## 3 Project Sites
### Highway 84 (Verdigre) Site  
Highway 84 is a Nebraska state route connecting the town of Verdigre, Nebraska to Bloomfield, Nebraska. The soil condition is primarily fluvial deposits transported from the Rocky Mountains as shown in Figure 4a. However, the nearby Niobrara and Missouri Rivers, located about 10 to 20 km away from this site, washed out and spread the underlain  
Figure 5: Scene of I-180 Slope. (**a**) Crack on I-180 Slope. (**b**) DSS System Installed on I-180 Slope.  
Figure 6: Arrangement of DSS Cable and Posts.  
marine sourced shales (Pierre Shale), which contained expansive clay minerals. It is also noted that the layers of chalk, the source of slippery layers, are easily found along the cliffs of the nearby Missouri river. Classification of clayey soils in this site was classified as CH to CL (the borderline of highly compressible clay and low compressible clay). These soils are called \"Blue Clay\" by local residents, which are in fact a mixture of loosely packed Pierre shales and dark clayey soils. This area is a region of Nebraska that is well-known for various slow landslides caused by the unique soil characteristics and topography.  
Minor cracks appeared on the shoulder of Highway 84 in the summer of 2018 ([PERSON] et al., [29]), as shown in Figure 7, and they developed to noticeably wider ones afterward. Consequently, the Nebraska Department of Transportation (NDOT) decided to remove the crack-infested field soils from the affected area and rebuild the slope, as shown in Figure 8a, with off-site borrow soils, which do not include expansive clay minerals. The brought-in soils have the classification symbol CL with fine content of 30%. In addition, Xanthan (a biopolymer)-based stabilized soils were applied at the bottom of the slope. The similar DSS system and PVC pipe anchors as the ones used for the I-180 site were installed along the slope, as shown in Figure 8b, inspired by the promising results and experience with the I-180 slope. Additionally, it was noted that the same crew conducted the UAS-based topography survey with a set of upgraded UAS equipment as follows.  
Figure 8: Cross Section and Pictue of Verdigre Slope. (**a**) Retrofitting design of the slope. (**b**) Anchors for DSS and UAS reference points.  
Figure 7: Initial Cracks Appeared on Highway 84.  
The UAS system for this site collected a sequence of high-resolution imagery data that might be used for the 3-D reconstruction of the slope geometry, using a Pix4 DMapper. This was accomplished through the use of a dedicated surveying UAS, a WingtraOne fixed-wing UAS platform with an onboard Sony 42 MP camera, and a built-in multi-frequency PPK (Post-Processing Kinematic) GNSS antenna of the UAS. The imagery data collected with this UAS were accurate enough without ground control, as this had been confirmed by the research team and others (e.g., [PERSON] et al., [30]). To achieve this accuracy without the use of GCPs, data were collected using the PPK technique, with one GNSS receiver on the UAS and the other being the static base station. The base station was logged continuously throughout the UAS flight, and the location of the base station was obtained using OPUS (Online Position User Service). Here, OPUS provided an accurate estimate of the base station, specifically for International Terrestrial.  
Reference Frame coordinates of 0.6 cm horizontally and 2.5 cm vertically. Due to the known accuracy of the PPK platform and the wet slope in the field, CPs were not deployed nor deemed necessary. For likewise reasons of wet and slippery slopes, the TS-based surveying technique was also not deployed for safety. The accuracy of the tagged pictures was 2 cm horizontally and 4 cm vertically, with the constructed point clouds having an average ground sampling distance of 1.04 cm.