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Effects of abandoned shallow bord-and-pillar coal workings on surface development

Abstract The relevance and potential effects of the factor of safety, time and subsidence are considered in the context of abandoned bord-and-pillar mines. Stability during the period of underground mining safety is normally assured by use of a safety factor, but calculation of the time to failure of the pillars when a mine is abandoned has not been adequately addressed in the literature. Coal pillar information gathered from a worldwide review was subjected to statistical analysis and a method for the assessment of stability, linking factor of safety to time, subsidence to rock mass rating and the extent of surface subsidence to peak pillar vertical stress was developed for spreadsheet application. Factors of safety for a proposed development should be site-specific to the form of development; general factors of safety should not be used. Where a desired factor of safety cannot be met measures to secure the site will be required.