## 2 Background  
### Geological Setting  
The Huangtupo landslide is located in Badong County, Hubei Province, on the southern bank of the Yangtze River, approximately 69 km from the Three Gorges Dam (see Figure 1a). As one of the largest and most destructive landslides within the TGRA, the Huangtupo landslide comprises several sliding masses: the No. 1 (northwest) and the No. 2 (northeast) riverside sliding masses, the No. 3 substation slide (southwest), and the No. 4 garden spot slide (southeast). The No. 1 riverside sliding mass can be further divided into two partially overlapping secondary sliding masses, designated as sliding masses No. 1-1 and No. 1-2, with volumes of approximately 13 and 5 million m\({}^{3}\), respectively [30]. In terms of topography, the landslide exhibits a steep morphology at both the front and rear sections, while the central portion is relatively gentle (Figure 1b,c). The elevation of the Huangtupo landslide ranges from 50 m to 600 m above sea level, with the shear outlets of the No. 1 and No. 2 sliding masses submerged in the Yangtze River, where the reservoir water level fluctuates between 145 m and 175 m [31]. The total area of the Huangtupo landslide is 1.35 km\({}^{2}\), with a total volume of 69.34 million m\({}^{3}\), meaning it can be categorized as a typical giant landslide within the TGRA.  
Monitoring data indicate that the No. 1 riverside sliding mass is the most actively deforming area within this large-scale landslide group. Consequently, it has garnered significant attention from governmental, scientific, engineering, and research communities. To systematically investigate the deformation mechanisms, failure processes, and potential deformation trends in the No. 1 riverside sliding mass, the China University of Geosciences established a comprehensive surface and subsurface multi-field monitoring system in the area in December 2012. This system includes an underground observation tunnel group within the No. 1 riverside sliding mass, consisting of one main tunnel and five branch tunnels, with a total length of 1.1 km and tunnel diameters ranging from 3 to 5 m. Additionally, various surface and subsurface monitoring instruments were installed to characterize the internal structure of the landslide and to monitor deep deformation processes.  
The Huangtupo landslide is located on the dipping strata of the Badong Group, Middle Triassic Formation (\(T_{2}b\)), which contains multiple weak layers and major rupture zones [32]. The material of the sliding mass consists of block rock, fractured rock, and disintegrated rock, along with gravel soils originating from the second and third segments of the Badong Formation (\(T_{2}b^{2}\), \(T_{2}b^{3}\)). The sliding zone of the riverside sliding masses is composed of grayish-yellow silty clays with gravel, while the sliding zone of the overlaying No. 3 and No. 4 landslides is formed by brownish-red silty clay with gravel. The underlying stable bedrock of the Huangtupo landslide consists of limestone and dolomitic limestone from the Jialingjiang Group and Lower Triassic Formation (\(T_{1}j\)), as well as interlayered clastic and carbonate rocks from the Badong Formation (\(T_{2}b^{3}\)) [33]. The dip direction of the bedrock is N25\({}^{\circ}\) E, with a dip angle varying between 30\({}^{\circ}\) and 50\({}^{\circ}\). The sliding zone generally follows the dip direction of the underlying bedrock [30], as illustrated in Figure 2.  
Figure 1: Location (a), photo (b), and plan view (c) of the Huangtupo landslide.