## 3 Results and Discussion
### Close-Range Photogrammetry  
A dense point cloud with 47,742,682 points was created by processing 396 photos, 16 selected reference points (visible in all camera positions), and 129 control points. The resulting 3D model consisted of a mesh with 9,548,536 faces and 4,775,132 vertices, as shown in Figure 13. The calculated RMS (Root Mean Square) reprojection error of 0.0802754 (0.538642 pixels) shows a precise alignment between the reconstructed 3D points and their corresponding 2D positions in the images and confirms the high quality and accuracy of the reconstruction. Rhinoceros 7 software was used to create a worn cross-section on profile 4-4 of the double disc cutter from the dense point cloud and compare it with the original cross-section. Figure 14 shows constructions of the original profile and the worn profile, wherein the geometry of the worn profile is clearly recognizable. The worn surface of the disc cutter can be seen in Figure 15, in which a detailed view of the original and the worn profiles of the cutter ring can be seen. It was not possible to physically obtain the new disc cutter, so the original profile is based on the original technical drawing. The profile of a worn cutter ring tip differs considerably from its original profile. The cyclically loaded contact between steel and rock material during the rolling motion of the disc cutter leads to fatigue of the steel, which manifests itself in the formation of microcracks and the gradual removal of the steel material from the disc cutter ring. Due to its constant contact with the drilled rock, the tip of the disc cutter is exposed to a high level of material removal. Figure 13: A 3D model of a worn disc cutter, created in Agisoft Metashape 1.6.3 software. The profiles of the original and worn disc cutters were further used to create 3D models of the original and worn double disc cutters. For the sake of simplicity, the models were constructed without the shaft of the disc cutter, as this is unnecessary for the volume comparison as it remains unchanged and is not subject to wear. It was also assumed that the worn profile, as shown in Figure 15, is the same throughout the disc cutter. The volumes of the original and the worn disc cutter were then determined. The difference between the two volumes (original and worn) is 1.4548 dm\({}^{3}\). Considering the density of steel (7.85 kg/dm\({}^{3}\)), the calculated mass loss is therefore 11.42 kg. The comparison between the mass loss determined by weighing (11.8 kg) and the mass loss determined by three-dimensional modeling from the point cloud shows that both values are very comparable and lie within a 4% deviation. Three-dimensional models of the original and the worn disc cutters are shown in Figure 16.  
In general, the wear of the double disc cutter used in this study was normal or uniform. This type of wear is desirable in tunneling with TBM technology as it allows for the most efficient use of the cutting tool. In addition, normal wear ensures a predictable drilling process and provides a possible plan for replacing worn disc cutters. Normal disc wear is generally the most common type of wear in TBM tunneling and occurs in about 75% to 90% of cases [35; 40; 61]. Looking at the original and the worn profiles of the disc cutter used in this study (Figure 15), the contact pressure on the disc cutter ring appears to increase during wear with the same tunneling parameters and the same geology. The width of the original ring tip of the disc cutter is much greater than the width of the worn ring tip. According to [PERSON]'s theory [62; 63; 64] of contact pressures, this means that the disc cutter is exposed to ever greater contact stresses with increasing wear. This can be claimed for the case study in this research, in which the wear of the disc cutter was uniform. Figure 14: Original cross-section (\(\O\) 330.2 mm) and worn cross-section (\(\O\) 274.2 mm) of the double disc cutter.