Various natural and man-made factors influence sinkhole hazards in urban areas. Key natural contributors, including soil permeability and geological and hydrogeological conditions, and anthropogenic factors such as defective sewer and water pipes and metro tunnels, play a significant role. In Tehran, internal erosion has been identified as one of the major causes of sinkhole formation. This process can result from both natural and human-made factors due to the special characteristics of the city. Certain areas of Tehran naturally consist of mixtures of fine and coarse-grained soils, highly susceptible to suffusion. Additionally, vast sections of the city are covered by non-engineered fills of varying thicknesses, typically uncompacted and thus vulnerable to internal erosion. In addition, sewer and water pipe networks spread throughout the city and are the primary source of soil erosion due to high-pressure liquid (water or sewage). Despite the recognized importance of soil erodibility in sinkhole formation, no prior study has attempted to map the erodibility of Tehran's soils. This paper aims to incorporate soil erodibility into sinkhole susceptibility maps for Tehran. The potential for internal erosion is assessed based on key geotechnical properties, including soil gradation, Standard Penetration Test (SPT) results, and the thickness of non-engineered fills. Two methods—Frequency Ratio and Simple Additive Weighting—are applied to develop these susceptibility maps. Results show that the Frequency Ratio method, which determines the weight of each factor directly from the sinkhole occurrence database, produced more accurate maps than Simple Additive Weighting. This study provides valuable insights into the relationship between soil erodibility and sinkhole susceptibility in urban environments, contributing to improved hazard assessment and mitigation strategies in Tehran.

Sinkhole Hazards; Internal Erosion; Suffusion; Non-engineered fills; Susceptibility Mapping