A Study of Earthquake Induced Liquefaction Potential Risk based on Standard Penetration Test (SPT) N-Value

Abstract

Bhutan lies within the seismically active Himalayan belt; however, the country currently lacks a national seismic zonation map, and structural designers rely on the seismic zonation map of India for earthquake-resistant design. As per the seismic zonation map of India, the northeastern region of India is classified under Seismic Zone V, which represents the highest level of seismic activity. Given Bhutan’s similar tectonic and geological setting, the country is also exposed to significant seismic risk. Despite this vulnerability, there is very limited research on the liquefaction potential of foundation soils in Bhutan, which is an important factor influencing the stability and performance of infrastructure during earthquakes. Assessing liquefaction risk is therefore essential for improving the resilience and sustainability of infrastructure development in the country. In this study, the liquefaction potential of the foundation soil is evaluated based on the Standard Penetration Test (SPT) N-values. The analysis is based on data maintained by the Bhutan Standards Bureau (BSB) on the Standards Penetration Test N-value and other related geotechnical parameters. The liquefaction potential is assessed based on a simplified stress-based method with an assumption that the fineness of the soil is less than 5% (FC<5%) and the soil is expected to be 100% saturated during the summer monsoon season. The findings indicate that the liquefaction is likely to occur in areas such as Paro, Thimphu, Wangdue Phodrang, Samtse, Gelephu, and Gedu for an earthquake of magnitude as low as 5.5 at a foundation depth of 1.5 m. These findings highlight the importance of incorporating liquefaction potential assessment, ground improvement measures, and appropriate foundation design in future infrastructure projects.

This study contributes to the limited geotechnical engineering literature in Bhutan and provides preliminary insights that can support engineers, planners, and policymakers in improving earthquake resilience and infrastructure safety.