Constructing a Stormwater Storage Facility in a Glacial Valley Using CMGC Project Delivery-Paper

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MnDOT is constructing a deep stormwater storage facility to reduce flooding on TH35W near downtown Minneapolis, where an undersized storm sewer has caused traffic delays and safety concerns. The facility comprises six circular cells, each 42 feet in diameter, with a total storage capacity of about 14 acre-feet.<br /><br />The constrained site, high groundwater, and glacial deposits containing cobbles and boulders made conventional open excavation impractical. The project used top-down construction with diaphragm walls extending roughly 113 feet below ground. Geotechnical investigations—including borings, surface-wave geophysics, and pump tests—helped characterize a buried bedrock valley and assess groundwater conditions.<br /><br />MnDOT selected Construction Manager/General Contractor (CMGC) delivery to involve the contractor during design, improve constructability, and manage subsurface risks collaboratively while retaining owner control of the design. A shared risk register and project risk pool established how unexpected conditions and mitigation costs would be handled. For example, the contractor’s base price assumed no obstructions; additional work to remove encountered cobbles and boulders was paid through the risk pool. Extra concrete needed because of wall deviations was also addressed through agreed unit pricing.<br /><br />The team selected external deep wells and a redundant pumping system to depressurize the ground during excavation. Additional pumps proved necessary, and the predefined risk-pool process enabled their timely installation. Other adaptations included an early work package for the working platform and soil nail wall, added vertical soil nails to protect an existing noise wall, and independent cell walls designed to work in compression.<br /><br />The cell structures were completed in 2022, with overall project completion anticipated in fall 2023. The project demonstrates how early contractor involvement, thorough geotechnical investigation, and explicit risk-sharing can help address uncertain ground conditions and maintain progress on complex infrastructure work.

Keywords

stormwater storage

flood mitigation

Minneapolis

TH35W

diaphragm walls

geotechnical investigation

groundwater management

CMGC delivery

risk sharing

deep excavation