Seismic site classification and site period mapping of Chennai City using geophysical and geotechnical data

Subsurface conditions play a major role in the damage potential of earthquakes and the seismic soil amplification of a site which is a critical factor affecting the level of ground shaking. Shear wave velocity (Vs) of the soil layer is an important parameter influencing the amplification behaviour of the site. Site characterization in calculating seismic hazards is usually based on the near-surface shear wave velocity values. The average shear wave velocity of the top 30m of the soil, referred to as (Vs)30 is commonly adopted by competent building codes to classify the sites for earthquake resistant design of structures and in general it is widely used in microzonation studies. In the present study, the shear wave velocity of soil layers was measured at 30 locations in Chennai City by Multichannel Analysis of Surface Waves (MASW) test. In addition, nearly 300 borehole data were used to estimate Vs based on the correlations between Vs and SPT-N values for Chennai developed by authors earlier. Merging of MASW test results with borehole data yields sufficient coverage of Vs to develop a new site classification map for Chennai based on the NEHRP standard. It is found that part of the city belong to site D category (stiff soil). The developed site period map reveals that the fundamental site period varies in the range of 0.03 to 0.6s, thus the soil conditions in Chennai pose a potential threat during earthquakes to low rise buildings (less than 6 storeys) which are densely distributed throughout the city. © 2010 Elsevier B.V.