PS logging for site response analysis in Dhaka City

Abstract

The purpose of this research is to measure shear wave velocity of different areas of Dhaka city using PS Logging and estimate the site amplification of those areas based on shear wave velocity. For this purpose, seventeen locations were selected in Dhaka city. Among them, PS logging was carried out at ten locations by the author and data of rest seven locations were collected from CDMP. Most of the sites are located on reclaimed areas which have filling sand up to a depth of 15 to 20m. Original soil is encountered at the surface in some places. In every location, Standard Penetration Test was carried out up to a depth of 30m and later in the same hole a 3inch PVC pipe was installed. Finally, shear wave velocities were estimated using Suspension PS Logging equipment in Down-hole technique.

Using 189 pairs of values of corrected SPT N and Shear Wave Velocity from seventeen locations, a correlation among Shear Wave Velocity, SPT N Value and Depth was developed and proposed for Dhaka city. Perfect correlation between variables would result in an “r” of 1.0. The coefficient of determination (r2) is 0.45, which indicates a “r” value of 0.67. This means a significant amount of scatter exists in the measured data.

For estimating site amplification for those seventeen locations, acceleration computer program DEEPSOIL was used. With soil layer depth, unit weight () and shear wave velocity (Vs) as inputs, soil amplification by equivalent linear analysis was estimated. Four input motions (The Imperial Valley Earthquake; the Kobe Earthquake; the Kocaeli Earthquake and the Northridge Earthquake) were used in these analyses, which were scaled to 0.19g value for bedrock in the Dhaka region. From the detailed site specific analysis, it was observed that Imperial Valley Earthquake produces the highest (2.46g) peak spectral acceleration (PSA) for the site East Nandipara and the Northridge Earthquake produces the lowest (0.002g) peak spectral acceleration (PSA) for most of the sites. The peak ground acceleration values at surface were observed to be in the range of 0.11g (Kocaeli) for the site Ashulia Jubok Project to as high as 0.66g (Northridge) for the site East Nandipara. The Amplification Factor of different locations were found in the range of 0.47 (Kocaeli) for the site Gulsan-2 to as high as 4.69 (Northridge) for the site East Nandipara. Four Site Amplification maps were prepared for Dhaka City for the above four input motions.