## 4 Results and Discussions
### Identification of Slope Hazards and Field Validations
Figure 10 illustrates the decomposition of vertical deformation and down-slope deformation for both westward and eastward slopes [37, 38]. The unit deformation vectors along down-slope or vertical direction and the corresponding projections to the LOS direction for four situations are shown in the figure. Then, the possible movement process can be traced back from the LOS deformation according to Figure 10. According to Figure 7, P1-P3 and P5-P6 are covering the areas of westward slope. In this case, vertical and down-slope deformation are both negative value in LOS direction (away from the satellite) in any slope dip angle. Unlike other slopes, the dip direction of P4 is mainly westward. As ascending orbit data are used in the study, for westward slope, a vertical deformation on the slope always appears to be away from the satellite in the LOS direction (see Figure 10). However, a down-slope deformation can be either close to or away from the satellite when the slope dip angle is larger or smaller than the incidence angle. In terms of P4, the dip angle is around 34\({}^{\circ}\)\(\sim\)39\({}^{\circ}\) and less than the average incidence angle 43.1\({}^{\circ}\). Thus, the down-slope deformation dominated deformation is positive value in LOS direction. Back to Figure 9, it is suggested that the area may experiences downward movement with decreasing LOS deformation (during 7 October to 30 December 2020) and down-slope movement with increasing LOS deformation (after 28 February 2021). P6 also shows obvious abnormal deformation areas, which are mainly distributed on the north side of the slope, as shown in Figure 8, with the concrete crest drain ditch as the boundary. It can be observed in Figure 9 that the deformation value of ps-6-2 fluctuates slightly in a small range, but the point ps-6-1 keeps decreasing. To verify identification results, the field investigation has been carried out from 14 May to 17 May 2021. Typical photographs are presented in Figures 11 and 12 for potential hazards P1-P6. Figure 9: Time-series deformation for selected PS points. Figure 7.0 Field validation photographs of potential hazards P1-P5. (**a**) Joint crack at the 2 nd-level slope. (**b**) Shallow landslide on top of the 2 nd-level slope. (**c**) Full view of P2. (**d**) Excavation on the slope. (**e**) Slope surface washout. (**f**) Vegetation cover on slope. Figure 7.1 Ground deformation decomposition illustration. Figure 11a show obvious joint cracks on the second-level slope with a width of about 1 cm and an extension of about 5 m. The cracks are located roughly at the same area where the deformation velocity of InSAR results is the largest. In addition, shallow landslides were also found near the sidewalk (Figure 11b). From Figure 11c,f, dense vegetation can be seen on the slope for both P2 and P5, but there was no obvious sign of slope deformation or instability. Therefore, the possibility of slope hazards P2 and P5 is ruled out. Field investigations showed that there were obvious excavation traces on the slope surface (see Figure 11d). Based on consultations from highway maintenance staffs, the slope surface was damaged by artificial excavation around January 2021. It can be indicated that the highway slope deformation identification based on InSAR technology can well reflect the dynamic status of the slope. Through field survey on P4, it is found that the slope has been well vegetated, and there are local surface washout and cracks on the slope (Figure 11e). Figure 7.2 Field validation photographs of potential hazard P6. (**a**) Shallow landslide on top of the 2 nd-level slope. (**b**) Full view on south-side of drain ditch. (**c**) Cracks on crest drain ditch. (**d**) Full view on north-side of drain ditch. layer exposed on the 3 rd-level and 4 th-level slope, and it has been stabilized by a flexible protection net combined with grid beams (Figure 12b).