Seismic Rehabilitation of Sardis Dam

Salah-Mars, Said; Mejia, Lelio H.; Fleming, Robert; Forrest, Wayne; Stacy, Samuel; ,

Seismic Rehabilitation of Sardis Dam

by Said Salah-Mars, Woodward-Clyde Consultants, Oakland, United States,
Lelio H. Mejia, Woodward-Clyde Consultants, Oakland, United States,
Robert Fleming, Woodward-Clyde Consultants, Oakland, United States,
Wayne Forrest, Woodward-Clyde Consultants, Oakland, United States,
Samuel Stacy, Woodward-Clyde Consultants, Oakland, United States,

Document Type: Proceeding Paper

Part of: Geotechnical Practice in Dam Rehabilitation

Abstract:

Extensive subsurface investigation of the embankment dam revealed the existence of a weak layer of silt and soft clay in the original topstratum clay underlaying the upstream shell of Sardis Dam (Mississippi). Based on the evaluation of the ground motions at the site due to an earthquake on the New Madrid source region, it was estimated that the weak layer would suffer substantial strength loss. The potential strength loss of the weak layer could trigger a slope failure of the upstream shell of the dam. Different alternatives to improve the shear strength of the weak layer were investigated. They included deep soil-cement mixing, stone columns and vibro-compaction, drilled piers, and driven piles. A dam rehabilitation scheme based on reinforcing the upstream shell of the dam and its foundation using driven piles is presented herein. Compared to the other alternatives, driven prestressed concrete piles offered the most reliable and practical remediation; better quality control; and had potentially less uncertainty of the composite strength obtained. Prestressed concrete piles have a higher strength and more shear resistance compared to other alternatives. Based on fluctuations in reservoir level, the reinforcement technique would need to be installed from floating plant. Driven piles offer the most feasible alternative based on the difficulties of installing reinforcements such as piers and stone columns underwater. In addition, driven piles had the added benefit of some densification of the upper sand shell as a result of pile driving operations. The remediation procedure consists of driving piles through the sand shell and weak layer and into the dense substratum sand. The piles will provide resistance to the potential sliding mass (triggered by liquefaction of the weak layer) by transmitting the loads to the dense substratum sand. Two- and three-dimensional nonlinear finite element analyses were performed to analyze the soil-pile behavior and interaction effects.

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