## 5 Discussion
### Scarp and Slope Deformation  
Figure 12a shows the interpreted results concerning the landforms in the north section of the Dongao Tunnel. The development of two erosion gullies and scattered scarps was observed. The scarp indicated by the arrow was about 236 m away from the north portal of the tunnel, with its upper apex between the south-bound lane and the north-bound lane of the tunnel. This remarkable scarp had a length of approximately 70 m and a relief of 8 m. Such features on a clinometric map were inferred to be related to a deep-seated gravitational deformation, and we checked on-site which debris was piled-up under the scarp (Figure 9). A twisted slate with open cleavage was also present (Figure 9). The north-bound lane of the tunnel was crossing the slope with deep-seated gravitational deformation about 50 m below the surface. Observation of excavated faces during tunneling revealed that closer to the scarp, the rock mass was more broken, and the weathering was greater. The Geological Strength Index (GSI) of rock mass exposed on the excavation face was reduced from 25-50 in the general sections outside the deformed slope to 10-25 within the area that was affected by gravitational deformation. Figure 12b shows the excavation face of the north-bound lane (riverside) where the tunnel was passing through the boundary of the gravitational deformation slope. The cleavage of slate was approximately parallel to the excavation face, what had an obviously stained surface. The rock mass had toppled over with a variation of dip of up to 30\({}^{\circ}\), causing the opening of the discontinuity to exceed 10 cm. The south-bound lane of the tunnel (hillside) was about 30 m west of its north-bound lane and it did not pass through the gravitationally deformed slope. The GSI value of rock masses that corresponded to the section where the south-bound lane of the tunnel passed through the gravitationally deformed slope was in the range of 20-35 and differed slightly from that in both adjacent sections (25-45 in the south section and 30-55 in the north section). The image captured from the excavation face within the section adjacent to the gravitationally deformed slope revealed the staining of rock masses and the displacement of discontinuities, without any opening thereof (Figure 12c). Based on the locations of the scarps that were observed on the slope and the variations of the discontinuities of the rock masses that were exposed on the excavation face of the tunnel (Figure 12b,c), Figure 12d schematically depicts the deep-seated gravitational deformation of the slope with its possibly slipping surface. The scope in which the rock mass had a high degree of weathering and the attitude of the tilted discontinuity that were affected by the deep-seated gravitational deformation exceeded the scope in which scarps were identified in the clinometric map. The rock mass in the area adjacent to the slope that was significantly affected by a gravitational deformation might still be disturbed with a moderate shear deformation; some relatively small scarps on the slope might not have been precisely identified; or some scarps that were related to the gravitational deformation slope might have been damaged by weathering and erosion, making them unidentifiable in the high-resolution clinometric map. Figure 12e,f shows the features of rock masses that were about 24 m and 47 m below the erosion gullies. The closeness of the excavation of the tunnel to the erosion gully was positively correlated to the degree of weathering of the rock mass that was exposed on the excavation face and, therefore, to the filling of the fissures and fractures and to the proportion of the filling that was clay. Figure 12e shows an image of the excavation face below an erosion gully in the Suao Formation. The slate had weathered to yellowish, and no slickenside was observed on any joint surface. Figure 12f shows an image captured from the excavation face under the erosion gully on the northern edge of the Xiaomaoshan Fault zone.