The strength of hard-rock pillars

Observations of pillar failures in Canadian hard-rock mines indicate that t he dominant mode of failure is progressive slabbing and spalling. Empirical formulas developed for the stability of hard-rock pillars suggest that the pillar strength is directly related to the pillar width-to-height ratio an d that failure is seldom observed in pillars where the width-to-height rati o is greater than 2. Two-dimensional finite element analyses using conventi onal Hoek-Brown parameters for typical hard-rock pillars (Geological Streng th Index of 40, 60 and 80) predicted rib-pillar failure envelopes that did not agree with the empirical pillar-failure envelopes. It is suggested that the conventional Hoek-Brown failure envelopes over predict the strength of hard-rock pillars because the failure process is fundamentally controlled by a cohesion-loss process in which the frictional strength component is no t mobilized. Two-dimensional elastic analyses were carried out using the Ho ek-Brown brittle parameters which only relies on the cohesive strength of t he rock mass. The predicted pillar strength curves were generally found to be in agreement with the observed empirical failure envelopes. (C) 2000 Els evier Science Ltd. All rights reserved.