A massive landslide damaged NHPC's Teesta V hydropower station on 20th August 2024 at Balutar, Sikkim Himalayas

A gigantic landslide occurred at Dipu-Dara near Singtam of Himalayan State Sikkim above the powerhouse of the NHPC's (National Hydroelectric Power Corporation) Teesta stage V dam power station on August 20, 2024. The Gas Insulated Switchgear building of the powerhouse was partly smashed and this landslide also affected the six residential buildings. The large cracks developed along the Singtam-Dikchu road which is the significant lifeline of Gangtok Town as well as North Sikkim. This study attempts to identify the principal causes and probable effects at the proximity region. Here, the geotechnical investigation has been done for slope stability using the limit equilibrium method (LEM). A total of three slopes (rock-debris) have been considered and the physical properties of the slopes have been systematically measured (slope material, angle, orientation, height, etc.) during a field survey in September 2024. SAR (C-band) imageries (Synthetic aperture radar, Sentine-1A) have been used here for InSAR coherence analysis before (09-08-2024) and after the event (21-08-2024). Results showed that most of the slopes (above 45°) along the riverside of Teesta are characterized by unconsolidated loose soil-forming materials of Phyllitic rock. At the time of GLOF, 2023, the slopes near the powerhouse were affected by the devastating flood through toe erosion. Here, this rock type experiences alternating dry and wet cycles which weaken its mechanical strength, develop cracks, and trigger slope failure. This 17 ​km NHPC headrace tunnel runs through fragile phyllite, schist, slate and quartzite rocks and it reduces the rock strength. Numerous past earthquake epicenters (the highest 5.45 magnitude, 2013) are also located between Dikchu and Signtam. The result of the LEM showed that the safety factor value of the landslide slope was 1.069, representing a little stable slope. This study will help policymakers for long-term sustainable hillslope as well as landslide management, particularly for the Himalayan tourism industry and border security.