1D nonlinear site response analysis of DMDP area, Bangladesh

Abstract

The purpose of this research is to perform 1D site response analysis using Standard Penetration Test (SPT) and shear-wave velocity of the Dhaka Metropolitan Development Plan area (DMDP), Bangladesh. Additionally, the result obtained from the 1D analysis has been validated through microtremor analysis result. For this purpose, fifty locations were selected in the study area and the selected locations maintain approximately a uniform distribution within the whole DMDP area. Most of the locations are located on reclaimed areas with sand filling up to a depth of 10 to 15m or more. The depth of boreholes varies between 30 to 85.5m depending on the SPT-N value of 65 or more in the last consecutive 3 layers. After completing SPT a 100mm PVC pipe was installed in the same hole to obtain shear-wave velocity using the seismic downhole technique. The shear-wave velocity beyond the borehole depth has been taken from CDMP 2009 data applying the closest pair of points between the test borehole to the CDMP test point. For estimating site responses for those selected fifty locations, a computer program DEEPSOIL was used. Soil layer depth, unit weight, shear wave velocity (Vs) and soil shear strength as inputs, soil site responses were estimated by equivalent linear and non-linear analysis. Total ten input motions were used in these analyses, which were scaled to 0.13g value for bedrock in the Dhaka region. Besides, a microtremor recording has been carried out at every location of those fifty borehole locations. Microtremor data has been collected according to the guideline of SESAME. Finally, a Horizontal to Vertical spectral Ratio (HVR) curve has been obtained of the site to compare with the 1D response analysis. From the detailed site-specific analysis, it was observed that the amplification factor of the study area ranges 3.1 to 5.5 for nonlinear analysis and 3.8 to 8.5 for equivalent linear analysis and the amplification factor obtained from microtremor analysis ranges 1.3 to 6.6. In areas of thick and soft Holocene deposit the amplification is more than the areas where shallow valleys of Pleistocene deposits with artificially filled sand or any clay in top layer. The peak spectral acceleration (PSA) for 50 sites varies between 0.41g to 0.80g in the case of nonlinear analysis whereas the range of PSA is 0.50g to 1.12g for equivalent linear analysis. It also observed that, the peak ground acceleration (PGA) of the study points ranges 0.17g to 0.26g for non-linear analysis and 0.22g to 0.43g for equivalent linear analysis.