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

During the past several decades, a segment of Interstate 35W (I-35W) at 42nd Street, located in Minneapolis, Minnesota, has experienced flooding during intense storm events. Flooding is caused by a combination of runoff from the upstream watershed and backflow through the downstream stormwater tunnel system. The Minnesota Department of Transportation (MnDOT) evaluated several stormwater storage facility (SSF) concepts to mitigate flooding. The sub-grade storage tanks are located adjacent to I-35 WB, between the highway shoulder and 2nd Avenue, which is elevated approximately 25’ above the highway. Prior to construction, there was an embankment covering the footprint of the new storage tanks, which supported earth loads and provided base reactions for a soundwall running parallel to 2nd Avenue. Therefore, an earth retention system would need to be installed to permit removal of the embankment. The project team elected to build a soil nail wall, which was constructed in lifts as the embankment removal progressed. Notable challenges related to design and construction of the soil nail wall were as follows: (1) elevations and angles of the soil nails to avoid conflict with existing utilities on the backside of the wall, and (2) sound wall base reactions including separate load cases for wind load acting in each direction to determine the governing scenario for the retaining wall. Design and construction consisted of six underground storage cells approximately 80 feet deep and 42 feet in diameter. The cells were constructed using slurry diaphragm walls and depressurization which allows for handling a high groundwater table (approximately 10-15 feet below highway grade) and highly variable glacial outwash soils in a congested construction area. This paper will discuss the design and construction considerations for the soil nail wall and underground storage tanks.