The use of remote sensing for discontinuity mapping and analysis in a large marble quarry, Carrara, Italy

This paper describes the data collection and the analysis in a quarry slope located in the Torano basin (Carrara, Apuan Alps - Italy). The slope is characterized by a remnant marble buttress from previous excavations, approximately 150m high and varying from 30 to 50m in width. As quarry slope stability analyses require a detailed knowledge of the rock mass and discontinuity proprieties a terrestrial photogrammetric survey, using an unmanned aerial vehicle, UAV, was carried out in order to increase the amount of data obtained through engineering-geological mapping. UAV mapping was chosen to overcome problems related to the elevation and steepness of the open pit walls and the limitations that are commonly encountered with traditional aerial and terrestrial photogrammetric techniques. The images were acquired with the UAV in vertical strips and oriented by the use of ground control points in order to derive three different photogrammetric models, one for each side of the buttress. The stereoscopic models were coupled with a high resolution point cloud acquired by ground-based LiDAR scans. Using this methodology also allowed the production of digital high-resolution orthophotos of all sides of the marble buttress. Remote sensing data allowed for characterizing both intact and brittle fractures in terms of attitude, persistence and spacing. This information was used to perform a kinematic stability analysis in order to identify the most probable failure mechanisms. The analysis was combined with GIS spatial processing with to the objective of highlighting the failure zones in relation to the actual morphology. In addition, finite element and distinct element models of the slope were constructed. The 3D finite element code, RocscienceTM RS3, was used to investigate the stress distribution in the slope and indicate areas of potential stress-induced damage due to the quarrying activities. The 3D distinct element code, ItascaTM 3DEC, was then used to build a discontinuum geomechanical model of the quarry slope. Three different discontinuum approaches were used. In approach 1 the main joint sets from the stereonet analysis were used to develop the geomechanical model. In approach 2 a deterministic model was constructed specifying the location, dip and dip direction of every fracture located in the area of interest. Finally, in approach 3 a geomechanical model was developed using the new discrete fracture network module implemented in 3DEC.