The economic, ecological and safe extraction of minerals from underground often requires the application of backfill. Currently the design and operation of backfill systems is largely based on practical experience rather than on well-researched engineering principles. This can lead to misjudgements of backfill performance with serious consequences concerning mine safety, mining system performance, mine profitability, and mineral deposit exploitation.
To obtain a better understanding of the interaction of backfill with the surrounding rock strata, a research project in an Austrian underground mine using backfill was launched. The investigated mine is an underground magnesite mine using a system of large stopes separated by slender pillars and filled with cemented backfill as mining method in the considered mining areas. In the up to 1000 m deep sections a system of 21 m high, 6 m wide and up to 80 m long stopes separated by 7 m wide pillars is applied. Only one stope is mined in a section at any time. After the stope has been backfilled the adjacent stope is excavated. This mine offers the opportunity to evaluate the influence of backfill on rock mass behaviour during different operation steps.
The focus of research was on investigating the effects of backfill on rock deformation and fracturing processes around mining excavations. Characteristic reference areas were selected based on geological and stress conditions and planned mining operations. The geology of the areas was carefully mapped, in situ discontinuity patterns analysed based on conventional discontinuity measurements and core drilling. Rock mass properties (laboratory and in situ) and backfill properties (laboratory and in situ) were determined. In addition, stress build up in the backfill was monitored in three directions using flat jack type pressure cells. Rock deformations and fracture patterns around mining excavations were monitored before, during and after placement of backfill using multiple rod extensometers and optical borehole observations. Similarly seismic velocity measurements to quantify changes in rock conditions were carried out. Particular attention was given to assess and document the effects of stoping activities on rock deformation around the mining excavations. This was necessary to account for the mining induced stress changes in the areas of interest.
The paper describes the results of the comprehensive programme of rock mechanics investigations. The results show that backfill placement in stopes has an immediate effect on lateral deformation of slender stope pillars. This is despite the fact that stress build up in the backfill is rather slow. This result is of practical significance as it puts into question some of the backfill requirements specified by the Austrian mining authority. Monitoring of rock deformation around mine tunnels several tens of metres away from the stoping area provides a good basis for assessing regional effects and assists in long-term stope planning investigations. An important outcome of the research project is that the instrumentation methodology used in this study has proven to be reliable, accurate and can be implemented by mining personnel.