Abstract:

Backfill is essential in underground mining for maintaining global rock mass stability. One of the geomechanical backfill design issues that presents ongoing challenges occurs when the backfill is undercut, as happens for Underhand Cut & Fill (UCF) mining methods, but can also occur for much larger spans in Long Hole Open Stoping, for example. Mitchell (1991) presented a method for estimating the required backfill strength for what was termed “sill mats”, and this motivated subsequent design charts by others (i.e. Stone, 1993; and Pakalnis and co-workers, 2005, 2017). More recently, numerical modelling has been used (Grabinsky et al, 2024a) to show that the Mitchell solution can significantly over-estimate required strengths, especially for backfill cross-sectional geometries where the span exceeds the height. Furthermore, a generalized approach has been identified (Grabinsky et al, 2024b) to characterize mining conditions for which significant backfill squeezing may occur on undercutting (e.g. Stillwater, Lucky Friday mines) from those where such closures can be ignored (e.g. Macassa mine). These new modelling approaches give stability predictions closely aligned with lower-bound strength case histories presented by Pakalnis et al and others, and so hold promise for optimizing future backfill undercut designs. However, to fully validate these new modelling approaches it is necessary to have well instrumented field case studies with which to compare the predictions of applied load versus progressive backfill failure, ideally through to collapse conditions. Developing a database of such case studies presents significant logistical and resource challenges.

The proposed paper will describe an ongoing field scale research project at Macassa mine intended to provide the first carefully monitored response of a UCF backfill to undercutting and subsequent over-pressurization with the intent to structurally fail the backfill mass in a controlled and safe manner. The theoretical and empirical background to current practice in UCF design will be reviewed to provide context for this project’s experimental design. The instrumentation will be fully discussed including installation details and methods for remote monitoring while the backfill is being undercut and subsequently over-pressurized. The Special Operating Procedures developed to ensure safety, especially during backfill failure, will be described. Finaly, the potential impact of the project’s outcomes on practical design, as well as limitations when considering design under other rock mass conditions, will be considered.

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