Karstified Bedrock, and the weak cover on it, is most important boundary condition in sinkhole formation at site conditions susceptible to dewatering. Such contacts provide a balance between the unconfined and granular aquifers, fed by surface water and the confined aquifer located at the base. Excessive groundwater withdrawal from one of these aquifers causes interaction throughout their contacts. Six drainage borehole further operated about 20 days of after the dewatering process, which increased 2-3 times compared with the amount discharge in the same period of the previous year, in the Çöllolar Open-pit Mine. The first one, week after the new boreholes put into operation, four sinkholes formed at the edge of the coal basin, at a distance of 200-350 m to the borehole occurred until June 2015. Later, four more sinkholes were formed, one in the same month, the other two in December 2015 and one in March 2017 despite the reduction of ground-water withdrawal. In this study, the hydrological, the result of numerical analyses of a conceptual modal built by considering geological, hydrogeological, geo-mechanical boundary conditions and dewatering process in the region are presented. As a result of the numerical analysis, it was determined that there is a significant decrease in pore water pressure due to extreme water withdrawal from the karstic aquifer and a significant increase in the vertical effective stresses on the sinkhole forming area. Moreover, internal erosion develops between the limestone and coal-bearing gyttja contacts, and the weak upper cover composed of young sedimentary layers collapses into karstic cavities in the base rock due to the gravity.
It is recorded that numerous karst-related sinkholes formed on the edge of the lowlands due to dewatering process. There are more than 4 000 sinkholes occurred due to dewatering on the coal mining sites between 1900 and 1980 years in Alabama, United States (Newton, 1984). Unconsolidated and weak sedimentary material on karstic cavities, migrate into the voids of underlying base rock by losing of buoyancy support. The increase in velocity of groundwater (internal erosion), lowering of groundwater level below unconsolidated cover, and surcharge are the most important reasons of sinkhole occurrence in similar areas. Susceptibility to sinkhole occurrence of an area depends, firstly, on the existence of carbonate base rocks containing karst structure and loose, weak and saturated cover on it. The elevation of groundwater level is generally related to the topography, geological characteristics, artificial recharge and water withdrawal. The geological, hydrological, geomorphological, hydrogeological and geo-mechanical conditions at the edge of the Afşin Elbistan Coal Basin, where the western slope of the Çöllolar Open Pit Box located resemble the features described above. In this study, the valid boundary conditions and the effects of dewatering for slope stability are explained and the results of the numerical model are evaluated considering the karst dewatering during February 2015 in studied area.
