The Significance of Nonlinear Characteristics on the Seismic Induced Pressures of Deeply Embedded NPP Structures

The conventional approach for treating the soil-structure interaction (SSI) effect is to apply various linear methodologies (e.g., substructure and direct methods). In the absence of any major nonlinear effect, linear methodology is sufficient to capture the SSI response parameters. However, for situations where structures are deeply embedded and/or buried (DEB) below grade and a sizeable inertial effect must be considered, the nonlinearity induced by the interaction at the soil/structure interface may potentially influence the SSI response. This is especially true for the seismic induced soil pressures. The effect of such nonlinear characteristics on SSI methodologies should be assessed to determine the extent to which the nonlinearity associated with DEB structures needs to be modeled. Sponsored by the US Nuclear Regulatory Commission (NRC), Brookhaven National Laboratory (BNL) is carrying out a research program to develop a technical basis to support the safety evaluation of deeply embedded and/or buried (DEB) structures as proposed for advanced reactor designs. In this paper, a study of the effect of the soil/structure interface on seismic induced pressures is presented. The nonlinear effect considered includes separation or slipping along the side soil/wall interface. This paper describes various finite element models developed using the LS-DYNA computer code to study the nonlinear effect associated with the soil/structure interface interaction of DEB structures subjected to seismic motion. It is found, based on results to date, that the seismic response characteristics of DEB structures are considerably influenced by the modeling technique for soil/structure interfaces. One of the prominent effects found is the soil/structure interface separations, which modify the soil pressure distributions in both circumferential and vertical directions, as well as their frequency content.