## 2 Background
### Deformation Characteristic of the Landslide  
Based on systematic investigations and multi-field monitoring data, the unstable area of the Huangtupo landslide is primarily located within the No. 1 riverside sliding mass, which continues to experience relatively rapid and continuous creep. The largest cumulative displacement recorded between January 2016 and July 2017 was observed on the northeast side of the landslide, reaching 90 mm [34; 35]. The latest monitoring network consists of eight surface GNSS monitoring points and six precision measurement points within the underground tunnel group, designed to capture deformation data from both the landslide surface and the tunnels. The spatial distribution of these monitoring points, along with field photographs of selected sites, is illustrated in Figures 1c and 3. Specifically, GNSS monitoring points P1, P2, and P3 are situated at the front of the landslide, while P4, P5, and P6 are located in the middle section, and P7 and P9 are positioned at the rear edge. Data for these monitoring points were collected from April 2019 to April 2024. Within the tunnel system, precision measurement point TZ002 is located in the No. 5 branch tunnel in the central part of the sliding mass; TZ011, TZ012, and TZ013 are situated in the No. 3 branch tunnel on the western side; and TZ006 and TZ007 are positioned in the main tunnel on the eastern side. Data collection for the tunnel monitoring points spanned from May 2022 to April 2024. The impoundment of the Three Gorges Reservoir commenced in 2003; the water level was first raised to its maximum design operating level of 175 m in October 2010. Since  
Figure 3: Photographs of partial deformation monitoring points in NO. 1 riverside sliding mass: (**a**) GNSS monitoring points; (**b**) precise measurement points inside the tunnel group. Figure 2: A typical cross-section of the Huangtupo landslide (A–A). then, it has fluctuated annually between 145 m and 175 m. During the flood season, from June to September, the reservoir water level is typically maintained near 145 m to regulate flooding. A comparison of accumulation deformation curves at different monitoring points reveals significant spatial variability. This statement is a qualitative description of the overall landslide deformation (Figure 4). The largest accumulative deformation, recorded during the monitoring period, occurred at P3 at the front of the landslide, reaching 156.5 mm. Deformation values at P1 and P2 in the front section were 92.4 mm and 108.9 mm, respectively. In the middle section, P4, P5, and P6 exhibited deformations of 65.7 mm, 92.4 mm, and 128.6 mm, respectively. At the rear edge, the deformations at P7 and P9 were 49.8 mm and 69.1 mm, respectively. These results demonstrate a progressive decrease in deformation from the front to the rear of the landslide and from the eastern side to the western side. Monitoring data within the tunnel similarly highlight the spatial variability in deep-seated deformation. On the eastern side, the displacements at TZ006 and TZ007 were 77.85 mm and 33.65 mm, respectively. They were significantly greater than the value of  
Figure 4: Time series monitoring data of No. 1 riverside sliding mass from 2019 to 2024. 26.40 mm recorded at TZ002 in the central area. TZ002 exhibited greater displacement than TZ011, TZ012, and TZ013 on the western side, which recorded deformations of 7.37 mm, 10.24 mm, and 22.83 mm, respectively. These deformation trends confirm that the eastern side of the No. 1 riverside sliding mass experiences greater overall deformation than the western side. Recent geological investigations further subdivide the No. 1 riverside sliding mass into two sub-sliding masses with varying sliding plane depths, designated as No. 1-1 and No. 1-2. On the eastern side, No. 1-1 has a sliding zone with a maximum burial depth of nearly 80 m, with sliding activity dating back approximately 100 ka. Conversely, No.