Mechanism of paleo landslide reactivation in the Himalayas: Insight into Kodari landslide of Sindhupalchwok District, Nepal

The reactivation of ancient landslides has become a significant issue in the Nepal Himalayas in recent years. Many people have lived on the historical accumulation of landslide debris for a long period and now face the risk of reactivation. This study examines the deformation mechanism of the Kodari reactivated landslide in Sindhupalchok District of Nepal, which began to reactivate in July 2015 and remains active. Soil samples from twenty distinct locations within the reactivated area were collected. A multistage direct shear test on unsaturated soil was performed to determine shear strength characteristics. The plasticity index and soil composition were derived from laboratory analysis. A comprehensive field examination elucidated the intricate details of the landslide mechanism.The soil samples demonstrate a cohesion value between 0.7 and 5.5, whereas the angle of internal friction ranges from 26 to 33.3. The soil displayed a plasticity index between 1.7 and 5.5. Of all the samples, seventeen are loose and non-compact, whereas three are dense. The results of the gradation research reveal that the soils are classified as sandy loam, exhibiting a relatively low plasticity index and low cohesion. The cumulative rainfall throughout the monsoon season from 2015 to 2023 varied between 2400 mm and 2900 mm. The precipitation levels during the early monsoon season surged significantly since 2019. The severe rainfall during the pre-monsoon period, following an extended dry season, resulted in surface deformation in the studied area. Moreover, unconsolidated soil exhibiting low cohesion and a low plasticity index underwent deformation due to the 7.8 magnitude Gorkha earthquake and subsequent aftershocks. 118 dwellings and 220 individuals are at risk of Kodari landslide reactivation.Keywords: Landslide reactivation, settlement threat, Nepal Himalaya, soil investigation