SAR Monitoring and Finite Element based Stability Modeling for the Zero Landslide in the Darjeeling Himalayas, India

Landslides are one of the recurring geological hazards in the Indian Himalayas, often leading to loss of life and economy. For the present study, the Zero landslide located in the Darjeeling Himalayas, India has been investigated. This landslide was first activated on July 16, 2014 and its subsequent occurrences have affected a total area of 1×10⁵ sq.m. Field investigations revealed that a local school building, its nearby roads and a few residential buildings are at risk from this landslide. Therefore, monitoring and stability modeling becomes imperative to assess the associated hazard level. For the studied case, the Small Baseline Subset Interferometric Synthetic Aperture Radar (SBAS-InSAR) technique was applied to monitor the surface level deformation. For this purpose, Sentinel-1 SLC images captured from January 2022 to November 2023 were collected, and processed using the HyP3 and OpenSARLab platforms. The SBAS-InSAR results revealed maximum subsidence i.e., Line-of-Sight (LOS) velocity (cm/y) of –8.2 and –11.5 for ascending and descending orbit directions, respectively. The presence of transverse tension cracks in the crown and flanks of this landslide supports the SBAS-InSAR results and indicate an active sliding. Furthermore, to assess the slope stability, continuum based two-dimensional finite element modeling (FEM) was carried out. For this, the Shear Strength Reduction (SSR) method was employed in the FE analysis to compute the safety factors for different scenarios. To incorporate material properties within the configured FEM, the Mohr-Coulomb strength criterion was used for soil overburden, and the Generalized Hoek-Brown strength criterion was used for bed-rock profile. The FE analysis revealed a critical Factor of Safety (FoS) value of 1.07 for dry condition and 0.78 for wet condition (R_u). The obtained FoS values suggest that the studied slope section is marginally stable in dry condition; however, instability may be induced during a rainfall event in future. Based on these findings, the design and implementation of landslide risk mitigation measures have been encouraged prior to any major landslide event at the study location.