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UBC Theses and Dissertations

Hard rock pillar strength estimation an applied empirical approach Lunder, Per John


Pillars are present in all hard rock mining operations and in order to effectively design these pillars, an estimate of the pillar strength is required. Two new pillar strength estimation methods for hard rock mine pillars are presented in this thesis. 31 pillar case histories of the database that was used to develop these new formulae were acquired during a cooperative study, entitled “Ground Stability Guidelines for the Extraction of Barrier Pillars in Hard Rock Mines”, between Westmin Resources Ltd. and The Canadian Centre for Mineral and Energy Technology (CANMET). 147 additional case histories were acquired from six documented hard rock pillar case studies in the literature, resulting in a combined database of 178 case histories. The combined database is comprised mainly of massive sulphide pillars with rock mass ratings of between 60% and 85%. Major structural features were not deemed to be an influence in pillar instability. Pillar stressess were calculated using either tributary area theory or numerical modelling methods. The factors determined to influence pillar strength for the combined database therefore are: • the average pillar confinement (which is a function of pillar geometry) • the unconfined compressive strength of the intact pillar material • the stresses that a pillar is subjected to The degree to which a pillar has failed is quantifiable using a pillar stability classification index which ranges from “1” (stable) to “5” (failed). The estimation of pillar stresses is preferably determined using threedimensional numerical modelling, but in some situations two-dimensional numerical modelling or tributary area theory may provide adequate results. It was concluded that the full size unconfined compressive strength of a pillar can be approximated by a strength size factor of 44 percent of the small scale unconfined compressive strength of intact pillar material. Two pillar strength formulae have been developed from the combined pillar database: “The Log-Power Shape Effect Formula” and “The Confinement Formula”. Both of the methods utilize the average pillar confinement. “The Log-Power Shape Effect Formula” is a refined shape effect formula which has a form similar to that proposed by researchers in the past. “The Confinement Formula” has a form that resembles the Mohr-Coulomb shear strength formula. The combined database was analyzed and the predicted strengths from “The Confinement Formula” was compared to the results for existing pillar strength methods (Hedley & Grant (1972), Bieniawski (1975), Salamon & Munro (1967), Obert & Duvall (1967), Hoek & Brown (1980)). “The Confinement Formula” is shown statistically to be the most reliable method of estimating the strength of the pillars that make up the combined database.

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