Fig. 1 Horizontal strain of surface above cross-cut (Shevelevskaya mine)
Fig. 2 Horizontal rock strain in cross-cut (Shevelevskaya mine)
Fig. 3 Horizontal strain of surface (Yu.A.Gagarin mine)
Features of profile distributions of horizontal strain are considered for a case with a two-meter measurement step. Extremely non-homogeneous rock mass strain distributions with peak-shaped concentrations are found in this case. Fourier analysis was performed to reveal deterministic component in these spatial distributions. The analysis resulted in periodograms of the highly non-homogeneous deformation. Basing on this study the conclusion may be made that a certain regularity is present in concentrations of deformation both near the opening and on the earth surface. The peak-shaped strain concentrations are due to natural hierarchical block joint structure which is in agreement with the influence of natural joints of various scale-levels on deformation of rock mass.
Processes of rock displacement under the effect of mining are of much importance among geomechanical factors for effective and safe rock exploration. Since engineering calculations of rock displacement are mainly based on regularities resulting from field measurements of rock displacement, methodological problems of prediction of rock displacement and deformation, processing and interpretation of the results are of a great importance for the practice [1]. Difficulties in solution of these problems are due to the fact that spatial distribution of rock strains found by measurements when the measurement step is reduced to several meters becomes sharply inhomogeneous and rather chaotic with acute strain peaks often having different signs. A question arises therefore whether these strain peaks have a meaning for the prognosis of dangerous geomechanical events, necessity and scale of strain averaging etc. We think that new solutions for processing and analysis of rock displacement measurements may involve implementation of methods for analysis of signals with complex structure basing on Fourier and wavelet analyses [2]. The paper considers results of Fourier analysis of strain profile distributions. Conclusions on spatial periodicity of induced strains of the rock are made.
The paper analyses profile strain distributions of earth surface made previously. Measurements of rock displacements were made at the Shevelevskaya and Yu.A.Gagarin mines (Donbass region). In the first case a special profile line of ground benchmarks at 2 m intervals was placed on the earth surface. In the second case a sequence of rigid wall benchmarks were placed at 2 m intervals and at a 0.5m depth from the rock exposure surface in cross-cut (the 176 m horizon). In the third case surface station No.158 (the Yu.A.Gagarin mine) was placed on the earth surface to monitor subsidence activation near the village. The active phase of rock displacement was induced by large-scale excavation at lower horizons. Horizontal displacements of pins with respect to each other were recorded during measurements. Accuracy of the displacement measurement was 0.1mm. Rock strains were calculated basing on the displacements. Magnitude of the effect of excavation in comparison with inter-benchmark distances suggested a rather homogeneous technogenic effect on rock mass under monitoring. The peak-shaped strain concentrations was caused by local rock mass inhomogeneity. Figures 1 to 3 demonstrate distribution of horizontal strains at about three years after benchmark installation. As seen, the strain distribution is markedly inhomogeneous with individual strain peaks having different signs. The earth surface monitoring was continued for more than a decade, changes in corresponding strain distributions were mainly due to rises in individual strain peaks.
Fig. 1 Horizontal strain of surface above cross-cut (Shevelevskaya mine)
Fig. 2 Horizontal rock strain in cross-cut (Shevelevskaya mine)
Fig. 3 Horizontal strain of surface (Yu.A.Gagarin mine)
As found by visual examination, sharp strain peaks usually correspond to natural cracks or contacts of rocks with markedly different properties that are activated under technogenic effect on rock mass. This phenomenon is clearly seen during bench formation in subsidence zone. Thus there is a reason to use strain distributions in analysis of regularities of natural joints.