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  • Stability Analysis of Old River Low Sill Structure

    Abstract: An updated stability analysis was performed on the Old River Low Sill Structure due to a change in the operating conditions from historic river sedimentation. Sedimentation of the Mississippi River channel since the 1973 spring flood has caused higher river stages at lower discharges. Numerical methods used included nonlinear analysis of pile group stability, seepage analyses, and limit equilibrium methods. The structure’s foundation was compromised during the 1973 flood, and emergency repairs were conducted to prevent scouring and undermining of the foundation by the flood scour. Rehabilitation included the reconstruction of a failed wing wall on the left abutment, rock and riprap fill in the forebay channel, and emergency grouting to fill the scour hole beneath the structure. An operating restriction was emplaced to limit the differential head across the structure due to flood damage. Taking these conditions into account, results from an updated analysis showed that full headwater uplift caused increased tension in the piles, while the increased body load caused increased compressive loads in the piles. Review of piezometric monitoring and the seepage analyses showed that full headwater uplift is unlikely, indicating the foundation grouting adequately sealed the scour hole beneath the structure. Analysis results exhibited lower magnitude compression and tension loads in the piles with design load cases compared to previous analyses. Recommendations from these analyses indicate that increased monitoring and additional investigation may support increasing the differential head limitation.