Influence of Regional Support Systems (Pillars and Backfill) on Local Areas and Internal

Safety in Mines Research Advisory Committee

Project Summary: GAP615

Applied research

Summary

The general influence of regional support on local stope conditions is not always well understood and a number of anomalous conditions, both positive and negative, have been reported in the vicinity of regional supports. Extensive ground motion and closure monitoring, studies of hangingwall conditions and numerical modelling analyses were carried out for a selection of different regional support (pillar and backfill) layout and reef horizon combinations. Although evidence arose from this work of differences in conditions possibly requiring different support design, it was felt that no absolute values could be provided for relevant design parameters. Rather, it was decided that a modified support design methodology be proposed that accounts for the additional influencing factors pertaining to the zone adjacent to regional support (Figure 1).

Figure 1Methodology for design of local support in vicinity of regional support

Principal findings

• Peak particle velocities (PPVs) increase with increasing perpendicular distance from the pillar (cf. Figure 2).
• PPVs recorded close to the pillar increase with the increase in pillar stress resulting from the face advance (cf. Figure 2).
• The reduction in PPVs from the face to the back area varies for different geotechnical areas.
• Generally, well placed backfill improves conditions in face areas if it is kept close to the face and conventionally designed working area support that fits in well with the backfilling/mining cycle is implemented.

Figure 2Schematic of relative magnitude (dashed arrow = increase) of PPVs as a function of proximity to face and strike stabilising pillar at TauTona mine

Conclusions

• Dynamic closure, resulting from events generating PPVs, is reduced in the vicinity of regional support (i.e. pillars and well consolidated backfill).
• Support units in the vicinity of strike stabilising pillars will be required to withstand less dynamic and quasi-static closure than units in panels further away from the pillars.
• A greater relative increase in PPVs will be encountered in the vicinity of strike pillars than in areas closer to the middle of the stope.
• Areas close to strike pillars, particularly gullies, have to sustain the cumulative effects of nearby events emanating from the pillar for the lifetime of the gullies.
• Conditions in gullies adjacent to backfilled panels are generally considered to be at least as good as if not better than those where conventional support is used, particularly under rockburst conditions.
• The PPVs recorded close to the face adjacent to the recently placed backfill are higher than the PPVs recorded further back in the areas where the backfill is already consolidated and provides a better support.
• To achieve the benefits of backfill, strict adherence to a well-established set of standards for both backfilling and working area support must be ensured.
• It is not possible to provide a generic design criterion for local area support in the vicinity of regional support, due to the variable nature of conditions that exist. It is, however, relevant to apply a methodology to the process of determining the criterion for each situation.