Pillar design for underground mining is typically done using empirical
formulae or numerical modelling. Practical experience and recent
literature, nevertheless, illustrate the shortcomings of these design
approaches. Ongoing monitoring of pillars is therefore recommended to
minimize the risk associated with these designs.
In a mine where a large number of pillars are cut, different pillar
strengths can be expected owing to variations in rock mass strength from
area to area. This problem is compounded by the fact that in mining
environments the pillars are not always cut according to the prescribed
dimensions. Although the original design using either empirical methods
or modelling may predict stable pillars, unstable pillars will be encountered
in reality. Very conservative designs with large factors of safety may
circumvent this problem, but this approach is uneconomical.
A possible solution to localized stability problems may be to enhance
the strength of a few unstable pillars. It may even be hypothesized that
reinforcing a few critical pillars may prevent ‘pillar runs’ on a much larger
scale. Rockbolting, strapping of pillars, and pillar shotcreting have
occasionally been used in the past as possible solutions. It appears that
these have not been successful in all cases and large collapses have
occurred in spite of the pillar remedial work.
The authors investigated the strapping of pillars by conducting
laboratory tests on cabled rock specimens. The results were qualitatively
compared to actual attempts of pillar reinforcement available in the
literature, as well as additional observations in an old haematite roomand-
pillar mine in Spain. Based on these results, the value of steel cabling
and mesh wrapped around pillars to improve stability is demonstrated.
Some cases where this approach will not be successful are also discussed.