Dynamic response of deep soft soil deposits under multidirectional earthquake loading

The analysis of site response to earthquake is of great importance for seismic design of structure. Although it has long been recognized that the ground is simultaneously subjected to both horizontal and vertical motions during a real earthquake, the effect of vertical motion is usually ignored in most site response analyses. Moreover, some studies show that there are significant differences in site responses for saturated and unsaturated soil deposits, and site response is influenced by variation of groundwater table. Therefore, these factors should be considerably considered in the dynamic site response analysis. Based on the results of geological investigation as well as laboratory and field tests conducted in Shanghai, a deep geotechnical profile with a depth of 280 m is analyzed in this paper. By using a verified, fully coupled and inelastic finite element procedure, a series of numerical analyses is carried out for a deep soft soil profile subjected to multidirectional earthquake loadings. Parameters of the model in the procedure are calibrated by field and laboratory tests. Numerical results demonstrate that the behavior of vertical amplification is quite different from the horizontal counterpart. Furthermore, the use of a single horizontal component as seismic input in site response analysis, neglecting the other horizontal component and the vertical one, may result in considerable underestimation of site response. Variation of groundwater table brings about a profound effect on both vertical and horizontal motions, its ratio, the response spectra at ground surface, and excess pore water pressure ratio. The analysis can capture a more complete and fundamental aspects of ground response for preliminary seismic assessment.

Research highlights
► Deep soil responses in Shanghai are investigated under 3-component seismic shaking rather than a single horizontal one.
► The verified numerical analyses take the effects of variation of ground water table on site response into consideration.
► The vertical ground motion can be considerably higher than 2/3 of the horizontal one.
► Three-component input motions can yield a larger site response than that by a single horizontal motion.
► Decreasing groundwater table leads to higher site response and liquefaction.