Geneza zapadliska w rejonie wschodniego zbocza wyrobiska kopalni piasków szklarskich w Osiecznicy

The “Osiecznica” sandpit exploits quartz sands withinthe North Sudetic Depression, SW Poland. In June 2002, near the upper edge of the eastern slope of the pit, the terrain sagged. It was originally assumed, that the sag resulted from vertical flows of Cretaceous, Cainozoic, and technological water. However, field investigation revealed a discontinuity of the undergound water flow within the eastern pit slope. Intitially horizontal filtration changed into vertical, locally turbulent flow, able to wash out up to 90% of sand fraction (suffusion). In the area of the eastern slope, there are underground washouts forming cavities over which terrian depression may originate. Such phenomena and underground water outflows from caverns at the slope base, as well as stream draining the pit floor do not pose any danger to the slope stability. Forestation of the first slope level will additionally stabilize it, despite the occurrence of underground caverns.