Site-specific ground response analysis of a proposed bridge site over Barak River along Silchar Bypass Road, India

Abstract

It is well known that the local soil condition is an important factor in changing the ground motion parameters during earthquake. Therefore, it becomes important to carry out site-specific earthquake risk study for the designing of earthquake-resistant structures due to future hazard. In the present study, a ground response analysis of a proposed bridge site over Barak River along Silchar Bypass Road is carried out to evaluate the local site effect with the real recorded earthquake ground motions available in this study area. Detailed soil investigation report in terms of SPT bore log profile is collected from the Government Agencies (Public Works Department, Govt. of Assam). In the absence of in situ shear wave velocity profile data, the Vs profiles are estimated from the known SPT-N value by proposing new empirical equations correlating Vs and SPT-N. After evaluation of the Vs 30 profiles, the site is then classified according to the provisions of National Earthquake Hazards Reduction Program (NEHRP). The input ground motions used for the analyses are in the form of acceleration time histories obtained from two recorded earthquakes such as Ms 7.2 (India–Burma Border earthquake, 1988) and Mw 6.4 (India–Burma Border earthquake, 1995). The modulus reduction and damping ratio curves are obtained from the different research works published considering the characteristics of subsurface profiles. The ground response analyses have resulted in the calculation of the ground motion parameters as well as the amplification factor, and response spectra at the ground surface for the site. The results have revealed that the ground motion at bed rock level has been highly amplified by the soil layers at three different frequency ranges from 2 to 8 Hz for both the earthquakes with a maximum amplification factor up to 4.6. The average surface level peak ground acceleration for the bridge site is found to be 0.25 and 0.18 g for the Ms = 7.2 and Mw = 6.4 earthquakes, respectively. Further, the averaged response spectrum of both the earthquakes for the bridge site is compared with the response spectrum of the Indian Standard (IS) 1893:2002. The design response spectrum for the bridge site is also developed considering NEHRP provisions. Therefore, the results would be very much useful for the designer, planner and the Govt. of Assam for the safety of such important bridge for earthquake-resistant design to mitigate seismic hazard.