ABSTRACT
Another approach is to select support that would be able to provide resistance high enough to completely prevent separation of roof strata. Limiting factor in the latter approach is stress on the floor and roof, which must not exceed strength of the immediate floor and roof rock. Geomechanical considerations during SHP design process are related to selection of magnitude of reaction, which the support exerts on the roof, and also with selection of its load during relaxation. When the values are set too low excessive convergence may take place. Values of support resistance that are too high may on the other hand cause failure of roof and floor rock. Complexity of roof control in mechanized longwall faces demonstrates itself, among other things, in the necessity to achieve stability of the roof beam in front of the support, and in its good breakability behind the support. In most cases, the strongest rock in the productive series is coal, therefore the problem amounts to establishing the thickness of the roof beam as a function of set requirements. In specific case of the first seam of “Stara Jama” pit the problem became even more complex due to the fact that the upper cut of the seam was mined out. The sill pillar that separates goaf from the floor cut by the seam, which is to be mined now, is formed by 0,8 m thick layer of coal that was left in foot during primary mining operations and 0,4 m thick tuff sandstone below it. The floor cut of the seam, which is to be taken out in secondary mining operation is 2,8-3,5 m thick. Since the thin roof beam was found not to be sufficiently strong, the aim of the research was to determine the thickness of coal layer that would need to be left in the roof in order to provide stability of excavation in front of the support. The problem was solved with the use of finite element numeric models. Phase 2 software was utilized (2D finite element program for calculating stresses and estimating support around underground excavation), Rock Engineering Group, University of Toronto (Ljubojev et al. 2000). The basic model of finite elements on which the research was carried out is shown in figure 3.
