# Liquidation of Shallow-Lying Post-Mining Excavations
## 1 Introduction  
The rock layers in which underground mining excavations have been made are in a certain state of stress caused by the weight of the layers lying above. The execution of mining excavations disturbs the original state of equilibrium, which causes the movement of rock mass particles and changes in stress [1]. The issue of the impact of underground mining on the surface is also of great importance in Poland, mainly due to the huge coal resources trapped in the protective pillars of Upper and Lower Silesia [2]. After a certain defined space in a coal seam has been exploited, the resulting excavation collapses: as the space is increasingly wider, the height of the caving also increases, but only up to a certain limit. The rocks, by crumbling and loosening, increase their volume so that at a certain height of caving, this increase completely fills the empty space after the coal seam has been extracted. The height of the caving is several times the thickness of the extracted seam [3]. At this height, the higher layers no longer collapse but only crack. This is the zone of cracks; its height is equal to the height of the caving zone. Above the crack zone, there is a deflection zone, which reaches the ground surface. At a sufficient depth, three zones are created above the seam being mined with caving: caving; cracks; and deflection. At a smaller depth of exploitation, the crack zone can reach the surface, whereas, at shallow exploitation, sinkholes are formed on the surface. This leads to a very important conclusion that for buildings and other objects on the surface, mining at small depths is the most dangerous because sinkholes or cracks are formed here; when mining at great depths, the ground only lowers, and sinkholes and cracks do not occur. The occurrence of sinkholes on the surfacedepends on the quality and type of rocks lying in the roof of the exploited deposit. Cracks, fissures, and slides occur most strongly in the part of the terrain that is located above the lower limit of exploitation. As a result of the extraction of the deposit in a certain area and filling the resulting void by the rocks of the overlying layers, there are displacements of the rock mass in the area above the extraction performed, reaching, if the deposit is extracted in a sufficiently large area, to the surface. As a result of these displacements, subsidence troughs are created above the selected part of the deposit at individual levels. Of particular interest is the subsidence basin that forms on the surface, where various types of objects are located. Depending on the size of the surface of the exploited part of the seam (deposit), the depth of the seam or the distance of the considered level from the seam, and the properties of the rock mass, one can distinguish between incomplete, complete, and overcomplete basins [4]. The results of many years of field observations and model experiments [5] allow for the distinction of direct influences, indirect influences, and secondary influences. Direct impacts occur as a result of filling the post-mining void (squeezing and filling with collapsing roof rocks). Indirect impacts occur as phenomena accompanying the displacements and deformations of the rock mass caused by direct impacts. These impacts include displacements and deformations caused by dewatering of the rock mass and changes in water conditions, changes in the structural properties of the subsoil of objects, and disturbance of slope stability. Secondary influences occur as a result of the activation of incompletely revealed influences of past exploitation caused by the impact of currently conducted exploitation. The share of indirect and secondary influences may be very significant in some cases, so they can significantly affect the displacement process, deforming the basin created as a result of direct influences of exploitation.