Instability mapping of Dhaka-Kasiani-Gopalganj railway line in Bangladesh with InSAR time series analysis

Previous studies on land subsidence identified Bangladesh and the Ganges-Brahmaputra Delta (GBD) region as a high-risk zone that is vulnerable to sea-level rise and climate change. This regional subsidence will affect the stability of linear rail infrastructure. The possible effect of this subsidence on rail infrastructure health was not scientifically examined before. Dhaka-Kasiani-Gopalganj Railway, which connects the capital city Dhaka to Gopalganj city in the southern part of Bangladesh, is chosen as a case study. This railway is about 150 km long, and most of the stations, except the ones within Dhaka city, have been newly constructed during the last few years (construction started in late 2016). In this study, SBAS-InSAR time-series analysis was used to extract LOS displacement along the 60 m buffer width of the railway embankment. Sentinel-1 A images from both ascending and descending passes were used covering the temporal resolution from January 2020 to October 2023. A mean displacement rate of around - 10 mm/year was observed for ascending satellite line-of-sight LOS direction and - 14 mm/year for descending LOS direction. The ascending LOS displacement rate was compared with GPS data to check the instability trend, and the result showed an agreement in LOS displacement trend. The 2.5D analysis was also conducted to compute the quasi-vertical component along railway line for better understanding of local instability. A mean quasi-vertical displacement rate of approximately - 16 mm/year was observed along the railway line, and it indicates that the railway embankment is subsiding in general. To enhance the accuracy and reliability of displacement interpretation, InSAR measurements were supplemented with contextual analysis incorporating land use/land cover (LULC) and soil data. In regions covered by "Crops" land cover type, a high rate of quasi-vertical displacement was observed indicating that railway line passing through paddy fields are highly prone to instability. Similarly, significant displacement rates were noted in "Deltaic silt" soil type, followed by "Alluvial silt and clay" and "Marsh clay and peat" type. Railway embankment built on these soil types should be prioritized for inspection. The threshold displacement velocity value was also defined based on the results so that it can serve as an early indicator of damage, aiding in the prioritization of inspection and maintenance work. This study aims to broaden the use of InSAR data in engineering practices in linear rail infrastructure health monitoring. Future research will focus on a detailed analysis of current damage conditions compared with satellite-based instability results.