Site amplification of the selected areas of Dhaka city based on shear wave velocity

Abstract

Risk to manmade structures due to seismically induced ground deformation has been evident from past earthquakes. Influence of site effects on strong ground motions has been ascertained by many seismologists and earthquake engineers. Mechanisms related to the local soil and rock properties have the capacity to influence ground motions. Depending on the ground characteristics, the ground shaking is influenced, which may result in the amplification (causing resonance) or attenuation. Amplification mechanisms control the frequency content of ground motions. Though direct waves were used for the study of ground motion, source, path and site characterization were recognised as the prime factors that affect earthquake ground motion. The purpose of this research is to estimate the site amplification of selected areas of Dhaka city based on shear wave velocity. For this, 10 (ten) locations has been selected in Dhaka city. In the selected locations, the depth of sand filling in areas varies from 2.0 to 6.0 m from existing ground level (EGL). The depth of clay layer varies from 4.0 m to 30.0 m. The depth of silty clay layer varies from 4.0 m to 6.0 m. The depth of clay layer varies from 4.0 m to 26.0 m. The depth of fine sand layer varies from 4.0 m to 20.0 m. The depth of dense sand layer varies from 4.0 m to 20.0 m from EGL. Shear wave velocities has been found out from the CPT equipment. The maximum value of shear wave velocity varies from 300 m/s to 810 m/s and minimum value of shear wave velocity varies from 50 m/s to 100 m/s. The average value of shear wave velocity varies from 164 m/s to 320 m/s. For estimating peak ground acceleration DEEPSOIL developed by Hashash et al. has been used. With soil layer depth, bulk density and damping as inputs, peak ground acceleration by equivalent linear analysis has been estimated. Four input motions (Gangtok data from Sikkim earthquake, Kobe data from Kobe earthquake, Northridge and Loma Prieta data) recorded in rock have been used in this analysis, which are scaled to 0.19g value. From the detailed site specific analysis, the PGA values at surface have been obtained in the range of 0.073g to 0.450g. The surface acceleration values have been very high (>0.2g) in the areas of Gulshan, Dakhin Khan, Mirpur and Mothertek. Values of 0.1g to 0.2g are estimated in different locations like Mugda, Uttara, Asulia, Mohammadrpur, and United City. These locations are characterized by clayey sand and mixture of sand, silt and clay. Peak ground acceleration has been observed to be very low (<0.1g) in the area of Kawran Bazar. These locations have soils with layers of silty sand and silty clays. The PGA values are useful for the ground response analysis. Amplification factors varies from 0.43 (Loma prieta) to 0.59 (Kobe) for Kawran Bazar, from 1.45 (Sikkim) to 2.18 (Kobe) for Gulshan, from 0.99 (Sikkim) to 1.24 (Kobe) for Mugda, from 1.03 (Sikkim) to 1.48 (Kobe) for Dakhin Khan, from 0.81 (Sikkim) to 1.20 (Kobe) for Uttara, from 0.34 (Loma prieta) to 0.76 (Kobe) for Asulia, from 1.12 (Loma prieta) to 1.67 (Kobe) for Mirpur, from 0.72 (Loma prieta) to 1.36 (Kobe) for Mohammadpur, from 1.85 (Sikkim) to 2.54 (Kobe) for Mothertek, from 0.53 (Loma Prieta) to 0.76 (Kobe) for United City. Peak Spectral Acceleration (PSA) varies from 0.0029g (Northridge) to 0.35g (Kobe) for Kawran Bazar, from 0.0028g (Northridge) to 1.21g (Kobe) for Gulshan, from 0.0028g (Northridge) to 0.66g (Kobe) for Mugda, from 0.0028g (Northridge) to 0.76g (Kobe) for Dakhin Khan, from 0.0029g (Northridge) to 0.93g (Loma prieta) for Uttara, from 0.0032g (Northridge) to 0.45g (Kobe) for Asulia, from 0.0028g (Northridge) to 0.92g (Kobe) for Mirpur, from 0.0030g (Northridge) to 0.90g (Kobe) for Mohammadpur, from 0.0029g (Northridge) to 1.67g (Loma prieta) for Mothertek, from 0.0028g (Northridge) to 0.46g (Kobe) for United City. The results provided in the seismic response analysis of Dhaka city could be used as guideline for risk assessment and management of future probable event.