Liquefaction Hazard Assessment with Integrated Deterministic and Performance-Based Approach for Coastal Megacities: A Case Study of Karachi, Pakistan

Abstract

This research investigates liquefaction hazard assessment in Karachi, the economic hub of Pakistan. Karachi's coastal region is increasingly vulnerable to liquefaction-related hazards, which pose a significant threat to the city's seismic resilience. Rapid, unplanned urbanization and increasing earthquake risk in this densely populated area underscore the critical need for a comprehensive assessment of liquefaction hazard, which remains largely unquantified in existing local research. In this research, a comprehensive assessment of liquefaction potential is provided for the Defence Housing Authority (DHA) Phase-VIII Karachi using a comprehensive framework. The initial geotechnical characterization identifies Holocene silty sand deposits (SM) with a relative density of 38\% and a shallow water table within 3 meters of the ground surface. Recent seismic hazard analysis, both probabilistic and deterministic, suggests that Karachi could experience a maximum PGA as high as 0.99g in some areas. The liquefaction potential was evaluated using advanced 3D finite element modeling in PLAXIS 3D software with the UBC3D-PLM Model, alongside an empirical simplified procedure. The simplified procedure consistently yields a Factor of Safety (FS) less than unity throughout the 9.14~m soil depth in both sand and silt layers, indicating high susceptibility of the coastal soil to liquefaction. Numerical modeling in PLAXIS 3D confirms these findings, showing a rapid increase in the pore water pressure ratio ($r_u$) to unity, with loss of soil strength and stiffness due to reduced effective stress. The soil is completely liquefied, with vertical settlement up to 0.125~m under a load of 100~kN/m\textsuperscript{2}. The research concludes that current infrastructure along the coastal belt of Karachi requires urgent geotechnical intervention, such as jet grouting or deep foundation transfer, to mitigate the catastrophic potential of seismically induced soil instability.