successful and the stopes are without support still stable, the block caving operations was due to the competent rock mass conditions of the magnesite not successful. However, lots of drifts were created for the haulage levels of the block cave. These drifts and the failed block cave itself can be problematic for the future mine because of its proximity to the abandoned mining areas. Figure 2 gives an overview on the old infrastructure and underground mining activities in the Bürgl mine. Infrastructure which is still active for the material from the Weissenstein quarry to the processing plant is colored in green. All the other infrastructure is blue. Areas where larger mining activities were conducted are red. 3.21 Feasible Mining Methods and Rock Engineering Key Elements The previous section highlighted, that the situation in Hochfilzen is rather complex for starting a new underground mine. The application of different mining methods was evaluated. As the deposit size is limited, it is important to extract as much as possible. Therefore, the adaptability of the mining method to adapt to the irregular geometry of the magnesite lenses in terms of thickness and lateral extent is a key criterion. Moreover, does the thickness of the magnesite lenses require the utilization of backfill as a working platform and to support the high excavation walls if a high percentage extraction is to be achieved. Another major target in Hochfilzen is the maintaining of the surface integrity. Therefore, sublevel caving where the surface is destabilized is not applicable. Due to economics and difficult logistics for cemented backfill cut and fill mining was also not feasible. As a result, there were two mining methods left over to mine in Bürgl, namely ● Sub level stoping ● Post pillar mining Related to these possible mining methods more detailed investigations were conducted. These investigations included some necessary rock engineering key elements namely ● the utilization of backfill ● the dimensions of the mining areas ● massive barrier pillars ● dimensions of stopes (rooms) and pillars ● the integration of old mining infrastructure 3.2.1 Utilization of Backfill The detailed investigations revealed that a mining system involving backfilling is the most appropriate method for mining in Bürgl. For the application of post pillar mining the usage of backfill is a key component, as it is function as the floor of a new mining level and is superior for stabilizing the local pillar. Also, for sublevel stoping backfill brings some advantages. In general, the utilization of backfill allows the increase of recovery, while having positive effects on the stability of local pillars which can be found in sublevel stoping as well as in post pillar mining. An additional advantage of backfill is the regional support of the comparable weak black shale in the hangingwall and the prevention of subsidence effects on the surface. Backfill must be available at any time. This is one of the
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