SUMMARY: The Stekenjokk Mine is the latest of the 15 producing mines of the Boliden Metall AB. Production started in 1975. The gently dipping ore body has a very complicated geometry due to intensive folding. At the planning stage a combination of room and pillar and cut and fill mining was anticipated. Qualitative finite element models were used to examine alternative mining layouts, pillar loading, probable ore extraction and influence of tectonic structure on pillar loading.
RÉSUMÉ: La mine de Stekenjokk est la plus recente des 15 mines exploitees par Boliden Metall AB. La production y a debute en 1975. Le filon, en pente douce, a une geometrie très complexe, due à la presence de nombreux plis. Au stade du projetage, une combinaison entre la methode des massifs longs et l 'extraction par taille et remblayage avait ete envisagee. Des modèles très elabores, selon la methode des segments limites, ont ici ete utilises pour etudier differentes configurations d'exploitation, la production potentielle de minerai, la charge exercee sur les piliers et 1'influence de 1a structure tectonique sur cette charge.
ZUSAMMENFASSUNG: Die Grube Stekenjokk ist die juengste der 15 in Produktion stehenden Gruben der Boliden Metall AB. Der Abbaubeginn erfolgte im Jahre 1975. Der flach einfallende Erzkörper hat auf Grund intensiver Faltung eine sehr komplizierte Form. In der Plannung wurde eine Kombination von Kammerbau und Firstenstossbau als Abbaumethode vorgesehen. Qualitative Finite-Elementmodellen wurden angewandt um alternative Auslegungen des Abbaues, Pfeilerbelastung, wahrscheinliche Erzausbeute und Einfluss tektonischer Strukturen auf die Pfeilerbelastung zu beurteilen.
BACKGROUND The Stekenjokk Mine, which came into production in 1975, is the latest of the approximately 15 producing mines of the Boliden Metall AB. Boliden Metall AB Bollden Metall AB is responsible for the major share of sulphide ore mining in Sweden. At present 11 million tons of ore are mined annually. This yields an annual production of about 170,000 tons of copper concentrates, 100,000 tons of lead concentrates, 160,000 tons of zinc concentrates and 360,000 tons of pyrite concentrates. At the smelter, the Rönnskar Works at Skelleftehamn, the production of copper amounts to about 60 percent of the domestic Swedish consumption, production is equal to the consumption of silver, and exceeds it for lead, gold, arsenic and selenium. The number of employees is about 5,800 and the turnover for 1977 was Skr 1,214 million.
THE ORE BODY When the decision was taken in 1973 to develop the Stekenjokk deposit into a mine, the mine planners were faced with the problem of devising a mining method which allows to use highly mechanized equipment in an ore body with a very difficult and highly variable geometry. Previous extensive exploration work had indicated an ore body of approximately 1500 m length and 200 m in width dipping approximately 130 to the south between shaft and surface. There is a continuation of the ore body to greater depth (figure 1).
MINING METHODS A review of possible mining methods led to the conclusion that a combination of room and pillar (R +P) mining and cut and fill mining (C +F) could be suitable for this ore body. Room and pillar mining implies partial extraction of the ore body. Pillars of ore are left to support the overlying strata, whereas the excavations ("rooms") created by ore extraction remain open. At a later stage the rooms may be filled with sand or tailings to provide long term support for the pillars. R +P was planned to be applied to the flat lying, central parts of the ore body. Preliminary design was based on square pillars with a width of 5 m placed in a square pattern with 12 m side length, resulting in 7 m wide rooms. This layout implies a loss of ore left in pillars of 17 %. The steeper, peripheral parts in cross section (figure 2) were to be mined by cut and fill mining. By this method the ore is removed by mining 3 - 5 m high slices of ore, starting at the bottom. Once one slice is mined the opening thus created is filled with deslimed tailings from the ore concentrator. Other mining methods or combinations of mining were not to be excluded from consideration, e.g. total extraction of the R +P area by a longwall mining method and continuous refilling of the mined out area.
ROCK MECHANICS ASPECTS OF SELECTING A MINING METHOD From a rock mechanics view the following questions required clarification:loading on pillars. especially near boundary between R +P and C +F, and on the fill
estimation of ore losses in pillars
effect of alternative design (e.g. use of rib pillars) on ore extraction and loading conditions
influence of mining on shaft and surface
influence of geologic
tectonic features such as intensive folding and the occurrence of a graphitic phyllite between ore body and surface.