An Overview of Chimney Subsidence Above Coal Mines

Chimney subsidence can cause surface subsidence (ground settlement) in the form of a pit or sag when there is a mine roof collapse in a mine entry. Surface subsidence occurs when the roof collapse propagates upward and does not choke itself off due to the associated volume expansion of the rubblized fallen materials and where no sufficiently competent rock is present to bridge the cavity. This paper contains a review and evaluation of reported case history data associated with occurrences of surface subsidence related to chimney subsidence. This analysis included investigating the chimney subsidence height potential including relative to the overburden rock conditions, extraction height, mine depth, the width of entry and the roof rock fall distance. Also, this paper provides a summary of the results of an assessment of the bridging capacity of various more common competent coal measures in the roof. This numerical analysis was performed considering linear arch conditions. Parameters which were investigated in this analysis included the competent roof rock strength, thickness, depth, and spanning distance.