Liquefaction hazard mapping and relationship between probability of liquefaction and factor of safety for Kolkata city, India

Rapidly increasing population of Kolkata metropolitan city has led to encroachment in the marshy areas, mainly Salt Lake and Rajarhat in the outskirts of the city. Haphazard and unplanned construction in the city and on marshy land-filled regions has rendered the city vulnerable to liquefaction in the event of earthquakes. Liquefaction potential in terms of factor of safety (FS) for silty clay and silty sand has been evaluated by deterministic procedures of Boulanger and Idriss (Evaluating the potential for liquefaction or cyclic failure of silts and clays. Centre for Geotechnical Modelling. Report No. UCD/CGM-04/01, 2004; CPT and SPT based liquefaction triggering procedures. Centre for Geotechnical Modelling. Report No. UCD/CGM-14/01, 2014), respectively, for the study area. Probability of liquefaction (P-L) values were determined using first order second moment (FOSM) reliability method and using ArcGIS 10.4.1 software, liquefaction hazard maps were constructed based on factor of safety and probability of liquefaction values at depths 7 m, 15 m and 25 m. Using nonlinear regression analysis, an attempt has been made to correlate P-L with its corresponding FS values. Logistic function was utilized and P-L-FS relations along with their fitted curves were obtained for silty clay, silty sand as well as for all soils considered in the present study. Sensitivity analysis was conducted on random variables, undrained shear strength (S-u), N-value for equivalent clean sand ((N-1)(60CS)), total overburden pressure (sigma(v)), effective overburden pressure (sigma'(v)), maximum horizontal acceleration (a(max)), stress reduction factor (r(d)) and magnitude scaling factor (MSF), by increasing their coefficient of variation by 10%, 20% and 30%. Sensitivity index for all the parameters were determined which showed that S-u and ((N-1)(60CS) are the most sensitive while sigma(v) and sigma'(v) being the least sensitive parameters.