Grabinsky, M, Jafari, M & Thompson, B 2026, 'An update to the Grabinsky et al. (2021) plug strength method', in AB Fourie, M Horta, M Oliveira & S Wilson (eds), Paste 2026: Proceedings of the 28th International Conference on Paste, Thickened and Filtered Tailings, Australian Centre for Geomechanics, Perth, pp. 1-12, https://doi.org/10.36487/ACG_repo/2655_16
(https://papers.acg.uwa.edu.au/p/2655_16_Grabinsky/)
Abstract: A common backfilling strategy for tall (e.g. longhole) open stopes is to pour in 2 stages, i.e. to first pour a ‘plug’ of backfill to a few metres above the undercut’s brow, followed by a ‘main’ backfill pour after a suitable cure period. The purpose of the plug is to gain sufficient strength to protect the backfill containment structure in the undercut from the additional stresses due to the main pour. Grabinsky et al. (2021) proposed an analysis method based on a failure mechanism motivated by the Prandtl approach to design structural footings in clays. The method was numerically calibrated and then verified using 4 case studies (3 continuous pours, and one staged pour) in heavily instrumented stopes with complementary strength data from field and laboratory experiments. The authors have used the method extensively to define plug strength for two-stage pouring strategies. They are also aware that the method has been applied by others for a wide range of mining conditions, many of which do not strictly conform to the original assumptions. Therefore, enhancements and extensions to the original method need to be considered.
Sound justification now exists for the assumption that cohesion, c = ½UCS. For draining backfill (i.e. hydraulic fill) friction can also be accounted for which significantly stabilises the backfill plugs. It is not recommended that the plug strength method be applied to waste rock berms, and some alternate plug design criteria are suggested. Irregular geometric effects are addressed, including a new shape factor to be used when rectangular (versus square) undercut drift cross-sections are encountered. Transitional geometry between the undercut and main stope is shown to not have any additional stabilising effect. These changes significantly expand the scope of application for the plug strength method. While these improvements provide increased efficiency, the inherent conservatism of the design approach is reduced, and so it is therefore necessary to consider an appropriate strength factor. Instrumentation is also recommended as necessary to verify plug strength assumptions as part of any design application.

Keywords: cemented paste backfill, plug strength, two-stage pour