UC San Diego

For large relatively stiff structures, soil structure interaction (SSI) plays a major role in dictating the overall seismic response. In light of recent strong seismic excitation affecting such structures, three-dimensional response as well as nonlinear soil behavior are among the areas of increased interest. As such, a series of numerical studies are conducted to shed more light on the involved SSI mechanisms. Amongst those studies is a comparison of the equivalent linear and nonlinear soil formulations in evaluating the seismic response of large embedded structures. Depending on the level of attained nonlinear response, influence of the following modeling considerations is discussed: i) employing the nonlinear versus linear soil formulation, ii) initial own-weight lateral earth pressure stress-state, and iii) the soil-structure interface characteristics. Both formulations generally resulted in remarkably close estimates of structural response.

An opportunity to investigate the SSI mechanisms of large embedded structures due to low amplitude shaking was permitted by the availability of seismic data from an instrumented test site at Higashi-dori, Japan. The compiled data set includes the recorded accelerations, for two downhole arrays, and the response of a 1/10th scale twin reactor. The extracted site properties are shown to provide a reasonable match to the recorded data. Using these properties parametric computational studies are conducted to illustrate salient mechanisms associated with the seismic response of such large embedded structural systems.

Furthermore, an opportunity to investigate the seismic response of the Fukushima nuclear reactors due to strong shaking was facilitated by data recorded during the magnitude 9.1 Tōhoku earthquake. Linear and nonlinear response of the ground was evaluated using system identification techniques. During the strong shaking, a clear and significant reduction in stiffness was observed within the upper soil strata. Of special interest was the response of Unit 6, which was the most heavily instrumented of the reactors. Response at the base of Unit 6 was compared to that of the nearby downhole array. Amplification of motion along the height of Unit 6 was evaluated, exhibiting the primary role of rocking response.