Abstract:
This paper gives an overview of the difficulties associated with
determining the strength of hard-rock pillars. Although a number
of pillar design tools are available, pillar collapses still occur. Recent
examples of large-scale pillar collapses in South Africa suggest that
these were caused by weak partings that traversed the pillars.
Currently two different methods are used to determine the strength
of pillars, namely, empirical equations derived from back analyses
of failed and stable cases, and numerical modelling tools using
appropriate failure criteria. The paper illustrates that both
techniques have their limitations and additional work is required to
obtain a better understanding of pillar strength.
Empirical methods based on observations of pillar behaviour in
a given geotechnical setting are popular and easy to use, but care
should be exercised that the results are not inappropriately extrapolated
beyond the environment in which they are established. An
example is the Hedley and Grant formula (derived for the Canadian
uranium mines), which has been used for many years in the South
African platinum and chrome mines (albeit with some adaptation of
the K-value). Very few collapses have been reported in South Africa
for layouts designed using this formula, suggesting that in some
cases it might yield estimates of pillar strength that are too conservative.
As an alternative, some engineers strongly advocate the use of
numerical techniques to determine pillar strength. A close
examination unfortunately reveals that these techniques also rely
on many assumptions. An area where numerical modelling is
invaluable, however, is in determining pillar stresses accurately and
for studying specific pillar failure mechanisms, such as the influence
of weak partings on pillar strength.
In conclusion, it appears that neither empirical techniques nor
numerical modelling can be used solely to provide a solid basis for
conducting pillar design. It is therefore recommended that both
these techniques should be utilized to obtain the best possible
insight into a given design problem. Owing to the uncertainties
regarding pillar strength and loading stiffness, monitoring in trial
mining sections and in established mining areas is also an essential
tool to test the stability of pillar layouts in particular geotechnical
areas.
