Abstract:

Consistency between actual in-mine and modeled ground response to mining is essential if a model is to be used as a mine design tool. A new procedure is presented in this paper that infers Mohr-Coulomb floor properties that will synchronize floor heave behavior between mine and model. The procedure works where the presence of floor heave depends on pillar size, as has been observed in a deep western U.S. longwall coal mine. In this case, floor heave was observed near pillars approximately 52.4 m (172 ft) wide but was absent around pillars approximately 23 m (76 ft) wide, as the longwall passed. Pillars were idealized in FLAC3D models as a single square column that includes the roof, pillar, and floor. Coal in mine pillars was modeled as a Hoek-Brown material with parameters tuned to match one of Bieniawski’s in situ compression tests, scaled to an unconfined compressive strength of 6.2 MPa (900 psi). Models of various pillar shapes replicated Bieniawski pillar strength equation estimates for pillar width-to-height ratios up to 8. Mohr-Coulomb properties in the floor were then varied to define a boundary between heave and no-heave floor response to loading for each pillar size. The intersection of these solution sets served to bound Mohr-Coulomb properties for the mine floor. A representative set of properties within this region reduced capacity of the 52.4-m (172-ft) wide pillar, which has a width-to-height ratio of 17, by 46% compared to a linear extrapolation of Bieniawski’s equation. Pillar capacities with inferred floor properties are easily fit to the Holland-Gaddy and Maleki empirical equations. This procedure provides an alternative, and significantly different, extrapolation of coal pillar capacity that correctly models observed entry behavior, yet requires only careful observation to ascertain.

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