Methodology for site-response studies using multi-channel analysis of surface wave technique in Dehradun city

Two-dimensional shear wave velocity investigation using the multi-channel analysis of surface waves helps identify lateral and vertical variations of shear wave velocity at shallow depth. This information is required for predicting the ground motion response to earthquakes in areas underlain by thick soil cover. At one such area in Dehradun, at the foothills of the Himalaya, shear wave velocities of the near-surface soil were determined for 50 locations covering almost all representative units with respect to variation in local geology and geomorphology. Based on the average shear wave velocities of the upper 30 m of the soil, sites located in the south-southwestern and central part of the city are predominantly classified as class 'D' (180-360 m/s) except a few locations like Clement Town (site no. 43) and Majra (site no. 48), which can be classified as class 'E' due to very low shear wave velocity, in accordance with the NEHRP 1997 provision. The northern part of Debradun city shows high velocities, ranging from 300 m/s at the surface to more than 700 m/s at depth of 30 m. These sites located in the northern and eastern parts of the city with average shear wave velocity (Vs) values more than 360 m/s have been classified as class 'C' site (360-760 m/s). The characteristic period estimated for each site of the soil column varies from 1.5 to 3.12 Hz. Based on the shear wave velocity, input motion, static and dynamic properties of different soil layers, site-response spectrum and amplification functions have been derived. The response spectrum suggests spectral acceleration value for two-storey structures of 3 to 8 times higher than the peak ground acceleration at the bed rock level, i.e. 0.05 g. The analysis also suggests, peak amplification at 3-4, 2-2.5 and 1-1.5 Hz in the northern, central and south-southwestern parts of the city respectively.