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ABSTRACT: The Minnesota Department of Transportation (MnDOT) evaluated several concepts to minimize flooding at a low point of Trunk Highway 35W (TH35W) south of downtown Minneapolis. A deep stormwater storage facility (SSF) with lift station was selected and carried through final design. Given the restricted project footprint, and being directly adjacent to an active interstate, open excavation construction methods were not feasible so circular cells composed of diaphragm walls (d-walls) were utilized as the structure walls and also to facilitate mass excavation of soil within the cells. The footprint of the SSF is located near the edge of a glacial erosional channel which contains glacial alluvial deposits including cobbles and boulders. Understanding the ground conditions was critical in assessing constructability and determining the cost to complete the project. Multiple phases of geotechnical investigation were conducted including geophysics (Multichannel Analysis of Surface Waves), borings (mud-rotary and rotosonic), and pump testing. This information supported the development of a geotechnical data report (GDR) and a geotechnical interpretive report (GIR) which were part of the project contract documents. Obstructions (including cobbles and boulders) were encountered at some of the borings conducted and therefore were anticipated to be encountered during construction. To minimize contingency carried in the Contractor’ s bid, it was decided by the Owner that the Contractor should assume no obstructions during d-wall construction, and obstructions encountered during construction would be paid using a project risk pool. The funds in the risk pool would be informed by the GDR and GIR. Obstructions were encountered during d-wall construction which impacted production and schedule. Methods of identifying, tracking, and paying for obstructions were agreed upon by the team prior to construction. The cells of the structure were successfully completed in 2022 with final completion of the SSF anticipated for the summer of 2023.
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