Floor heave events in underground mining often happen suddenly and cause risk to workers and equipment. Reducing the risk of floor heave must be addressed in mining layout design. To improve that design one must understand the significant parameters that are correlated to the occurrence of floor heave. To accomplish that goal, researchers from the National Institute for Occupational Safety and Health (NIOSH) completed a numerical study using FLAC3D, taking advantage of its strain softening and ubiquitous joint capabilities to better simulate the floor heave and caving mechanisms involved at an actual mine site. In the study, two models having different uniform pillar sizes in a three-entry gate road system were used to investigate the effect of pillar size and other parameters on floor heave. In the models, a sandstone layer of various thicknesses was placed in the roof at various distances above the coal seam. The models had a brittle stratum just below the coal seam. All other stratigraphic members were one material, designated "rest of strata." Other model parameters that were varied included sandstone and rest of strata elastic moduli and the friction angle of interfaces at the top and bottom of the coal seam. The coal and immediate floor were ubiquitous joint materials with strain softening, which would better simulate the floor heave failure mechanism. Loading was from the overburden as a result of panel excavation. Normalized floor heave was negatively correlated with the ratio of average vertical strain of the two pillars of each model's gate road system. To be clear, pillars that were strained nearly equally resulted in higher normalized floor heave than when average vertical pillar strains of a gate road were very different. This measure was represented by a ratio of the higher average vertical pillar strain divided by the lower average vertical pillar strain. The pillar strain ratio had positive correlation with pillar width, and it had negative correlation with two parameters-the distance of the sandstone above the seam and the modulus of the rock in the overburden other than sandstone. Overall, modeled excavation of one longwall panel resulted in positive correlation of normalized floor heave to distance of the sandstone above the coal seam and negative correlation with pillar width. This result has implications for optimum design of pillars in gate road systems to minimize floor heave.
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