Effect of rise of ground water table on seismicground response of soft soil in Shanghai

A dynamic model is developed based on the results of geological investigations as well as laboratory and field tests in Shanghai. The Biot's dynamic consolidation theory and reduced order bounding surface model are coupled to study the effect of the rise of groundwater table on the seismic ground response of soft soil undergoing 3-D seismic base excitations (which are obtained from Taft). The effect of the rise of groundwater table on the horizontal and vertical amplification factors, the ratio of vertical to horizontal peak acceleration, the characteristics of acceleration and response spectrum at ground surface as well as the maximum pore water pressure and pore water pressure ratio along the depth of soil layer are studied. The results show that the rise of groundwater table has dramatically different influences on the horizontal and vertical amplification effect as well as the characteristics of acceleration and response spectrum at ground surface. As the result of the rise of groundwater table, the amplification effect increases in horizontal direction but decreases in vertical direction. The ratio of vertical to horizontal acceleration decreases. The filtering effect on the acceleration with high frequency and the amplification effect on the acceleration with long period increase. The range of liquefiable soil near ground surface extends and hazard risk increases. Key words: Biot's dynamic consolidation theory; seismic response analysis; reduced order bounding surface model; rise of groundwater table; 3-directional seismic base excitation