An investigation into the use of backfill to reinforce pillars in hard rock bord and pillar layouts

Abstract

The production of tailings is inherent to mining and minerals processing and will remain so for the foreseeable future. Catastrophic tailings dam failures, such as those at Canada’s Mount Polley in 2014 and Brazil’s Samarco and Brumadinho in 2015 and 2019, are a reminder that more needs to be done to safeguard lives, improve production, and protect the environment. As the global population increases, so does the demand for the metals and minerals required for modern life. With increased demand and supply comes the challenge of extracting greater value from low grade deposits, mining deeper ore bodies, and increased mineralogical complexity, all of which have the potential to lead to a higher production of tailings. This study was an investigation of the potential benefits of using the tailings as backfill in bord and pillar mines with specific emphasis on its use to increase the strength of the pillars and reduce the volume of tailings stored on surface. The literature indicated that backfill is extensively used in Chinese coal mines in conjunction with high extraction mining methods, mostly for environmental considerations. However, it is not commonly used in hard rock bord and pillar mines. This study explored the use of backfill to increase the extraction ratio in hard rock bord and pillar mines at increasing depths. The typical decrease in extraction ratio with increasing depth is illustrated in the dissertation with the aid of a simple analytical model, which emphasises the need to study backfill as an option to ensure viable mining operations in future. To simulate the effect of backfill confinement on pillar strength, an extension of the TEXAN limit equilibrium model was explored in this study. The model can simulate the failure of the pillars as well as the effect of confinement applied on the edge of the pillars. A simple plane strain model of the limit equilibrium model, with the confinement component, is derived in this study to study the attributes of the model. This was used to illustrate the effect of the backfill confining stress on the normal pillar stress in the failed zone of the pillar. An increase in confinement results in a substantial increase in stress carried by the failed pillars. This was also illustrated by conducting numerical modelling of an idealised bord and pillar layout with increasing levels of confinement. The intact core of the pillars increased in size for a larger magnitude of confinement and the resulting average pillar stress (APS) of the pillars also increased correspondingly. These preliminary models indicated that the limit equilibrium model with the confinement component behaves as expected. The magnitude of confinement that will be exerted by backfill on the pillar sidewalls is unknown. This study used a combination of literature sources and earth pressure theory to estimate the level of confinement that can be expected when placing backfill. Lateral earth pressure refers to the pressure that soil exerts in the horizontal direction, and it is an important parameter for the design of geotechnical engineering structures such as retaining walls. The analysis and historic studies indicated that a value of 0.05 MPa is a possible realistic value of confinement that will be exerted by the backfill. Numerical modelling of an actual platinum mine layout was done to illustrate the beneficial effect of backfill on pillar stability at greater depths. This illustrated that the zone of failure decreases for increasing confinement and there is a substantial reduction for a 2 MPa confinement value. It is nevertheless doubtful whether such a magnitude of stress will be exerted by the backfill unless there is substantial pillar dilation. Calibration of the actual confinement exerted by the backfill needs to be done as future work. Trial backfill sections in mines with sensitive instrumentation in the backfill will, therefore, be required to quantify this effect. In summary, the confinement limit equilibrium model proved to be a valuable approach to simulate the effect of backfill confinement on pillar strength. Mining areas where the pillar strength is reduced due to the presence of weak layers may benefit significantly from the placement of backfill. Additionally, the use of backfill will also reduce the requirement for tailings storage on surface, thus minimising the risk of environmental damage.