diversion from the spring. According to experts in the geosynthetics field, nothing like the reverse bathtub had been attempted before, making the solution’s ability to meet the following five design objectives all the more critical:

• ensure groundwater flow to the spring was neither obstructed nor diminished
• provide an escape valve for groundwater elevations higher than the design elevation
• provide a water-tight barrier between the groundwater and the road subgrade
• provide a collection and monitoring system for groundwater leaks into road subgrade
• provide a collection and monitoring system for groundwater losses during periods when groundwater levels rise above the design elevation