Excavation stability analysis in an underground marble quarry in the Apuan Alps (Italy): Application of terrestrial LIDAR, conventional methods and numerical modeling

The excavation of tunnels in a rock mass changes the existing stress state and initiates deformation processes that, in some cases, can cause instability. Stability investigations, using methodologies capable of verifying both the safety conditions and of representing new scenarios in underground works can be greatly enhanced through the use of high-resolution 3D images acquired in combination with ground-based LiDAR. Data from ground-based LiDAR allows the extraction of geological and structural characteristics of the rock mass such as geometries, attitudes, persistence and spacing of joints, even in inaccessible areas. High definition terrestrial laser scanning was adopted in an underground marble quarry, called "Romana", located in the Apuan Alps (Italy), as a support for the geological, geostructural and engineering geological study. Discontinuities were identified and mapped in a deterministic way by using the Leica TruView plug-in which manages laser scanning data and allows representation of the fractures in a georeferenced way either by arcs or by a sequence of aligned points. Given the large quantity of information collected, data were filtered and added to a geodatabase that, after GIS processing and additional in situ engineering geological surveys, was used to create thematic maps illustrating the fracture trends. In this way detailed documentation of the geomechanical and geostructural characteristics of the discontinuities was obtained and subsequently used to model the face stability in different zones of the tunnels. Preliminary analyses were performed by means of numerical modeling in order to calculate approximate stress values around the excavation, and to understand how they change when perturbed. The simulations were carried out using both finite element, (Phase2; without discontinuities), and distinct element methods (UDEC; including discontinuities derived deterministically from remote sensing data). The calculated stress values were then used to perform a 3D stability analysis of excavation rock wedges using block theory software, Unwedge. Model results showed that the presence of discontinuities considerably affects both the stress orientation and stress magnitude around the excavation zone, and must be taken into account to ensure accurate stability analyses. In this context the use of the most recent technologies of remote sensing, together with traditional engineering geological surveys, can provide an important and fundamental contribution. Results of the study to date have been analyzed and discussed with the managers of the underground quarry in order to optimize the planning of future excavations.