| Most underground coal mines in the U.S., both now and in the past, use room and pillar mining methods. The onset of subsidence over abandoned room and pillar mines may be delayed for many years after mining and has been considered difficult, if not impossible, to predict. The objective of this research was to develop empirical methods of predicting the onset of subsidence. Site-specific mining and geologic data were collected on 80 subsidence events. Statistical analysis of this data indicated that subsidence characteristics are strongly influenced by the competence of materials in the lower half of the overburden. The type of subsidence, whether it will be of pit or sag type, is determined by seam depth, the percentage of unconsolidated material (soil) in the overburden, and the percentage of competent material (mainly limestone and sandstone) in the lower half of the overburden. For sag subsidence, the delay between mining and subsidence is influenced primarily by pillar stress and by the competence of material in the lower half of the overburden. Maximum sag subsidence delays, for a given pillar stress, are associated with those sites having 20 percent, or less, competent strata in the bottom half of the overburden. Considerable variation is found in those sites having more than 20 percent competent materials in the lower half of the overburden. |
