2125 PLANNING OF A MINING CONCEPT FOR AN UNDERGROUND OPERATION IN A CHALLENGING ALPINE ENVIRONMENT * P. Gams1, C. Stranzl2 and T. Fruehwirt1 1 Technical University of Leoben, Department of Mineral Resources Engineering, Chair of Mining Engineering and Mineral Economics, 8700 Leoben, Austria (*Presenting author: patrick.gams@unileoben.ac.at) 2 Global Raw Material and Geology, Veitsch-Radex GmbH & Co OG, Austria ABSTRACT Magnesite is extracted and processed to refractories in Hochfilzen located in the Austrian Alps since 1958. Active mining is currently taking place in an open pit mine. Already in the 1960s underground mining was tested in the Bürgl deposit in parallel to the open pit operation. 10 years ago, a core-drilling program was conducted to update the model of the Bürgl deposit for a feasibility study regarding reopening an underground mine. The whole deposit consists of four larger magnesite lenses. Based on the promising results of that program, Montanuniversitaet Leoben and RHI Magnesita started to plan the reactivation of underground extraction of magnesite in Hochfilzen. There are a number of challenges such as the complex geology of the shallow deposit with a weak layer of porphyroid which can be seen as a large fault zone between two parts of the deposit, steep topography, tourism, seasonal operation above ground and environmental aspects. Due to the size, shape and geology of the deposit, the challenging mining environment and the need to preserve the integrity of the surface, it was decided from the very beginning that only a backfill based mining method is feasible. Based on that and on several rock engineering considerations, two feasible mining methods were identified, namely post-pillar mining and sublevel stoping with backfill. The latter was further developed. The rock engineering design for the sublevel stoping method was developed using empirical and numerical methods. Due to the geometry of the steep and complex deposit with a steep strike parallel large fault zone, the control of high shear stresses is crucial. For that reason, large dip pillars which take the shear loads are implemented between the mining blocks. Backfilling of stopes is introduced to ensure local and regional stability of the mine. Based on the chosen mining method, panel pillars can become several tenths of meters high and must be stabilized with backfill. Furthermore, it is possible that these pillars can be intersected by the porphyroid zone requiring additional support, which is obtained from the backfill. To ensure mine stability, a strict mining sequence, where mining of a stope is only allowed when the adjacent stopes have been backfilled, is applied. As a result, backfill material must be available when it is needed. The backfill material, which is dumped into the stopes as rockfill, is directly obtained from a nearby waste dump from former mining activities. Due to the alpine climatic environment, at an elevation of approx. 1600 m a.s.l., it is only feasible to excavate backfill during the summer months, which is critical for adhering to the strict mining sequence. Moreover, the underground storage capacity for backfill is limited. For that reason, a suitable method for the handling and storing of the backfill for the winter months is investigated. Due to the small size of the deposit, the life of mine and the yearly production are limited. As a result, the excavation of large mine infrastructure is not feasible and the integration of the already existing mine infrastructure is necessary. Other considerations such as transport systems for backfill and mined mineral are included in the planning process. This paper discusses the different phases of the planning process for the mining concept and gives an overview of the challenges and the current status of the project. KEYWORDS mine planning, feasibility study, alpine mining, stoping, rock mechanics
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