ASSESSMENT OF SOIL LIQUEFACTION POTENTIAL BY MICROTREMOR MEASUREMENTS IN LALITPUR

Abstract

Soil liquefaction is one of the major effects of earthquakes causing various hazards like slope failure, settlement, sand boils, lateral spreading and fissures. Several occurrences of liquefaction have been observed around the world resulting in foundation settlement, bearing capacity failure, collapse of structures, damage to infrastructures and human settlements, along with serious socioeconomic effects. Nepal has also experienced the events of liquefaction, most of which are documented in the Kathmandu Valley during 1934 Bihar Nepal earthquake and 2015 Gorkha earthquake. The fluvio-lacustrine deposit along with shallow ground water table make this place vulnerable to liquefaction. Several study regarding the liquefaction potential of Kathmandu Valley have been carried out, most of which are based on SPT-N values. Evaluation based on microtremor analysis and shear wave velocity is scarce. In this study, an attempt is made to evaluate the liquefaction potential of Lalitpur Metropolitan city based on Vulnerability index proposed by Nakamura (1997). Microtremor data of 39 stations in Lalitpur Metropolitan has been collected. These include the site of liquefaction occurrence observed during 2015 Gorkha Earthquake. Vulnerability index (Kg) of all the stations were calculated. A threshold value (Kgf) was also calculated and found out to be 21.85. Based on this threshold value, the study area is characterized as more vulnerable zone (Kg ≥ Kgf) and less vulnerable zone (Kg < Kgf). A map of the study area was prepared on this basis and was compared with the existing hazard maps and geological maps. A considerable agreement was found among the maps. The eastern part of study area was found to be more vulnerable to liquefaction than the western part. The Kg values were also found to be in good agreement with the borehole information as well as liquefaction potential indices (LPI). The study results demonstrate that the Nakamura Methodology is a quick and simple method to evaluate liquefaction potential and an alternative way to generate hazard maps. The methodology could be useful in places with scarce geotechnical data, and in situations with limited funds. However, detailed study upon availability of funds and abundant geotechnical data is must for finer characterization. Furthermore, this method can be used for quick characterization of large areas of land, lab based and penetrative field based analysis is a must for site specific liquefaction hazard analysis.