Safe conditions for coal mining under aquifers in the Velenje Coal Mine are the result of extensive research in the field of hydrogeology, drainage, geomechanics and planning. The area of research is called the Velenje basin, which contains a layer of lignite in the form of a slightly concave lens. Directly above the coal seam is a layer of marl or shale, called the insulation layer. Above the insulation layer are more or less permeable sands of the Pliocene era, which may contain accumulated water that poses a risk of water and sludge intrusion. In addition to Pliocene sands, triadic layers of different ages and lithologies located in the surface underneath also pose a risk of water intrusion.
The criteria for safe excavation under aquifers were developed in Velenje Coal Mine in order to prevent water bursting to the excavation areas. The criteria are still relevant and in use today. In order to comply with the criteria for safe excavation under aquifers, the permitted excavation heights must be calculated. Such calculation demands the following data: water pressure in the first sands, the height of excavation below the surface, the thickness of the insulating layer and the method of excavation (the excavation location in the layer according to the course of the demolition process).
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