Best practices in continuously (or not continuously) pouring paste backfill

Abstract

Cemented paste is established as a widely used backfill material. Mines using this type of backfill initially use conservative pouring strategies whereby a 'plug' is poured to a height exceeding the (containment) barricade and brow. The plug is allowed to cure to an extent that the remainder of the stope can be poured without inducing significant additional pressure on the barricade. In the last decade, more mines are applying engineering principles including engineered barricade designs, barricade pressure monitoring, specific backfill plug strength designs, and quality assurance/quality control (QA/QC) plant-based protocols to evaluate and verify how the efficiency of backfill placement can be safely optimised.There are several examples within the literature where mines have demonstrated how continuous backfilling can be safely adopted into their respective standard operating procedures. There is, however, an absence of published field data for cases where high or inconsistently low pressures at barricades limit the advisability of continuous pouring. This can create a bias in expectations. We present case study data from mines where a range of barricade pressures leads to, at best, an unproven justification for continuous pouring. Risk profiles are heightened if non-ideal conditions exist (i.e. non-engineered barricades, new operations lacking in site-specific experience). Emphasis on the continual process of safely optimising backfilling efficiency is more helpful than a focus on the potential end result of continuous pouring. Indeed, we cite cases where operations have reverted to more conservative strategies when better appreciation of the risks of continuous pouring evolve with time or changing conditions.We have previously recommended initially collecting 'baseline' data to assess the range of barricade pressures under normal operating conditions, prior to a recommendation for continuous pour trials. This paper emphasises that although collection of baseline data can be time consuming, given the consequences of barricade failure, it is a necessary task to adequately define risks.As technology allows the more widespread use of instrumentation to fulfil previous 'use barricade pressure data to verify safe and efficient backfilling' recommendations, it is important to step back and review best practice approaches and, indeed, the context of when it is feasible and when it is not feasible for continuously backfilling or accelerated backfilling to be adopted. Critically, we emphasise that instrumentation is only part of the solution to ensure safe backfilling. Definition of adequate plug strength, proven by QA/QC in terms of early age strength testing, adequate barricade designs and potentially personnel exclusion zones are also necessary.