The paper aims to present a case of an open pit marble and underground quarry. The rock mass characterization has been based on laboratory mechanical tests on marble, geostructural exploration of joint sets, and in situ testing of rock stress by means of hydro fracturing tests. Numerical models of the quarry have been conceived to evaluate the behaviour of the excavation, and to relate the measured stress state of the rock mass to the numerical results. An evaluation of possible influence of the joints on the stopes stability is furthermore made.
Present day marble quarrying gives raise to large open pit and underground excavations. Very often both kind of excavations are jointly carried out at the mountain quarry sites and the resultant permanent voids can rapidly reach impressive sizes and unusual shapes. Ground Control should therefore be recognized as an integral aspect of quarry activities. However, unlike classical mining operations and excavations for civil purposes (Hoek & Brown, 1982, Brady & Brown, 1985), the monitoring and adoption of consistent design is not yet common practice in marble quarrying. Apart from the specific estimation of the stress levels -mostly for non deep excavations- the knowledge of the relative joint orientation, joint strength and simple basic schemes of falling or sliding blocks can be suitably exploited for the stability evaluation (Shi & Goodman, 1981, Warburton, 1981) taking into account the joint variability. This paper summarizes aspects of the geomechanical characterization, in situ testing and modelling on a particular marble quarry site, exploited by open pit and presently also operated underground. Results refer setting of the principal joint sets, laboratory mechanical testing of marble specimens and in situ estimation of rock stress by means of hydrofracturing tests. An analysis of the possible influence of the rock structure on underground stopes is made by means of the block theory. The measured stress values are compared with the stress computed by means of a 3D boundary element scheme reproducing the particular site configuration according both to the natural topography of the mountain surface and to the modified morphology produced by the open pit excavation, as well as the new underground stopes.
The quarry site belongs to one of the quarry basins exploiting the wide white marble deposit of the metamorphic complex of the Alpi Apuane range. The quarry is located in the hinge zone of an anticline (Pianza's anticline) and the previous open pit exploitation had deeply modified the original mountain morphology. The lowest quarry yard is at about 760m a.s.l., while the access to the new underground exploitation (at about 800m a.s.l.) has been opened in the lower part of a 90m subvertical rock cut, made into a highly dipping natural spur (Fig. 1). Apart from thin and discontinuous eluvium-colluvium deposits and the quarry waste dumps, covering at a large extent the mountain surface, the marble formation constitutes the outcropping rock almost everywhere in the quarry basin.