ASSESSMENT OF LIQUEFACTION POTENTIAL

Abstract

The study of earthquake-related soil or subsurface failure is inevitable due to increasing threat on the life and the property after an earthquake. Soil liquefaction following an earthquake is a major cause to devastations and damages to infrastructure and economic loss. Guwahati, one of the major cities located in the North Eastern region of India is burgeoning in every aspect with major infrastructures like educational institutions, hospitals, schools, tall buildings, flyovers, etc. thus acquiring subsurface information before any construction is very necessary. Guwahati city is vulnerable to be hit by high magnitude earthquakes like that of the 1897 or 1950 earthquake as Assam has a history of great earthquakes in the past. So, the study of liquefaction potential of Guwahati city is of utmost importance. In this report, assessment of liquefaction potential of Guwahati region (Assam) which lies on the seismically vulnerable region with Seismic Zone – V has been done for a higher magnitude earthquake with the knowledge that most of the part of the city has developed over filled alluvium deposits. In this regard, contour maps of ground water table and SPT-N values for 200 bore holes have been plotted. The critical cyclic stress ratio required to cause liquefaction and the cyclic stress ratio (CSR) induced by an earthquake have been evaluated using the simplified empirical method developed by Seed and Idriss (1971). The corresponding cyclic resistance ratio (CRR) has been calculated to evaluate factor of safety (FOS) of different layers of all the bore holes. To examine the liquefaction potential, LPI has been evaluated for the whole region. The above calculations have been done for two cases, i.e. maximum considered earthquake (MCE) and design basis earthquake (DBE). The contour maps of FOS at different depths and LPI have been plotted to see their variations throughout the city.