Abstract
The article presents an example of using the electrical resistivity tomography method to assess the condition of shallow rock mass layers in the area of linear discontinuous deformations created in the past due to underground mining activity. The research concerned the mining area of one of the Upper Silesian Coal Basin mines, where intensive mining operations have been conducted for several decades. In the considered area, linear discontinuous deformations were created in the form of ground steps. Their location is related to characteristic layout of deposit accessing roadways and extraction fronts in several coal seams. The article analyzes the geological structure of the deposit and the state of deformation of the rock mass caused by mining operations. In order to evaluate the hitherto impacts, appropriate calculations of the extraction influence were performed, assuming different views on the summation of horizontal strain in long time intervals. The calculations were carried out using the theory of W.Budryk - S. Knothe. Theoretical considerations were supplemented with geophysical surveys performed with using electrical resistivity tomography. Obtained results of the near-surface layers ERT imaging in the form of resistivity profiles confirm the existence of rock loosening zones in the areas of discontinuous deformations occurrence.
Highlights
Underground mining extraction causes two types of surface deformations : continuous where no visible breaks in near surface layers is visible and discontinuous - where discontinuity on the surface is observed and some relative movement on both sides of discontinuity occurred in vertical and/or horizontal direction
The aim of the study was to confirm the usability of ERT imaging for recognition of the threat of discontinuous deformation arising in the areas of intensive underground mining operations
The second important factor influencing the anomalies visible in ERT image is a shear-slip of fractured rocks in the vicinity of system of extraction edges, which revealed on the surface in the shape of linear discontinuous deformations
Summary
Underground mining extraction causes two types of surface deformations : continuous where no visible breaks in near surface layers is visible and discontinuous - where discontinuity on the surface is observed and some relative movement on both sides of discontinuity occurred in vertical and/or horizontal direction. Continuous deformations manifest on the surface in the shape of so-called subsidence troughs They always accompany underground mining - with deeper extraction they tend to be more smooth but occupy wider area on the surface [2, 7, 10]. In Upper Silesian Basin they are not as common as it was in the past, because of greater depth of extraction nowadays and connected with it different mechanism of their creation. It is usually tied with instability of old shallow workings caused by influences of presently performed deep extraction
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