From flooded mine workings the regulated descent of water with a large hydrostatic head can be ensured at reliable determination of the size of the target left between the flooded mine workings and the face of the slipway, through which the water is planned to be discharged. To calculate the size of the pillar it is necessary to know the compressive strength of rocks, which is usually determined by samples at natural moisture.

The zone of inelastic deformations resulting from mining operations, as is known, is characterized in most cases by cracking, developing in time. On the other hand, after a certain period of time, the fracturing of rocks may decrease due to their compaction, i.e. closure of cracks.

The initial parameters in solving problems related to the manifestations of rock pressure are rock strength indicators. The main of them is the compressive strength. As a rule, it is determined in the laboratory on samples of the rocks under study. There are also known production methods that allow to determine the ultimate strength directly on the surface of rock outcrop (face, wall of excavation, roof) or at some distance from the contour of excavation, in a borehole. However, these methods are imperfect, labor-intensive and complex ...