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.