Numerical simulations for prediction of seismic ground motion amplification: 2D vs 3D comparison for natural reliefs and earth dams

Local site conditions (i.e., dynamic properties of subsurface and bedrock materials, surface and buried morphology) play an important role on seismic ground motion amplification. Therefore, their effects may become crucial in several geological/geotechnical systems such as earth dams and natural reliefs for selection of ground motion to be used in the analysis of engineering problems. These systems are usually studied based on 2D numerical analyses. However, due to the complexity of geomorphological and geotechnical situations, frequently coupled 3D soil-topography amplification analyses should be conducted to realistically assess the dynamic response. In this research, large-scale numerical simulations are carried out with the finite difference computer code FLAC3D, that was used for both 2D and 3D analyses. Two case histories have been considered, the Palatino hill, a natural relief in the historical center of Rome, and the Angitola zoned dams in Southern Italy. In both cases visco-elastic linear analyses have been conducted considering models of increasing complexity, from homogeneous to heterogeneous ones with the inclusion of one or more additional layers, in order to distinguish between topographic and stratigraphic/valley amplification. In both cases Chang wavelet was used for input motion for three values of frequency (2Hz, 5Hz and 8Hz). The comparison between the results of 2D and 3D analyses has been carried out in terms of maximum accelerations and/or displacement in representative points. The numerical analyses have been preceded by a calibration study, comparing the results of 2D and 3D numerical analyses with closed form solutions. It is shown that 3D behavior can have a pronounced effect on the seismic response of the examined systems in terms of amplitude and shape of the acceleration/displacement profiles. In fact, 3D amplitudes can be much larger than 2D ones depending on the frequency of the input motion. The importance of vertical parasitic component of motion is highlighted, which again is frequency-dependent in that it increases with increasing frequency. For Angitola earth dams, the influence of narrow canyons in which the dams are built significantly contributes to the enhanced amplification of response, which is in agreement with the literature findings.