Mapping of Average Shear Wave Velocity for Bangalore Region: A Case Study

Abstract

Mapping the shear wave velocity profile is an important part in seismic hazard and microzonation studies. The shear wave velocity of soil in the city of Bangalore was mapped using the Multichannel Analysis of Surface Wave (MASW) technique. An empirical relationship was found between the Standard Penetration Test (SPT) corrected N value [Formula: see text] and measured shear wave velocity [Formula: see text]. The survey points were selected in such a way that the results represent the entire Bangalore region, covering an area of [Formula: see text]. Fifty-eight 1-D and 20 2-D MASW surveys were performed and their velocity profiles determined. The average shear wave velocity of Bangalore soils was evaluated for depths of [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]. The sub-soil classification was made for seismic local site effect evaluation based on average shear wave velocity of [Formula: see text] depth [Formula: see text] of sites using the National Earthquake Hazards Reduction Program (NEHRP) and International Building Code (IBC) classification. Mapping clearly indicates that the depth of soil obtained from MASW closely matches with the soil layers identified in SPT bore holes. Estimation of local site effects for an earthquake requires knowledge of the dynamic properties of soil, which is usually expressed in terms of shear wave velocity. Hence, to make use of abundant SPT data available on many geotechnical projects in Bangalore, an attempt was made to develop a relationship between Vs (m∕s) and [Formula: see text]. The measured shear wave velocity at 38 locations close to SPT boreholes was used to generate the correlation between the corrected N values and shear wave velocity. A power fit model correlation was developed with a regression coefficient [Formula: see text] of 0.84. This relationship between shear wave velocity and corrected SPT N values correlates well with the Japan Road Association equations.