Probabilistic-Based Seismic Slope Deformation Analysis of an Embankment Dam

South Carolina, USA

Slate Geotechnical Consultants performed a probabilistic-based, nonlinear seismic deformation analysis of a 50-foot high, 4000-foot long zoned embankment dam, in support of an ongoing seismic probabilistic risk assessment (SPRA) of an associated nuclear power plant in South Carolina. The general embankment cross section consists of a vertically aligned compacted clay core buttressed by compacted earthfill shells, which are founded on alluvial plain deposits of varying thickness.

Previous studies have assessed the potential for earthquake-induced liquefaction triggering in the alluvial plain deposits and have performed simplified seismic stability and deformation analyses to quantify the probability of dam instability and failure. The general consensus from these studies is that the alluvium is considered to be susceptible to liquefaction and may lose significant shear strength during the strong ground shaking postulated for the site. However, given the nature of those evaluations (limit-equilibrium slope stability and Newmark-type displacements), an assessment of deformation potential was limited.

The current study included a more rigorous evaluation of the magnitude of expected dam displacement/deformation related to extreme ground shaking events. Finite difference analytical software (FLAC) and contemporary constitutive models were used to estimate the strain potential of the alluvium and performance of the dam subject to several ground motion time histories at varying intensity (PGA) levels. Input material properties were established to represent the variability of the various embankment and foundation zones and then assigned in a Latin Hypercube Simulation (LHS) of ten, equal-probability realizations for analysis. The calculated crest deformations were then correlated to the potential for cracking within the embankment and, ultimately, the potential for dam failure/collapse. The results of the analysis will be used to further refine the fragility assessment of the potential for dam failure during strong ground shaking and determine future studies of the dam’s performance.