If coal seams are mined by longwall mining, the original stress balance in the rock mass is changed. The stress increases around the mined-out area. As a result, increased stress causes compression around the excavation. The compression around the excavation is shown by exact measurable subsidence. The impact of mining can be theoretically determined by a limiting impact angle. The paper deals with the assessment of mining in the ninth and seventh blocks at Karvina Mine Lazy Plant and its influence on the surface. The process of failure of the mining seam's rigid roof in both areas will be explained. On the basis of spatio-temporal relationships of real surface subsidence, mined longwalls, and geomechanical events, the character of overlying rock deformations can be evaluated. The character of rigid overlying strata will be evaluated by means of an inflexibility coefficient which has the advantage of including the main factors of influence: the thickness of the seam and the rigidity and compactness of the overlying strata. It is possible to specify the time of breakthrough by synchronous evaluation of seismic events at the mining seam's overlying strata because surface measurements are usually done once every half-year.

1 Introduction

In every case, measured subsidence at the surface occurs as a result of higher stress, which arises in the surroundings of mined areas (Jirankova 2010). Determination of the value of subsidence at the surface with regard to the extent and thickness of mined longwalls is important for distinguishing situations where the deformation of solid overlying strata occurs (Mučková et al. 2010). In many cases a strutting vault forms over the mined area and breakthrough of the whole thickness of the intact firm overburden does not occur. At the origin of the strutting vault this can lead to an enormous concentration load of formation and the occurrence of geomechanical events. But in cases where the intact overlying strata break, the breaking cannot extend further. Incurred overhangs of intact firm strata by its restraint to the no undermine overburden contribute to considerable additional load on the affected areas.

Dimensioning of the mined-out area in which get in concrete conditions to breakthrough unyielding hanging rock strata has a very important meaning for consideration of the changes in the state of stress in rock massif. It is possible to obtain the proportions of the mined-out area at the time of breakthrough by backward evaluation of mine surveys and seismic observations in the locality in which they exist (Jirankova et al. 2012). Backward evaluation also provides a survey of the break overburden earlier dig for seam which has substantial meaning for the correct interpretation of the actual evaluation break overburden at the same time dig for strata.

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