:ASSESSMENT OF LIQUEFACTION POTENTIAL OF KOLKATA SUBSOIL

Abstract

Soil liquefaction is one of the major problems in geotechnical earthquake engineering. Devastating effects of Soil Liquefaction has been observed from each and every Earthquake where large numbers of structures were damaged due to this phenomenon. No structural design is adequate for preventing the failure of structure if Soil Liquefaction occurs. Kolkata is situated upon a deep alluvial deposit, deposited by river Hooghly (Ganga). The deposit is not very old. In Kolkata soil profile generally two types of soil deposit are present, Normal Kolkata deposit and River Channel deposit of these two types of soil, the River Channel deposit generally consists of fine to medium grain sand and most susceptible to liquefaction hazard. In the present study an extensive analysis has been done for determination of Liquefaction Hazard of Kolkata soil at different locations. The study comprises of evaluation of Liquefaction Potential of Kolkata city soil using Simplified Procedure suggested by Seed and Idriss and method suggested by Seed, Idriss and Arango. The borehole data were collected from different sources. The data available for the borehole were tabulated to assess the deficiencies in the database. The deficiencies arise since the data has been collected from different sources and each agency has not conducted tests or processed data in the same manner. A consequence of the above missing data was generated from neural nets developed specifically for interpolation/ extrapolation of parameters essential for evaluation of liquefaction iv potential. A computer program has been developed for determination of Liquefaction Potential for the methods mentioned earlier. The conservative liquefaction potential values were used in neural net for obtaining liquefaction potential value at different grid points. Contour maps have been plotted using liquefaction potential values for Kolkata city for the recommended PGA and ground accelerations from 0.05g to 0.25g at different depths. The study concludes with the facts that River channel deposit soils are most hazardous for liquefaction. Normal Kolkata deposits are relatively less hazardous than that of River channel deposit. Some precautions should be taken during construction upon those River channel deposits. The contour maps, would assist the designers in taking suitable decision regarding necessary sub soil treatment at different locality based on the expected acceleration at foundation level. u