## 5 Discussion
### Fault Zone  
Figure 10e is an image of the Xiaomaoshan Fault that was captured right after the excavation during tunneling. Disturbed by the fault, the rock mass was severely deformed and had a low strength. A fault gouge was seen on the excavation face. The discontinuity was difficult to identify. Ring excavation with a reserved core-ground part was performed to avoid collapse of the excavated face, but significant deformation of the sidewall of the tunnel was still observed. The rock mass in the zone of the Houishan Fault was more disturbed than that in the zone of the Xiaomaoshan Fault. Before the tunnel excavation, the pipe roof method must have been used to support the surrounding ground to protect the tunnel. The excavated face was immediately covered with shotcrete to mitigate its weathering. Therefore, only the images that were taken from both sides of the Houishan Fault are available, but images from the most severely sheared section of the Houishan Fault are not. Figure 10f shows the excavation face of the Dongao Tunnel in the section that was near, and was disturbed by, the Houishan Fault. Slate interbedded with metasandstone was exposed at the excavation face. The rock mass was loosely interspersed with mud, and quartz veins and calcite penetrated the weak discontinuity planes. Rock debris commonly fell from the excavation face following tunnel excavation. [PERSON] and [PERSON] (1997, 2009) [33, 34] indicated that the strata on both sides of the Xiaomaoshan Fault were difficult to distinguish in the field due to their similar lithologies, and a fault gouge and breccia that was composed of quartz and flint were indicators for the location of the Houishan Fault. Benefiting from the capability of LiDAR to partly penetrate vegetation and thus depict the ground features more realistically, we found that the zones of the Xiaomaoshan Fault and the Houishan Fault exhibited in LiDAR DEM landforms different from adjacent areas. Compared with the 3D terrain model shown in Figure 10a, the Xiaomaoshan Fault zone had a texture that was much rougher than that of the adjacent Suao Formation and Nansuao Formation (Figure 10b). Such a rough texture was obvious in the slope where the fault intersected, but not in the ridge and river side, perhaps because a rigorous erosion had eroded the local geomorphic features caused by faulting in the latter two areas. The Houishan Fault was surrounded by slate and schist on both sides. Its fault zone had developed more corroded gullies, which had severely damaged the geomorphic characteristics. The rough texture of the fault zone in LiDAR DEM could only be observed near the elevation of the quarry road, and it was no longer visible below the slope belly (Figure 10b). Nevertheless, the feature of rough texture can be an indicator for fault traces (Figure 6) and provide a basis for field investigations. Based on the locations of the Xiaomaoshan Fault and the Houishan Fault that were interpreted from the high-resolution clinometric and hillshade maps, as well as the geological observations on-site and geological information obtained from the excavation face of the Dongao Tunnel, we plotted the lithological distribution along the tunnel and at the locations of the encountered faults, further refining the geological map (Figure 11). The tunnel excavation revealed the locations of the disturbed zones of the Xiaomaoshan Fault and the Houishan Fault, and provided information about the vertical extension of these faults. The disturbance zones of the Xiaomaoshan Fault and the Houishan Fault along the tunnel direction ranged 153-164 m and 173-177 m, respectively--close to those interpreted from high-resolution clinometric and hillshade maps (118-130 m and 102-158 m, respectively), but different from those indicated in the geological map (30 m and 60 m, respectively) that was determined using conventional methods of geological investigation, and surveying and mapping. Figure 11: Lithological distribution along the north section of the Dongao Tunnel (**a**) and a refined geological map (**b**).