2127 is intensely sheared to a fault gauge. Its thickness reaches 60 m in uppermost outcrops, with depth thickness is diminishing, concomitantly the rock quality is deteriorating. It is the biggest structural element at Bürgl site. Spielbergdolomit and massive dolomites of the Hochhörndler Complex are hosting the metasomatic and thus shape-wise highly irregular magnesite lenses. They are of sparry type, generally dip concordantly with the host rock at an angle of 30 to 70° towards south. They reach a thickness up to 80 m and extend for hundreds of meters in strike and dip direction. At Bürgl, four magnesite ore bodies of significant volume are known. Three of the main lenses are in footwall position to the sheared Porphyroid layer, one is in hangingwall position, both with direct contacts. The ore bodies are beside primary and secondary dolomites, practically monomineralic, finegrained (grain size below 5 mm) and of intense coloring (brown, grey, red, black). Rarely, magnesite shows relict bedding and normally behaves isotropic with a distinct little porosity and UCS up to 180 MPa. 2.2.2 Backfill Regulative framework does not exclude mining methods with caving or remaining open voids in particular; nevertheless, permitting process is beneficial in avoiding that. For that reason, the mining company has decided to exclude such methods from planning efforts. Huge dumps from the mining period starting 1957 until 1971 are remaining at Bürgl site. From economic as well as material properties it is the best medium available for backfill. As products have evolved over the years CaO cut-offs are less stringent nowadays, meaning magnesite that had to be dumped in the past is usable today. It seems feasible that in the coarse fraction magnesite could be sorted out using NIR technology and just the reject as well as fines used as backfill. This potential could mitigate some of the financial burden related to the backfill operation. 2.2.3 Alpine environment The mining site is in extremely steep, high alpine area with harsh climatic conditions. Weissenstein open pit is only operational during snow-free periods. Accessibility and avalanche danger are limiting factors. Also, for future backfill extraction at Bürgl site this will be very true; it is thus anticipated to extract and sort the backfill during summer months. Backfill should be stored for the winter months in the abandoned Bürgl open pit connecting it with a draw point chute to the future underground mine. 2.2.4 Tourism The mining site is in an area of multiple land use by tourism and agriculture, causing further constrains for the operation. 3. ROCK ENGINEERING CONSIDERATIONS 3.1 Rock Engineering Challenges 3.1.1 Complex Geological Setting As described in the previous section, the geology of the deposit is unique. The foot wall consists of stable dolomite. Above this lies the first magnesite lens, which is separated from the magnesite above it by the porphyroid layer. The valley floor is formed of less stable black shale. This shale can be considered to have low strength. For this reason, it can be assumed that it has no regional load-bearing effect. As the distance between the future stopes and the surface is comparatively small. Similarly, to the black shale, the porphyroid is also low-strength and has a limited load bearing capacity. Additionally, the porphyroid is mylonitised in areas less than 1 m thick and has very low strength. Like the dolomite in the hanging wall, the magnesite is stable. However, the low-strength porphyroid can
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