The Himalayan state of Sikkim is prone to some of the world’s largest landslides, which have caused catastrophic damage to lives, properties, and infrastructures in the region. The settlements along the steep valley sides are particularly subject to frequent rainfall-triggered landslide events during the monsoon season. The region has also experienced smaller rock slope failures (RSF) after the 2011 Sikkim earthquake. The surface displacement field is a critical observable for determining landslide depth and constraining failure mechanisms to develop effective mitigation techniques that minimise landslide damage. In the present study, the persistent scatterers InSAR (PSI) method is employed to process the series of Sentinel 1-A/B synthetic aperture radar (SAR) images acquired between 2015 and 2021 along ascending and descending orbits for the selected areas in Gangtok, Sikkim, to detect potentially active, landslide-prone areas. InSAR-derived ground surface displacements and their spatio-temporal evolutions are combined with field investigations to better understand the state of activity and landslide risk assessment. Field investigations confirm the ongoing ground surface displacements revealed by the InSAR results. Some urban areas have been completely abandoned due to the structural damage to residential housing, schools, and office buildings caused by displacement. This paper relates the geotechnical investigations carried out on the ground to the data obtained through interferometric synthetic aperture radar (InSAR), focusing on the triggering mechanisms. A strong correlation between seasonal rainfall and landslide acceleration, as well as predisposing geological-structural setting, suggest a causative mechanism of the landslides.

Chittagong, the second largest city of Bangladesh, is mostly developed in the hilly areas. Hills are mostly covered with loose and weathered tertiary sedimentary rocks susceptible to landslides. Generally, during heavy rainfall within a short time, water infiltrates into the loose rocks and soils, which increase the pore water pressure, and finally exceeds the shear strength of the materials to initiate landslides. Landslides history, slope characteristics, geology and geotechnical properties have been considered to divide Chittagong City and surrounding areas into four landslide hazard zones. Zone I in the east of Chittagong City persists as the most susceptible area for landslides, and the city becomes gradually less susceptible further westward up to zone IV. Prevention of landslides in most cases requires costly engineering solutions. Therefore, mitigation through timely evacuation of people from hazardous areas might be a good solution to combat landslides. Two automated rain gauges are installed at high risk landslide zones with built-in rainfall threshold values. Threshold values for early warning of landslides are set after statistical analysis of past landslides in relation to total amount of rainfall and local geology. The threshold values of rainfall are set to 100 mm in 3 h, 200 mm in 24 h and 350 mm in 72 h.

Chittagong, the second largest city of Bangladesh is mostly developed on the hilly areas. Hills are mostly covered with loose and weathered Tertiary sedimentary rocks susceptible to landslides. Deforestation and hill cutting are also accelerating the destructive process of landslides considerably. Generally, due to heavy rainfall, within a short period of time, water easily infiltrates into the loose rocks and soils which increase the pore water pressure and finally exceeds the shear strength of the materials to initiate landslides under the influence of gravity. On the basis of landslides history, slope characteristics, geology and geotechnical properties Chittagong City and surroundings have been divided into four landslide hazard zones. East of Chittagong City, zone-I is most susceptible to landslides and the city is progressively less susceptible further westward up to zone IV. Batali hill, Bayjeed Bostami, Chittagong Cantonment, Chittagong University Campus and their surrounding areas have been identified as most susceptible areas to landslides. Prevention of landslides in most cases requires costly engineering solutions. Therefore, mitigation through timely evacuation of people from landslides susceptible areas is the best solution to combat landslides. Two automated rain gauges installed in high risk zone with built-in rainfall threshold values. Threshold values for early warning of landslides are 100 mm in 3 h, 200 mm in 24 h and 350 mm in 72 h. These rain gauges send real time early warning to the registered mobile phones so that first responding organizations can take action straight away before landslides happen.