Analysis and prediction of subsidence caused by longwall mining using numerical techniques.

There have been several procedures for predicting subsidence caused by underground mining activities. This study presents a new approach called displacement equilibrium approach based on the history matching concept using the finite element method for the prediction of maximum subsidence. The general concept of this method is based on the assumption that total roof collapse occurs behind mine face as it advances. This assumption appears to be true for almost all longwall mining conditions. In this approach, the displacements at the mine roof are prescribed to be equal to, or a certain percentage of the coal seam thickness. Displacements and stresses in the overburden are then computed. Material bulking due to roof collapse and subsequent failure of overlaying strata is introduced as well. This study indicates that the magnitude of maximum subsidence can be predicted reasonably well using the displacement approach. In addition, reasonable predictions for shape of the subsidence profile can be obtained by introducing cracks in the overburden by using interface elements in the finite element formulation. Alternatively, cracks in the overburden can be simulated by using the modified displacement discontinuity method. In the displacement discontinuity method a number of discontinuity segments with unknown displacements magnitude are placed along the boundaries of the region to be analyzed. A computer code was developed to implement the modified displacement discontinuity method for simulating discrete cracks in the overburden. This program was then used in the analysis of subsidence at a number of longwall mine panels. This study indicates that inclined cracks propogating from the mine rib sides toward the surface would improve the predicted subsidence and strain profiles significantly, when compared to field profiles and predictions based on the analysis without cracks. The influence of surface topography and seam inclination on predicted deformation values at the ground surface was also investigated. This study indicates that location of predicted maximum subsidence and shape of the subsidence profile are significantly influenced by the surface topography and seam inclination. Surface topography also has significant influence on the magnitude of predicted horizontal displacements and strain values. (Abstract shortened with permission of author.)