## 5 Discussion
### Lithology  
Figure 10 shows the 3D terrain model, digital elevation model, and images of excavated faces of Dongao Tunnel in its northern portal section. The 3D models clearly showed the erosion gullies that developed near the Xiaomaoshan Fault and the Houishan Fault. The flow paths of these gullies were obviously concentrated in the lower part of the mountain, and many bifurcations in the elevation above the belly part of the mountain were observed. In addition, the fractured geology close to the faults was conducive to the development of erosion gullies, but these erosion gullies were not necessarily the geomorphic features of a fault, as was evidenced by the existence of erosion gullies. The dislocations of ridges and the spatial distribution of gullies provided information about the location and horizontal extension of a fault, but the vertical extensibility of the fault could not be fully determined from the geomorphic features.  
Figure 10c is an image that was captured during the NN177 rounding of the Dongao Tunnel, when tunneling was begin carried out in the Suao Formation, north of the Xiaomaoshan Fault. (The first and second Ns represent the north-bound lane and the north-working site, respectively; the number is the number of the rounding cycle during tunneling.) The cleavage of the slate approximately parallel to the excavated face had a high persistence, and the slate was yellowish-brown due to rust staining and frequent infilling with mud. The joint set that dipped to the east with high angles (on the left in the image) had a moderate spacing (20-60 cm), high persistence, and obvious rust staining. Joints were typically opened with filling materials. Figure 10d shows an image that was captured during the NN1020 rounding, when tunneling was being carried out in the Dongao Schist formation, south of the Houishan Fault. The greenschist was occasionally mixed with black schist with highly persistent schist, approximately parallel to the excavation face. Fractures in the schist were filled with mud, quartz veins, and carbonate minerals. Multiple sets of joints had moderate to wide spacing and low persistence. The uniaxial compressive strength of the slate of the Suao Formation was typically less than 25 MPa, which corresponded to a weak rock; that of the Dongao Schist was in the range of 25-50 MPa, which corresponded to a medium-strength rock. Metasandstone, which typically has a relatively high strength, was observed in a few parts of the Dongao Schist formation. The geological data obtained by the excavation of the Dongao Tunnel were consistent with the geomorphic features of the two types of strata shown in Figure 4: Slate had a low resistance to weathering. As a result of weathering, it formed flaky or broken rock masses and then gradually weathered further into soil, so the ridge lines were mostly arc-shaped. The schist had high resistance to weathering; the persistence of schist exceeded that of other joint sets, and the quartz veins and carbonate minerals that filled the fissures of schist were also highly resistant to weathering. The attitudes of schist and their appearance after weathering remained prominent geomorphic features and were captured by the fine topographic surveying and mapping using an airborne-based LiDAR.  
Figure 10: 3D terrain model (**a**), digital elevation model (**b**), and images of the excavation face captured during tunneling in the slate of Suao Formation (**c**), the schist of Dongao Schist Formation (**d**), the Xiaomaoshan Fault (**e**), and the disturbed zone of the Houishan Fault (**f**).