Liquefaction Susceptibility Mapping in West Bengal with emphasis on its Capital City Kolkata under the Impact of a few Great Earthquakes triggered from the Himalaya and Northeast India

Abstract

A host of great historical earthquakes from the Himalayas and Northeast India reportedly triggered liquefaction with the surface manifestation of sand boil, ground subsidence and lateral spreading in West Bengal and its capital city Kolkata located in the alluvium-rich Ganga-Brahmaputra river system, thus presenting a strong case towards systematic liquefaction potential analysis for this terrain using multivariate techniques based on a large Geophysical and Geotechnical data base. An integrated computational protocol has provided site classification of the terrain following standard nomenclature and its characterization in terms of absolute and generic spectral site amplification through equivalent linear/ non-linear geotechnical response spectral modelling as an intermediate step towards Liquefaction Potential and Risk assessment of the region. The large Geotechnical database is used for estimating Cyclic Stress Ratio (CSR) and Cyclic Resistance Ratio (CRR), which further delivered Factor of Safety (FOS), Liquefaction Potential Index (LPI), Probability of Liquefaction (PL), and Liquefaction Risk Index (IR) in the State and its capital Kolkata. The State including Kolkata have been classified into ‘Severe’, ‘High’, ‘Moderate’ and ‘Non-liquefiable’ zones based on LPI distribution while the liquefaction risk map classified the terrain into ‘Low (IR ≤20)’, ‘High (2030)’ Risk Zones. An intensely liquefiable stratum with FOS<1 is identified in the 5-15m depth region consisting of coarse-grained variants of sand, silty-sand and clayey-silty sand with an approximately 0.5-12.7m deep groundwater condition. An understanding of the liquefaction potential and its associated risk will act as catalyst in reducing structural vulnerability of the terrain by improving sediment strength.

Keywords

• liquefaction
• factor of safety
• probability of liquefaction
• liquefaction risk