Monitoraggio geomeccanico e misurazione dello stato di sollecitazione in cave di marmo a cielo aperto ed in sotterraneo

The mechanization of marble cutting operations thrusts the enlargement of quarry exploitations. This trend makes necessary the geomechanical control and computations of the static condition both at the exploitation workplaces and of the residual rock structures (HOEK & BRAY, 1981, HOEK & BROWN, 1982, BRADY & BROWN, 1985, BIENIAWSKI, 1989). Planning of the investigations is driven by the specific geostructural and mechanical rock mass characteristics of the quarry sites in respect to the natural morphology and the excavation geometries (CRAVERO et al., 2003). For instance in open pit quarrying, the monitoring of the structurally controlled deformation of high excavated rock faces is relevant for safety and planning of further quarry developments. The paper reports an experience of monitoring and computations at an open pit quarry. The quarry site belongs to the Alpi Apuane marble basin (Carrara marble), where past exploitation operations left in place a parallelepiped marble spur ?90m high, horizontal section ?30mx20m and hosting old drifts (Fig. 1). This residual rock structure is intersected by different rock joints and a sub-vertical large fault located on the back side of the spur. Evident joint traces appear on the excavation and drift surfaces, and a monitoring plan was defined which allows to detect the influence of rock fractures on the deformation behaviour of the marble spur. Groups of different measuring devices (1D or 3D crack gages, MPBX extensometers, load cells and piezometer) were arranged on the surface and in the old drifts (Fig. 2) while a wireless transmission system conveys the measured data to the quarry technical office (DUNNICLIFF, 1993). Hydraulic fracturing was furthermore applied in two vertical boreholes located on the two opposite sides of the big fault to detect the stress condition (HAIMSON, 1993, AMADEI & STEPHANSSON, 1997). Computations were made to estimate the influence of the rock mass structure and of the environmental condition on the static behaviour of the rock spur. The Block Theory (BT) technique (GOODMAN & SHI, 1985). was applied to detect potentially unstable rock blocks defined by rock joint intersections in the marble spur (Fig. 3). FEM modelling, made by using the commercial software Phase, allowed to clarify the possible role played by the fault and by different stress or water load conditions on the deformation mechanism shown by the spur (Fig. 4). The experimental control along with the analysis of unwanted deformation and rock block instabilities was motivated not only by the need of controlling the evolution of the displacements, but also suggested setting up remediation countermeasures against a possible unstable trend of the marble spur.