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Nine Mile Creek Watershed Causeway
Minnesota Department of Transportation
As part of a Minnesota Department of Transportation (MnDOT) design‐build project, Ames Construction and its project partner companies constructed a causeway in 2017 through a lowland area in the Nine Mile Creek watershed. Located along US Highway 169, in the southwest metropolitan area of Minneapolis, the causeway replaced an aging bridge. The major components of the causeway included a columnsupported embankment (CSE), two 3,000‐ft‐long parallel retaining walls, 22 box culverts, and new roadway pavement. Wetland impacts were also a significant consideration for the project. A column‐supported embankment was used to support the causeway utilizing nearly 5,000 Controlled Modulus Columns (CMC) and a load transfer platform (LTP), which permitted greater column spacing.
MnDOT asked Itasca to analyze the column supported embankment in order to better understand the behavior and performance of this complex system.
Itasca developed a FLAC3D model incorporating all of the major causeway components and structural support elements. The advanced PH soil model was utilized, calibrated directly from the CPT tests and using shear wave velocity to evaluate soil deformability. The CMC columns were modeled using embedded pile elements (bending and shear) on top a cable element (axial force). The plastic moment in the piles was updated during modeling, based on the cable axial forces and a specified trust-moment diagram. A circular shell was also added to the head and base of each pile in order to have a better representation of the local soil-structure interaction. The geogrids inside the LTP are modeled with cable elements in order to specify a limit force and not develop compression. These cables are rigidly connected to a geogrid element only for developing the interaction with the soil. Nearly one million structural elements were used for this model.
Numerical modeling revealed an unexpected outcome with a large deformation in the East wall due to lateral forces acting on the relatively narrow East LTP. Additional lateral restraint elements were added to much of the of the causeway within the embankment and, in some critical areas, these elements were included at two elevations. Additional FLAC3D modeling by MnDOT indicated that these additional elements were critical to ensuring the embankment system performing as a unified system. FLAC3D numerical modeling was critical to discovering this unusual potential failure mode and assessing the efficacy of the proposed mitigation strategy.