Three-dimensional reconstruction of high-resolution images of existing tunnels for geo-structural monitoring and inspection purposes.

Geotechnical and structural monitoring and inspection of existing linear infrastructures, with particular focus on tunnels, have gained a great prominence in recent years. In Italy, a new regulation for inspection, monitoring and maintenance of tunnels has been issued and a massive inspection plan is carried out to guarantee a constant maintenance and safe condition of existing tunnels. The relevance of geo-structural monitoring lies in the possibility of controlling the interaction between soil and structure and in prevention of damaging infrastructure systems because of deterioration. Monitoring also helps the prevention of natural disaster effects, according to the rising attention paid to hydrogeological risk in the country in the last decades. The Italian territory is vulnerable to earthquakes, floods, and landslides, which are the major hazards that can involve human settlements, constructions, and big infrastructures such as tunnels. An opportunity arose for the possibility of working on huge amounts of data and develop accurate methods of elaboration and visualisation of the most significant information for inspection and maintenance planning purposes. For this work, innovative methods and mobile survey technologies were used to get linear images and 3D point cloud data of some highway tunnels in central Italy, which are characterised by relevant structural deteriorations and cracks. High-resolution black and white images of tunnel linings were captured through a mobile system composed by line cameras, lamps for a correct illumination of the tunnel surface and positioning system. To represent the whole tunnel surface, 4 runs were performed: right and left wall and right and left ceiling. High-density point clouds of the tunnel were acquired by a mobile laser scanner mounted on a vehicle. The combination of 2D high-resolution images and 3D data can have a significant impact on the data visualisation and presentation in order to have a comprehensive representation of the actual state-of-the-art of the infrastructure. The 3D representation of the tunnel from 2D linear images is accomplished through the reconstruction of a 3D geometrical model of the tunnel section through a tool for the automatic elaboration and management of images. This automatic calculation algorithm provides the three-dimensional reconstruction of the infrastructure through 2D high-resolution images, in order to have the best representation and visualization of the elements inside the tunnel. The major advantage of the tool is the possibility of identifying and evaluating structural defects and cracks in the tunnel surface directly on the 3D model and of better understanding the effects on the infrastructure caused by deformation events in the geological context. It can also provide the comparison of subsequent surveys for monitoring. This work represents an innovative means affecting fundamental aspects of existing tunnels management and monitoring, e.g., the investigation of deformation phenomena, temporal evolution of deteriorations and cracks, causes identification/prevention, soil-structure interaction studies, geological hazard risk reduction.