Evaluation of Liquefaction Return Period for Bangalore Based on Standard Penetration Test Data: Performance Based Approach

Problem statement: The conventional liquefaction evaluation is based on a deterministic approach. However in this method the uncertainty in the earthquake loading is not properly taken into account. However recent research in this field indicates that this uncertainty in the earthquake loading has to be considered in the liquefaction potential evaluation. Moreover the evaluation of liquefaction return period is not possible in the conventional deterministic methods. This study explained the methods for evaluating the probability of liquefaction and the return period of liquefaction based on probabilistic approach. Approach: In this study the geotechnical data was collected from 450 bore holes in Bangalore, India, covering an area of 220 km². The seismic hazard analysis for the study area is carried out using Probabilistic Seismic Hazard Analysis (PSHA) and the peak acceleration at ground surface was evaluated for site class-D after considering local site effects. For assessment of site class, shear wave velocity profiles in the city had been obtained using Multichannel Analysis of Surface Waves (MASW) survey. Based on this data the probabilistic liquefaction analysis was done to evaluate the probability of liquefaction in the study area. Based on the performance based approach the liquefaction return period for the study area was also evaluated. Results: The results showed the variation of liquefaction susceptibility for the study area. The corrected standard penetration values required to prevent the liquefaction for return periods of 475 and 2500 years were also presented here. Conclusion/Recommendations: The spatial variation of probability of liquefaction and the factor of safety against liquefaction evaluated using the two methods match well for the study area.