Liquefaction in Kathmandu Valley During 2015 Nepal Earthquake

Abstract eng:
On 25 April 2015, an earthquake of magnitude 7.8 occurred in Gorkha, Nepal, an actively deforming central Himalayan mountain range. The epicenter was approximately 80 km northwest of capital city Kathmandu. Nepal lies on the boundary between the two massive Indian and Eurasian tectonic plates that collided to form the Himalayas. The continuous subduction of the Indian plate under the Eurasian plate caused the earthquake which resulted in widespread damage of civil infrastructure such as buildings, temples, bridges, highways and liquefaction occurred mainly in the central part of the country which includes Kathmandu, Lalitpur, Bhaktapur, Sindupalchowk, Dolakha and Gorkha districts. This paper compiles the occurrences of liquefaction and liquefaction-induced damage in various regions of the Kathmandu Valley during the 2015 Gorkha Earthquake and related aftershocks. Kathmandu Valley was once a large lake and the ground conditions comprise of lake and river sediments known as lacustrine deposits which are susceptible to liquefaction. The lacustrine deposits are up to about 500 m thick and form interbedded layers of clay, silt, sand and gravel which are variable in extent and thickness. Liquefaction potential in these deposits has been studied and pointed out by many researchers in the past. Clear evidence of liquefaction in the form of sand boils, lateral spreading and ground failures were observed in many regions of the Kathmandu Valley following the Gorkha Earthquake. The location of ground failures induced by liquefaction was limited to edges of Kathmandu basin than that in the basin interior due to the basin edge amplification effects. However, no major ground failure occurred and the extent of damage due to liquefaction was rather limited. One reason might be the low intensity of the ground motion (PGA ranging from 0.16-0.2g) which may have generated cyclic stress ratios (CSR) marginally higher than cyclic resistance ratios (CRR) locally and therefore rather limited to “marginal” liquefaction. Other reason might be the groundwater level; which was substantially low due to the dry season and continuous groundwater extraction.

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