Estimation of local site effects and seismic vulnerability using geotechnical dataset at flyover site Agartala India

Considering future earthquakes, the vulnerability of Agartala city is following an increasing trend mainly due to the recent surge in population density and significant infrastructure developments. The recent moderate intensity earthquake at Tripura, namely the 2017 Dhalai Earthquake (M-w 5.7), with its epicentre being the adjoining district of Agartala, resulted in liquefaction cases, sand blows and lateral spreads, which may be an alarm for the geotechnical engineering fraternity. The present study attempts to evaluate local soil sites' effect in the form of developing site-specific ground motions using stochastic point-source program SMSIM based on past scenario earthquakes in this region and performing one-dimensional nonlinear ground response analysis (1D GRA) of a recently constructed flyover site of 2.3 km length at Agartala city. Further, the seismic vulnerability of the flyover structure incorporating the local site effect is also assessed herein. The results obtained are explained in terms of surface acceleration time history, the ratio of shear stress to effective vertical stress with depth, acceleration response spectrum, Fourier amplitude ratio with frequency, etc. 1D GRA presented contrasting results with higher values of acceleration in areas with stiffer soils due to attenuation of the seismic waves. In general, the amplification ratio (A(f)) evaluated from the present study indicated that the central portion of the flyover yielded high values and a high fundamental frequency value of 6.0 Hz, which might be detrimental for low-lying buildings. The generated synthetic motions of scenario earthquakes will be helpful for researchers and designers for earthquake resistant assessment or design of geotechnical structures in north-eastern India or similar sites in the absence of site-specific data. Finally, this study sheds important observations on seismic design guidelines of the structure located in this region, which may revamp the existing codal procedures.