Estimation of earthquake induced liquefaction potential of selected areas of Dhaka city based on shear wave velocity

Abstract

The purpose of this research is to estimate the earthquake induced liquefaction potential of selected reclaimed areas of Dhaka city based on both Standard Penetration Test (SPT) and Shear Wave Velocity. Sixteen borings have been conducted and seventy six boreholc data have bcen collected from nine selected locations of Dhaka city. Small Scale Microtremor Measurements (SSMM) have been conducted at the same locations to determine shear wave velocity. The filling depth of the reclaimed areas varies from 1.5 to 13.5 m from existing ground level (EGL). The field SPT N-value of the filling depth varies from I to 13. Mean grain size of the soil in the filling depth varies from 0.10 to 0.32 mm. Fines content of the soil in the filling depth varies from 6 to 48%. Shear wave velocity of the filling layer varies from 70-125 m/sec. In some locations organic layer (thickness: 3.0-7.5 m; SPT N-value: 1-4 and organic content: 12-24%) is found just beneath the filling sand. For the liquefaction analyses, the values of pcak ground acceleration, am" and the moment magnitude, Mw have been taken as 0.15g and 7.0, respectively. Liquefaction analysis based on SPT (Japanese Code of Bridge Design, 1990; Chinese criterion by Ishihara, 1990 and Seed- Idriss simplified procedure, 1971) and shear wave velocity (Andrus & Stokoe, 2000 and Yunmin et aI., 2005) is conducted for forty-nine borehole locations. Othcr forty-three borehole locations are found not to be susceptible to liquefaction since the soil profiles in those locations are mostly clayey. According to the analysis based on Japanese Code of Bridge Design (1990), liquefiable soils have been found at twenty-two locations. Liquefiable depth has been found to vary from 4.5 to 12 m from EGL. According to the analysis based on Chinese criterion by Ishihara (1990), liquefiable soils have been found at forty locations. Liquefiable depth is found to vary from 6 to 13.5 m from EGL. According to the analysis based on Seed-Idriss simplified procedure (1971), liquefiable soils have been found at fortyeight locations. Liquefiable depth is found to vary from 6 to 13.5 m from EGL. According to the method proposed by Andrus & Stokoe (2000), liquefiable soils have been found at thirteen locations out of sixteen locations. Liquefiable depth is found at 6 m from EGL. According to method proposed by Yunmin et al. (2005), there are no liquefiable soil strata in researched borehole locations. It is seen that there is a probability of liquefaction to occur in reclaimed areas of Dhaka city especially for the locations reclaimed by dredged soil up to the filling depth. It is found that estimated results ofliquefaction potential of the studied locations are different for different methods. Therefore, liquefaction analysis of susceptible soils may be carried out using different methods and conclusion about the liquefaction susceptibility may be drawn using appropriate engineering judgment. The un-corrected SPT N-value, un-corrected shear wave velocity, mean grain size (0'0) and fines content (F,) of liquefiable soil varies from 2 to 11,70 to 125 m/sec, 0.14 to 0.19 mm and 12 to 28%, respectively. A correlation between shear wave velocity and SPT N-value has been developed in this study. Study may be conducted to prepare guidelines for reclamation procedure to avoid possible liquefaction of reclaimed areas. Laboratory tests such as cyclic loading test or shaking table test may be conducted to determine the cyclic strength of the soils and further validating the empirical methods of this research.