Violent failures of coal pillars, known in practice as coal minebumps, have long been a subject of investigation. Many fieldinvestigations have considered geological conditions that createhigh stress in the pillar to be the main causative factor leading tobumps. In recent years, constraint at interfaces has been observedto be associated with the occurrence of coal bursts.This paper shows how interface friction and width-to-height(W/H) ratio affect the degree of violence of coal specimen failure.A series of UCS tests on 178 coal samples from bump-prone (Utah,called UT coal) and non-bump-prone (West Virginia, called WVcoal) mines were conducted. Three violent failure parameters—peak sound pressure level, core zone failure, and ultimate stress—were used to assess the violence of failure. The degree of violencewere investigated at varying interface frictions (high: μ = 0.40,medium: μ = 0.22, and low: μ = 0.13) and W/H ratios (W/H = 2,3, 4, 5, 6, 7, 8, 10, and 12). A simulation series of modeled coalspecimen under uniaxial loading was also conducted using thefinite element method to observe the equivalent plastic straindistribution experienced by the modeled coal specimens.It was found that the violence of coal specimen failure dependson W/H ratio and interface friction. Interface friction has moresignificant influence on the violence of failure at high W/H ratiosthan it does at low W/H ratios.
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