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A "Cascading Pillar Failure" can occur in certain room-and-pillar mines when one pillar in a mine layout fails which transfers its load to neighboring pillars causing them to fail, and so forth. Whether failure occurs in a stable, nonviolent manner or in an unstable violent manner is governed by the local mine stiffness stability criterion. To apply this stability criterion, a boundary-element-method computer program with a strain-softening material model calculates the local mine stiffness and the post-failure pillar stiffness. The behavior of computer simulations changes depending on whether the model satisfies or violates this stability criterion. Example analyses illustrate how the computer program can simulate stable and unstable failures in mines.
Cascading Pillar Failure (CPF) (Swanson and Boler, 1995) in room-and-pillar mines can go by many other names such as "progressive pillar failure", "massive roof collapse", "domino-type failure", or "pillar run". In this kind of failure, when one pillar collapses, the load that it carried transfers rapidly to its neighbors causing them to fail and so forth. This failure mechanism can lead to the rapid collapse of very large mine areas. In mild cases, only a few tens of pillars might fail; however, in extreme cases, hundreds, even thousands of pillars can fail. CPF can have catastrophic effects on a mine, and sometimes these effects pose a greater health and safety risk than the underlying ground control problem. Usually, the CPF induces a devastating airblast due to displacement of air from the collapse area. An airblast can totally disrupt the ventilation system at a mine by destroying ventilation stoppings, seals, and fan housings. Flying debris can seriously injure or kill mining personnel. The CPF might also fracture a large volume of rock in the pillars and immediate roof and floor. A methane explosion might result from the CPF.
Many case studies exist of CPF in coal mines. The most infamous example (Bryan et al., 1966) is the Coalbrooke Colliery in South Africa where 437 miners perished when 2 km2 of the mine collapsed within a few minutes on January 21,1960. Chase et al. (1994) document 5 recent cases of CPF in U.S. coal mines, and they reference many other case studies. CPF has also happened in many metal and nonmetal mines in the U.S. Straskraba and Abel (1994) describe the failure of a large room-and- pillar copper mine, and Swanson and Boler (1995) analyze the failure of a room-and-pillar evaporite mine. In addition, there are probable examples of CPF in gold, limestone, potash and other industrial minerals. Close scrutiny of these failures reveals several commonalities such as 1) extraction ratios are usually more than 60%, 2) width-to-height ratio of panel pillars is always less than 3 for coal mine failures, usually much less than 1 in the metal mines, and may be less than 2 in the nonmetal mine failures, 3) substantial barrier pillars with width-to- height ratios more than 10 are usually absent from the mine layout.
