Stability assessment of entry roofs in underlying multiple seam mines

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Virginia Tech

Entry roofs in underlying multiple seam mines are vulnerable to failure under the influence of stress transferred from the upper seam workings. Overburden depth, innerburden thickness, percentage of hard rock in the innerburden and relative location of pillars in the two seams are some of the factors known to influence stress transfer and ensuing instability in the lower seam. This study was undertaken to quantify the effects of these factors and to develop a comprehensive roof stability assessment model capable of predicting any instability in the lower seam entry roofs under given conditions of geology and loading.

The finite element analysis technique was used to model a range of loading conditions and innerburden lithology. The innerburden thickness was varied from 50 to 20 feet. The innerburden lithology was varied in terms of percentage of sandstone and its location in the innerburden. Various loading conditions were simulated by changing the relative location of pillars in the two seams and by varying overburden depths from 300 feet to 1500 feet. Excessive horizontal stresses and unsymmetric loading were also simulated. The effects of these factors were quantified in terms of safety factors across the entry span.

The finite element results were used to obtain curve-fitted equations for computing safety factors at roof corner and midspan for given conditions of innerburden thickness, percentage of sandstone in specified location, overburden depth and pillar arrangement. These results were verified by back analysis of case studies and then incorporated into a comprehensive roof stability assessment model.