EFFECTS OF GROUNDWATER LEVEL FLUCTUATION ON SUPERSTRUCTURE’S SEISMIC BEHAVIOUR IN DHAKA CITY

During an earthquake, site surface ground motion is different in terms of amplitude and frequency content in comparison to the bedrock motion. These changes are due to dynamic response of subsoil layers. This dynamic behaviour is a function of strain level of soil and pore presser level. In Dhaka city, during the monsoon seasons, because of high precipitation, the ground water level goes up in some areas. This, in turn, may result in a higher pore water pressure during an earthquake, which consequently filters some of the high frequency content of the earthquake motion and amplifies some other frequencies. In this study, the effects of such changes (both in ground motion frequency content and in amplification factor) and their effects on superstructure’s behaviour are calculated. At first stage, the site soil profile is modelled and analysed by use of one dimensional nonlinear site response analysis. The soil profile is modelled using an equivalent system of spring and dashpot as shown in Figure 1.The deformation of subsoil layers due to earthquake shaking generates irreversible deformation in soil that results in excess pore pressures development. This excess pore pressure reduces the soil effective stress and soil strength. The model proposed by Matasovic and Vucetic (1995) is used in this study to account for pore pressure as a function of number of cycles and cyclic strain

Keywords:

Nonlinear site effect , Time History analysis , structural performance , BNBC 2017 , pore pressure