Impact of urban construction land gradient expansion on land subsidence in mountainous cities

The expansion of construction land on slopes in mountainous cities like Lanzhou has addressed the shortage of flat land but compromised slope stability, leading to uneven land subsidence and risks to infrastructure. This study assessed the land subsidence before and after urban expansion in five areas of Lanzhou by using slope spectrum construction method and gradient expansion intensity measurement that integrated with SBAS-InSAR technology. The results show that construction land on slopes over 20° increased significantly, accounting for 16% of new construction land. The average slope spectrum index was 4.02, with the upper slope limit rising by 8.2°. The land subsidence rate threshold increased by 10 mm/a, and the proportion of pixels experiencing subsidence greater than 5 mm/year rose from 3.63% to 5.24%. Increased construction intensity on slopes caused higher and faster subsidence, which diminished with greater distance from the expansion areas. Areas with slopes between 10° and 25° saw the greatest acceleration in subsidence. Geological composition, building density, groundwater exploitation, and cut-and-fill thickness collectively influence land subsidence rates. This study provides a scientific basis for mitigating geological disaster risks and promoting safe urban development in mountainous cities.