Impact of geotechnical factors on secondary extraction of coal

The literature review identified geotechnical factors impacting on unplanned secondary coal extraction. These factors are grouped into nine broad classes of factors; namely, stratigraphy, rock /coal engineering properties, spontaneous combustion, discontinuities, igneous intrusions, collapse of previous workings, hydrology, stress environment and the primary mining parameters. It was found that for underground extraction stratigraphy, igneous intrusions, collapse of previous workings and the stress environment have the most impact on the rock mass behaviour (i.e. the roof caving and rock burst potential, rib and pillar stability, floor heave and roof / surface subsidence), roof support (both on a mine- and panel- scale) and explosions/ignitions (gas conditions, as well as spontaneous combustion). For open cast extraction the stratigraphy, igneous intrusions, collapse of previous workings, the stress environment and the primary mining parameters had greatest impact on the rock mass behaviour (slope and high wall stability and floor heave) and explosions / ignitions.
Selected collieries were then visited in order to quantify the impact the factors may have on resource/reserve estimation and safety. This resulted in the identification of additional geotechnical factors with impact on secondary extraction, as well as a tenth class of factors – secondary mining parameters. The factors impacting on safety, as well as which exploitation phase (i.e. mine planning and/or production) they affect, were identified. These factors belong to all ten classes; those with most impact are primary and secondary mining factors and stratigraphy. The collieries quantified the factors according to their impact on safety; a quantification of 10 implies the factor has a high negative impact, while a quantification of 0 implies the factor has no negative impact. The factors were evaluated considering the selection of a suitable mining method (open cast or underground) and the negative impact on underground and open cast extraction. Those impacting most on underground extraction are the sequence of pillar extraction, the competency of the roof, the caving mechanism employed, whether more than one seam is to be extracted, the secondary safety factor (before re-mining takes place), the sequence of fender extraction, whether the surface infrastructure requires to be protected or not and the condition of the pillars. The factors with greatest impact on open cast extraction are spontaneous combustion (geology, coal and mining components), standing water bodies, vertical and lateral coal quality variation and the size and geometry of the remaining reserves.
The potential impact of geotechnical factors on the classification of a deposit as a resource or reserve (according to the coal-specific portion of the SAMREC Code) was determined. It was found that certain factors will prevent the upgrading of a resource to reserve status (i.e. those factors affecting the economic viability of a deposit, the reserve accessibility and reserve sterilization), while others may have a similar effect if unexpected coal losses occur during mining (e.g. factors that affect dilution, contamination and yield reduction).
In summary, standing water bodies, spontaneous combustion (the geology, coal and mining components), the competency of the roof, the condition of pillars, the time elapsed since the primary extraction, and the presence of dykes and sills are the most critical geotechnical factors in terms of safety, mining method selection, open cast and underground operations and with regard to potential adverse effects on the resource / reserve classification.
It would be misleading to present a list of factors that are critical for each operation, as every site is unique. However, this report can serve to identify which factors are most likely to be important and to educate individuals in the existence, effect and impact geotechnical factors can have on secondary extraction. They will then be in a position to judge which factors will be significant in that particular operation and to conduct the necessary further investigations. Chapter 9 gives further information regarding application of the findings of this report.
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