Abstract
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 Afsin-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 Afsin-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.
Highlights
In unconsolidated geological environments, the drawdown of the groundwater level results in two different types of displacement: settlement and collapse
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
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
Summary
The drawdown of the groundwater level results in two different types of displacement: settlement and collapse. As the effective stress increases, consolidation settlement takes place especially in cohesive grounds Another factor causing the ground to settle down is the decrease in level of piezometric surface in the confined aquifers. The occurence of cover type sinkholes is generally caused by the erosion, transport and failure of the soils that overlie cavernous rock. Because of its low strength compared to rock, which, if left over a cave can still be strong enough to stand for a long period, a soil arch over a void is inherently unstable and its collapse can occur rapidly Their underlying mechanisms are extremely intricate and have traditionally received more attention from the geotechnical communities [8]-[11]. An index, which describes the susceptibility of sinkhole formation, was calculated for areas where similar boundary conditions apply; and a map, which shows potential areas of sinkhole occurance in the study area, is produced
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