Mechanics of mine backfill

Mine backfilling is the process of filling large underground mining voids ("stopes") with a combination of tailings, water and small amounts of cement, to promote regional stability. Stopes are often in excess of 20 m x 20 m in plan dimensions and 40-50 m tall, and can be filled within a week. Barricades are constructed in all tunnels ("drives") that access the stope to contain the backfill material. In recent years, a significant number of failures of mine backfill barricades have occurred, resulting in the inrush of slurry backfill into the mine workings. In addition, sampling has shown material strengths in situ to be far greater than equivalent mixes cured in the laboratory (indicating the potential for reducing the cement content). The purpose of this thesis is to apply soil mechanics principles to the mine backfill deposition process with the intent of providing some insight into these issues. In many cases, filling, consolidation and cement hydration all take place at a similar timescale, and therefore, to understand the cemented mine backfill deposition process it was necessary to appropriately couple these activities. Developing appropriate models for these mechanisms, and coupling them into a finite element code, forms the core of this thesis. Firstly, the fundamental processes involved in the cementing mine backfill deposition process are investigated and represented using theory founded on basic physical observations. Using this theory, one- and two-dimensional finite element models (called CeMinTaCo and Minefill-2D, respectively) are developed to fully couple each of the individual mechanisms. A centrifuge experiment was undertaken to investigate the interaction between consolidation and total stress distribution in a cementing soil. The results of this experiment were also used to verify the performance of Minefill2D. Due to scale effects, the centrifuge experiment was unable to fully couple the interaction of the cement hydration and consolidation timescales. To achieve this, a full scale field experiment was undertaken. The simulated behaviour achieved using Minefill-2D (with independently derived material properties) provided a good representation of the consolidation behaviour. Finally, a sensitivity study carried out using Minefill-2D is presented. This study enables some useful suggestions to be provided for managing the risk of excessive barricade stress, and for preparing laboratory samples to more appropriately represent in situ curing conditions.