Numerical evaluation of natural periods and mode shapes of earth dams for probabilistic seismic hazard analysis applications

The evaluation of natural periods and related mode shapes of earth dams represents a critical issue when performing structure-specific probabilistic seismic hazard analyses (PSHA). The identification of critical scenario events, using techniques such as disaggregation of the seismic hazard, and the calculation of a suitable target spectrum for ground motion selection and scaling procedures (e.g., the conditional mean spectrum), require at least the knowledge of the fundamental period of the system. This problem can be solved using analytical, numerical, and/or empirical techniques. We present several linear elastic modal analyses for an earth dam located in Southern Italy, using a numerical solution of the generalized eigenvalue problem obtained by the finite element method (FEM). Our numerical experiments are performed, testing various assumptions on boundary conditions, degree of saturation, and the distribution of geotechnical characteristics of the dam’s materials. We then compare our results against existing analytical solutions. We show that ignoring soil–structure interaction effects due to the flexibility of the dam foundation (i.e., under the assumption of fixed base) can lead to a substantial underestimation of the fundamental period of the dam. This effect should be carefully addressed when modal analysis results are used in PSHA-related applications.