2131 for the future mine. On the other hand, from the point of view of rock mechanics and mine planning, the presence of drifts and mining areas in specific parts of the deposit can be disadvantageous. Existing drifts intersect areas potential pillars and tunnels. When it comes to sublevel stoping it is necessary to adapt the elevation of the new sublevels to the existing ones, to eliminate unfavorable intersections of them. Especially mining areas, where caving was tried, have to be excluded from mining. 3.2.5 Comparison of mining methods Based on initial dimensioning, a sublevel stoping and a post pillar layout, were developed in Surpac. Besides the extraction areas, the design also included the development which has to be excavated for a safe and productive underground mine. Different indicators were compared to find the most appropriate mining method to mine the Bürgl deposit. The indicators which were used to compare the mining methods were: ● Recovery of deposit ● Development costs per ton mined ● Flexibility and selectivity ● Mining costs ● Productivity ● Backfill sequence ● Possibility of integration of old mine infrastructure ● Ventilation The comprehensive comparison of the two mining methods based on the above indicators resulted in the further development of a sub-level stoping layout. The decisive factors were the higher productivity, safety and lower development costs per ton magnesite mined. 4. FURTHER DEVELOPMENT OF MINING SYSTEM - SUBLEVEL STOPING As previously outlined, sublevel-stoping with backfill is regarded as the optimal mining method for the magnesite deposit in the Bürgl underground mine. Using this method, mining is carried out in retreat, with stopes located between two superimposed levels – an upper level (top drift) and a lower level (bottom drift) – being mined. Subsequent to the mining of the stope, backfilling using rock fill from the historical waste dumps located nearby the mine is conducted. The stopes are mined in a strict sequence which is based on rock mechanics and operational considerations. Consequently, mining can commence in upwards direction, as the top drift becomes the foot drift of the superimposed upper stope. 4.1 Sublevel Stoping Layout The dimensions of the stopes are generally determined by the rock mechanics properties, quality control requirements, and machinery. After the decision to continue using sublevel stoping, an additional rock mass classification campaign after Bieniawski (1989) and Barton et al. (1974) was started in the historical underground workings. The results were the base for the more detailed dimensioning of the sublevel stoping layout. The first task was to determine the extension of the stopes. Therefore, again the method using the Mathews stability graph was applied. The results showed that the stopes could be bigger than initially expected and calculated for the first mine design. One important requirement was the minimum distance of 10 m between the roof of the bottom drift and the floor of the top drift, regarding safety. The drifts have a geometry of 5 m x 5 m. In combination with minimum distance, this results in a minimum stope height of 15 m. Another safety related parameter was the stope length, with a maximum length of 80 m. Including selectivity and machinery considerations following dimension were determined for the stopes: ● Stope width = 12 m ● Stope height = min. 15 m – max. 24 m ● Stope length = max. 80 m
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