These features were caused by natural and anthropogenic factors, and occur in zones with thin roofs, where pillars were affected by pillar robbing,or on locations with inappropriate sewerage systems above quarries. The formation of sinkholes was often reported in spring during years with high moisture contents in the overburden, caused by high groundwater recharge or above average precipitation. With increasing time since quarry abandonment, quarries become more susceptible to pillar creep and bending of the roof.
Hence, if no appropriate mitigation measures are taken to reduce such deterioration processes, the number of sinkholes is expected to increase.