Mining Publication: The Effects of Roof and Floor Interface Slip on Coal Pillar Behavior

Designing coal pillars to provide resistance against overburden and gob loads has long been an aim of rock mechanics engineers. This requirement has become more imperative as greater overburdens are encountered and when mining in stiff coal-bearing strata. Current design procedures rely on theories of coal pillar behavior that take into consideration a common hypothesis. This hypothesis states that the elastic core is surrounded by an inelastic yield zone. The distribution of stress at low-to-moderate pillar loads has been effectively defined by this hypothesis. However, it suffers greatly when applied to large width-to-height (w/h >10) coal pillars under considerable overburden (>500 m). In these situations, the hypothesis says the elastic core can achieve unrealistic stress states, giving the pillars extremely high calculated strength. A growing body of field studies has shown this is not the case. It has become clear that some other mechanism must be involved. This U.S. Bureau of Mines paper discusses the importance of an interface slip mechanism between the coalbed and the surrounding strata in controlling the extent and pattern of stresses and deformations in a coal pillar.