Liquefaction potential assessment of Guwahati city using first-order second-moment method

Soil liquefaction is the failure of the soil due to the sudden increase in a pore water pressure causing the effective stress to reduce significantly thus losing shear strength, with the resulting effect causing the fluid type behavior of the soil. Guwahati city, the capital of Assam, lies in the northeastern region of India and this whole region is considered seismically very active (seismically sixth position globally). The whole region is categorized in Zone-V according to the seismic zoning map of India as per IS 1893:2002 (Part 1). This region witnessed/experienced several major and great earthquakes in the past such as 1897 Shillong and 1950 Assam earthquakes. The soil deposits of the city mainly consist of an alluvial type of Holocene age and the presence of shallow groundwater table makes it vulnerable to soil liquefaction. In this study, an assessment of soil liquefaction potential of Guwahati city is performed based on the methods proposed by Youd and Idriss (J Geotech Geoenviron Eng 127:297, 2001) and Idriss and Boulanger (2010; CPT- and SPT-based liquefaction triggering procedures, 2014) using standard penetration test data. The evaluation of liquefaction potential is carried out for 82 borehole sites considering the Great 1897 Shillong earthquake of M_w 8.1 with a peak ground acceleration of 0.36 g. In the deterministic approach, the various parameters are involved in the evaluation of liquefaction potential, and the uncertainties of the input parameters as well as model cause the dissimilarity of the result. For instance, the same input parameters for both the models show different factor of safety. Hence, a comprehensive probability approach considering the uncertainty of parameters is essential for the evaluation of liquefaction susceptibility. In this study, reliability analysis for both the models based on first- order second-moment method has been used. The Reliability Index based on input parameters such as cyclic resistance ratio and cyclic stress ratio computed for both the models and subsequent liquefaction probability are established. The result shows the city is in most vulnerable condition even up to 15 m depth considering both the methods proposed by Youd and Idriss (J Geotech Geoenviron Eng 127:297, 2001) and Idriss and Boulanger (2010; CPT- and SPT-based liquefaction triggering procedures, 2014). Therefore, extra measure should be taken while constructing structure in and around the study area.