92-6 Mechanism of Ancient Mega Landslide Reactivation in the Himalaya: Insight into Kodari Landslide of Sindhupalchwok District, Nepal

Session: Landslide Inventory Mapping and Next Steps: Assessing Susceptibility, Hazard Models, Risk, and Policy

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Abstract:

The reactivation of ancient landslides has become a significant issue in the Nepal Himalaya 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. An estimated loss of approximately 2 million US dollars was incurred due to reactivation on the Kodari Highway section in the year 2016 alone. 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 noncompact, 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 compositional analysis of soil indicates a higher silt-sand composition ratio in the reactivated area. 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 as a result of the 7.8 magnitude Gorkha earthquake and its subsequent aftershocks. A total of 118 dwellings and 220 individuals are at risk of landslide reactivation in the research area.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8267

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