Non-contact rock mass survey by means of airborne photogrammetry and Infrared Thermography

The application of non-contact technologies is herein presented as a test for a scientific procedure aimed at providing a technological solution for the rock mass survey in areas affected by poor logistics for field campaigns. Either high cliff sectors, or sub-vertical coastal cliffs or, more simply, areas that cannot be reached for a field rock mass survey, represent a challenge to retrieve geostructural data in the perspective of a geomechanical characterization or stability analysis. For this study, two technologies were coupled to achieve a reliable model of rock masses. In particular, applied technologies are the aerial photogrammetric survey by Unmanned Aerial Vehicle (UAV) and the Infrared Thermography (IRT). The first represents a reference for applications aimed at rockfall stability studies, while the second is a relatively pioneering methodology exploiting the thermal radiation emitted by the rock mass and falling within the infrared portion of the electromagnetic spectrum. A three-dimensional model of the surveyed rock masses was built starting from the definition of dense point clouds and related elaborations. Thanks to the preliminary georeferencing of the UAV survey, discontinuity spatial orientation could be extracted by employing different algorithms, thus achieving their dip-dip direction values to be plotted on stereograms and statistically grouped. IRT surveys allowed the study of the distribution of the surface temperatures along the framed rock face. This is linked both to the different rock mass conditions (wet or weathered rock, presence of vegetation) and to the main geomechanical features of discontinuities such as persistence and aperture. Based on IRT outcomes, the innovative geomechanical parameters of Thermal spacing and Thermal RQD (Rock Quality Designation) were estimated with the aim of finding a potential, non-contact, alternative to the conventional procedures for the evaluation of the loose rock volumes. By matching the information achieved by the two surveying methodologies, a geomechanical model of the remotely surveyed rock mass was achieved proving the good adherence of the remote data to reality. Such outcome represents the implementation of the scientific experience on a key geomechanical topic, as well as a step forward in the integration of methodologies based on different principles but well matching if focused on a common scope.