ICGCM Papers:
Pillar Design Workshop
 
 
Spatial Variability of Coal Strength and its Implications for Pillar Design
Pillar Mechanics Workshop Proceedings 2010
Spatial Variability of Coal Strength and its Implications for Pillar Design
by
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Murali M. M Gadde, Senior Manager, R&D Peabody Energy Co. St. Louis, MO
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[Book] Pillar Mechanics Workshop Proceedings 2010
[Topical Area] Pillar Design Workshop
[Author] 
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Murali M. M Gadde, Senior Manager, R&D Peabody Energy Co. St. Louis, MO
[Abstract] 
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Over the past four decades, much research has been done on the mechanics and design of underground coal mine pillars. Significant insights into factors governing pillar strength and loading were developed during this period. Well-tested empirical and semi-empirical approaches have been developed by all major coal producing countries to size pillars while considering specific local and regional geologies. Because of this, desirably, today’s empirical research on pillar design has a narrow focus with the principal aim of refining the standardized pillar design methodology that the industry in each country has adopted. At the same time, new frontiers are being explored with the use of advanced numerical modeling methodologies for pillar design. Despite these advancements, it is not uncommon that such fundamental issues as the validity of using compressive strength of coal for pillar design spur intense debate within the research community. There appears to be two principal reasons for the controversy over the use of compressive strength for pillar design: significant variability seen in the lab tests and ill-developed procedures to compute the in-situ strength. Both these concerns are highly valid as numerous empirical analyses have demonstrated in the past. In this paper, however, it will show that the variability seen in the laboratory compressive strength of coal is not such a big factor in estimating the “average” behavior of a coal pillar as it was thought in the past. Using some lab testing data from a coal mine, it will be shown that the uniaxial compressive strength variability noticed on a pillar-scale is no different from that observed on the mine-scale. Similarly, with the use of numerical modeling it will be demonstrated that accounting for such “local” variability of strength within a coal pillar will lead to similar estimate of pillar strength as that obtained from the average laboratory strength. With the help of some failed and stable pillar cases from the same mine, it will also be shown that significant differences exist between the seam-specific in-situ strength and the “national average” established in the past based on a larger database of case histories. Finally, the pillar cases from the studied mine also demonstrate that the popular Gaddy’s equation to estimate in-situ strength of coal is a reasonable first approximation and provides a decent explanation of the conditions noticed at the mine.