Structure from Motion photogrammetry to characterize underground rock masses: Experiences from two real tunnels

A new methodology to identify discontinuity sets at the tunnel face based on the Structure from Motion (SfM) photogrammetric technique is proposed. The work focuses on the performance of this technique when employed to characterize the ground mass under real tunneling conditions, illustrating its possibilities and analyzing several aspects that affect the quality of the obtained results. By means of a set of overlapping photographs from the tunnel face, SfM constructs a 3D point cloud model, from which discontinuities are identified using a discontinuity set extractor software. To orientate and scale the digital model, an easy-to-use "portable orientation template", specifically developed for this work, is employed. The proposed methodology is applied to two real tunnels under construction in Northern Spain. Its results are compared with those obtained with a traditional analysis based on manual compass measurements. Results show that the SfM methodology provides an adequate characterization of the structure of the rock mass, identifying the same number of discontinuity sets as the compass measurements approach and with differences in orientation that are within the uncertainty range associated to manual measurements. Only one sub-horizontal set presented higher orientation differences, but this is mainly due to the presence of shotcrete at the face. In addition to the advantages of a "distant" measurement technique-e.g., health and safety advantages, capability to characterize unreachable areas, etc.-, as well as to the advantage of its reduced cost, the proposed SfM methodology and its associated tools allow one to represent planes associated to each discontinuity set back into the original 3D digital point model, and to perform detailed analyses that clarify and improve the obtained results. Finally, an analysis about the minimum number of photographs needed to adequately characterize the tunnel face is conducted, with results showing that around 15 good quality photographs are enough for tunnel faces with excavated areas of about 50 m(2).