Geomechanical and structural - geological characterization of marble for underground quarrying from a road gallery in central Alpi Apuane (northern Appennines, Italy)

A new road tunnel is planned in the central Alpi Apuane, a well-known region for its marble outcrops where the Carrara marble is quarried. The tunnel will cross marble of the "arabescato" type, of great commercial value and therefore it is also interesting to test the possibility to open underground quarries linked with the road galley. The area is characterized by a complicate geological structure resulting from two phases of folding and metamorphism the prolongation of outcropping marble bodies in the underground is thus not straightforward and the planning of underground quarries must follow detailed structural-geological investigations. After structural-geologic field work we reconstruct the three dimensional geometry of geological structures, extrapolate the underground geometry of the most valuable marble types and propose a location for underground quarries. Geomechanical properties of rock masses that we are going to cross with tunneling have been investigated considering their position in the geological structures. Four classes of rock mass were defined using the Geological Strength Index. The expected stress state near the excavation has been investigated through the boundary element method (BEM). A parametric study was then developed decreasing the cohesion parameter by the decreasing of the rock mass quality. For the strength parameters previously assigned trough rock mass characterization, zones of overstress are of limited extent and the core of pillars has strength/stress ratios greater than 1.