Seismic Re-Evaluation of an Embankment Dam Potentially Susceptible to Soil Liquefaction: Evolution of the State of Practice

Analyses of the expected seismic performance of earthen embankment dams subject to potential liquefaction hazards are routinely performed for high hazard dams. However, the approaches employed for performing seismic deformation analyses of embankment dams have evolved significantly since the early 1970s.

This paper discusses seismic evaluation of an anonymous earthen embankment dam constructed in the 1950s and therefore not originally designed for seismic performance. Even though the dam shells and transition zones were constructed of potentially liquefiable materials with little effective compaction, an evaluation of the dam’s expected seismic performance in the late-1980s showed limited seismic deformation potential.

The current approaches with more recent ground motions, site characterization, and numerical modeling techniques, indicate significantly different seismic deformation potential and seismic vulnerability estimates.
The updated probabilistic seismic hazard analyses included the development of input motions using scenario-based target spectra. Site characterization included selecting an appropriate Becker Penetration Testing (BPT) to equivalent Standard Penetration Testing (SPT) conversion method based on an evaluation of three common methods currently in use, and then comparing the results with in-situ relative density testing and embankment construction history. Analysis cross-sections included discretization of looser soil sub-layers that were neglected in previous studies. The coupled liquefaction triggering and nonlinear seismic deformation analysis methods using FLAC were selected based on demonstrated predictive ability to evaluate seismic deformations in well-documented seismic dam performance case histories, including the performances of both the Upper and Lower San Fernando Dams in the 1971 San Fernando Earthquake. Finally, the analyses continued through post-shaking deformations. The stability of the deformed dam crest, with reduced upstream shell support, was studied by evaluating deformed meshes from FLAC, and considering capabilities and limitations of continuum analyses (e.g. FLAC, etc.) for large deformations. Considering limitations of FLAC, an assessment of the potential final post-earthquake deformed geometry was performed through a set of pre- and post-earthquake deformed shape analyses that predicted the potential runout distance and geometry of the analytically predicted liquefaction-induced upstream flow slide, and the geometry and stability of the remaining crest conditions. These updated analyses demonstrate that the recent evolution of procedures for seismic analyses of an embankment dam can be significant to properly assess the expected seismic performance of a major earthen dam.