Susceptibility mapping for sinkhole occurrence by GIS and SSI methods: A case study in Afsin-Elbistan coal basin

The major structural defects that cause the displacement in rock are discontinuities (fracture-joint-fault) and karstic cavities. Depending on the position and geometry of the karstic cavities, sinkholes occur both within the bedrock and in the cover layers. Occurance of sinkholes primarily depends on existence of carbonate rocks containing sub-surface karstic cavities and loose and weak cover layers. Following the landslides occured in 2011, it was planned to reduce the hydraulic head of the karstic aquifer which lies at the bottom of the coal-bearing layers in order to restart the mining activity in Afşin-Elbistan Coal Basin. For this purpose, dewatering wells were drilled in January 2015. Two weeks after the drilling, the first sinkhole occured at the southwestern edge of the basin and during the following six months four other sinkholes occured close to the first one. The geological, hydrological, hydrogeological and geo-mechanical models of the basin has been composed in order to understand the mechanism of sinkhole occurence in the Afşin-Elbistan Coal Basin. Thematic maps showing the spatial distribution of parameters which determine the formation of sinkholes were obtained by using a Geographic Information System (GIS) based analysis method. Analytical Hierarchy Process (AHP) approach, which is one of the multi-criteria decision-making analyzes, has been adopted in determining the impact and weight coefficients of each effective parameter which plays a role in occurance of the sinkholes. The Sinkhole Susceptibility Index (SSI) was calculated by using all parameters which were classified and weighted. The SSI refers to the susceptibility of sinkhole occurance. The higher value of the SSI means that the risk of potential occurrence of a sinkhole is high. Calculated SSI in the study area ranges from 9 to 110 and the higher values were obtained for the area corresponding to the margin of the basin where the sinkholes occurred.