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Displacements in the boundary selections of a subsidence trough on the basic of model tests performe |
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Displacements in the boundary selections of a subsidence trough on the basic of model tests performed on a vertically heterogeneous loose mmedium Authors: J. Cygan, J. Leśniak, J. Walaszczyk
The paper provides a brief review of model tests conducted in the Strata Mechanics Research Institute on vertically homogeneous loose media. The results of recent model tests on vertically heterogeneous loose media are discussed. Model tests on lose media were undertaken when new theories appeared that described rock deformations due to mining activities (Knothe 1953; Litwiniszyn 1953, 1956). The aim of those model tests was to verify the postulated hypotheses and to analyse the displacements due to boundary conditions reproducing various mining techniques. Tests revealed that the mining impacts in the loose media, such as sand, follow the normal distribution and so do the mining impacts in the rock mass (Litwiniszyn et al. 1956). This conclusion proves the usefulness of model tests on rock mass phenomena caused by mining operations. These findings in qualitative terms can be extended to rock mass, as well. The equipment used in model testing is described in detail in (Knothe 1980; Lesniak et al. 1989; Knothe et al. 2000). Application of a homogeneous, loose medium more or less compacted allowed for finding the differences in displacement processes occurring within such media. In qualitative terms these differences correspond to those encountered in the mining practice (during mining operations in undisturbed compact rock and rock mass disturbed by previous mining activities where loosening zones are observed: cavingins, cracking, stratification). These conclusions prompted the research work on loose, vertically heterogeneous media (horizontal bedding planes), made from more and less compact layers. Accordingly, the differences in displacement processes in rock mass disturbed by more and less extensive earlier mining operations can be observed. The findings might be utilised to prognosticate the surface impacts of mining operations. Six series of experiments on homogeneous and vertically heterogeneous media were performed, the number of layers considered in the experiments would vary. Subsidences and horizontal displacements in the boundary section of the subsidence trough were analysed against the horizontal contours and against the vertical line for the vertically homogeneous loose medium (N) and compacted medium (Z) and for the vertically heterogeneous medium made of two layers differing in the degree of compaction (N-Z, Z-N). The medium made from three layers was considered, too (N-Z-N, Z-N-Z). The results lead us to the following conclusions: -the greatest subsidence and horizontal displacements are reported in the homogeneous, loose medium (N), the lowest values were obtained for the homogeneous compact medium (Z). In vertically heterogeneous media (N-Z, Z-N, N-Z-N, Z-N-Z) both subsidence and horizontal displacement assume the middle values (Fig. 3, Fig. 10); -In the vertically heterogeneous medium horizontal displacements are affected by the degree of compaction of the lower layer, in the direct vicinity of the spot where the boundary conditions are imposed (near the bottom of the model box); the layers lying higher up affect the horizontal displacements in a minor degree only (Fig. 4, Fig. 11); -The values of displacement near the surface in the vertically heterogeneous medium are not rela ted to the model height, in a minor degree are they affected by the actual proportion between the compacted and loose medium in the experiment (Fig. 3, Fig. 10). It is readily apparent that while prognosticating the impacts of mining operations the properties of various rock strata overlying the mined region must be taken into account. The strength of those impacts is associated with the stratum thickness and locality, the impacts are more pronounced when the layer lies near the mining level. Qualitative results might be utilised in the mining practice to prognosticte the impacts of projected mining operations, taking into account the mining history in the area.
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2011