Finite Element Models for Computing Seismic Induced Soil Pressures on Deeply Embedded NPP Structures

This paper discusses computations of seismic induced soil pressures using finite element (FE) models for deeply embedded and/or buried (DEB) stiff structures, such as those appearing in the conceptual designs of structures for advanced reactors. For DEB structures, the soil-structure interaction (SSI) effect is expected to have a strong influence on the estimate of the seismic induced soil pressures, especially for stiff structures embedded in soft soil strata. In this paper, two FE models are developed using the SASSI and LS-DYNA computer programs, representing respectively the substructure subtracting method and explicit FE algorithm. SASSI utilizes the wave propagation theory and the principle of superposition to treat the SSI phenomenon. In the LS-DYNA analysis, an attempt is made to apply the direct approach to the SSI effect, which treats the near field soil with an explicit FE mesh that is connected to a transmitting boundary to approximate wave propagation in the half-space. The structural model used for the study is derived from the characteristics of a conceptual design for an advanced reactor. The structure is founded in a soft soil overburden underlain by a rock and the input seismic motion is specified at rock outcrop and has a zero period acceleration (ZPA) equal to 0.3 g, typical of review level earthquakes for nuclear power plant structures in the Central and Eastern United States. Various depths of burial (DOB) for the structure are considered in the analysis to afford an assessment of the DOB effect on the seismic induced soil pressure estimates determined by these methods. Comparisons and discussions of the analysis results computed by the two approaches are provided.© 2006 ASME