The stress-change monitoring and geotechnical investigations carried out during mining in an experimental open stope of an underground talc mine located in Italy provided a better understanding of the mechanical behaviour of a soft, fractured rock mass at great depth. The back-analysed behaviour at different scales using numerical modelling led to the technical and economic optimization of design and reinforcement for future stopes, which will guarantee a further improvement in stability and safety within the mine, and greater profitability of the operation as a whole.
Le suivi de l'evolution des contraintes et les investigations gc!otechniqueseffectuc!espendant l'exploitation d'un chantier minier pilote realise dans une mine souterraine de talc italienne ont permis d'ameliorer notre comprehension du comportement mecanique d'un massif de roche tendre fracturee exploite à grande profondeur. La retro-analyse de ce comportement, à differentes echelles et à I'aide de modèles numeriques,conduit à une, optimisation technico-economiqaedu dimensionnement des futures chambres et de leur renforcement garantissant à la fois la stabilite et la securite dans les chantiers et une plus grande rentabilite de l'exploitation.
Aus den Beobachtungen von Spannungsvoranderungen und den geotechnischen Untersuchungen, die beim Abbau in einer Bergbauversuchsan1age in einem Untertagebau auf Talk in Italien durchgefuehrt wurden, ergab sich ein besseres Verstandnis des felsmechanischen Verhaltens eines Gebirges aus welchem klueftigen Gestein in groβer Tiefe. Die Rueckanalyse dieses Verhaltens in unterschiedlichen Maβstaben und anhand eines numerischen Modells fuehrt zur technish wirtschaftlichen Optimierung bei der Bemessung der kuenftigen Abbauraume und deren Ausbau, wodurch sowohl die Standfestigkeit und Sicherheit an den Abbauplatzen als auch ein höhere Rentabilitat fuer die Talkgewinnung gewahrleistet werden.
The Unitalc company exploits a talc deposit from the underground Brusada mine, near the village of Lanzada (Sondrio province) in the central Italian Alps. The orebody is a vein of extremely variable width (3–45 m), with a general N060° strike and a dip of 70° to the north, whose hostrock mostly consists of serpentinite. Like all other talc deposits in the region, this vein is a mineralized fault of very large extension. The mining method of vertical long-hole stoping is uncommon for this type of deposit, and has been chosen on environmental and economic criteria (pelizza et al., 1989). The principle is that stopes are excavated perpendicular to the vein direction, which are 35–50 m long, 6 m wide and 22 m high, separated by rib pillars of 25–30 m width. The deepest stopes, 600 m below ground surface, are backfilled with waste rock. Mining started in 1988 with a yearly talc production of about 30,000 tons. The constant preoccupation of the mine manager is to improve the profitability of this operation without endangering the safety of the mining staff and equipment. In particular to increase recovery (only about 20% at present), he examined the possibility to leave higher and wider stopes, combined with a reinforcement of the rib pillars by cable bolting. To this end, it was necessary to carry out the following works in an experimental stope:
A complete geostructural and geomechanical survey of the rock mass, including field and laboratory testing.
Monitoring of stress changes in the adjacent pillar during mining, using the Hydraulic Tests on Pre-existing Fractures (HTPF) method.
Measuring the variation and distribution of forces acting upon cable bolts that reinforce the stope walls.
Back-analysis and numerical modelling of the mined rockmass behaviour.
