MnDOT I-35W Stormwater Storage Facility

Project Narrative

Wet weather storm events on I-35W in Minneapolis has at times created surcharge conditions within the existing stormwater tunnels that result in manhole lids being blown off by water force. Minnesota DOT retained the team of Brierley Associates, Barr Engineering, and TKDA to develop and design a solution. Project construction was through MNDOT’s CMGC project delivery process by Kraemer/Nicholson Construction Company JV.

The I-35W stormwater tunnel system was constructed in the 1960s to carry stormwater runoff from the highway and the surrounding neighborhoods. Development in the 3,100-acre surface drainage area has continued, which has increased the volume of stormwater directed into the tunnels. Flooding of the highway occurs when the system is filled over its design capacity.

Brierley Associates was part of a team providing design services for a stormwater storage facility to mitigate flooding. Given the tight space constraints, an underground storage structure with the capacity of 14.7-acre-ft (4.79 million gallons) consisting of a series of 6 interlocking diaphragm walls approximately 46-ft in diameter and 80-ft deep in the ROW of I-35W was constructed.

The location of this stormwater storage facility presented unique geotechnical/construction challenges such as:

• a high groundwater table environment
• variable glacial geology
• unbalanced loading on the final structure
• the relationship of the new structure on the maintenance of the active service stormwater system
• a tight construction area within a dense residential and business corridor

The team coordinated with a major MnDOT project in the area which increased the number of lanes of I-35W near downtown Minneapolis. Solving these challenges required innovative staging and construction sequencing as well as geotechnical modeling. The photo on the top of this page is courtesy of Nicholson Construction. The video below is a simulation of the project developed by Barr Engineering.