In a previous study, the authors investigated the cable-strapping of pillars in room and pillar mines in order to solve local instability problems, by conducting laboratory tests. Based on experience in an old, small room-and-pillar haematite mine in Spain, the authors suggested a few steel cables placed around mesh-wrapped pillars as an effective means to improve pillar stability and control mine deformation. In the present study, and with the background of available laboratory uniaxial compressive strength tests on cabled samples and empirical knowledge of pillar behaviour, we present the results of numerical modelling of this type of pillar. Using FLAC and UDEC, it has been possible to develop models representing pillar response. These models add to the knowledge of pillar behaviour and improve our control of both standard and cable-strapped pillars, in terms of stress-strain evolution, plastic response, and the energy that the pillars are able to absorb.
Standard practice, as well as published data on pillar design, showed that neither empirical formulae nor numerical modelling can fine-tune pillar design. However, either of these techniques (or better, both together) can be used as a preliminary approach to pillar design, provided monitoring is then carried out to control pillar performance (Malan and Napier, 2014; Esterhuizen, Dolinar, and Ellenberger, 2011).
Rocks and rock masses tend to show variable behaviour and significant scale effects, so in a mine where a large number of pillars are generated in the same formation, a certain amount of variability in strength can always be expected. It is therefore not surprising that, even if by means of traditional approaches (tributary area and empirical formulae) and numerical models a design is reliable, in the mining practice (where pillars do not always have the prescribed dimensions and the rock has a variable nature) stability problems may arise – typically only in a few pillars, unless a very conservative approach was adopted.
