3.2.1. Geological Structure and Formation Lithology

The landslide research area belongs to the western part of the Yili Valley. The terrain is generally high in the north and low in the south. It is gradually inclined from the northeast to the southwest. The elevation of the area is between 620 and 3700 m. It is a block-like eroded and uplifted mountain, covered with gravel and loess layers, showing a low mountain grassland landscape. The landslide area is located on the southwestern side of the West Tianshan Youdi trough fold belt in the southwestern Tianshan fold of the Tianshan-Xing'an trough fold area. It belongs to the junction of the Boroconu Mountain Complex Anticline and Yili Block and is located 2.7 km south of the Nalati deep fault zone (Figure 2a). The rocks in the study area are mainly Ishikirik group tuff, tuff lava, and gray-green coarse sandstone in the Carboniferous system and basalt in the Dahala Junshan Formation of the Carboniferous system. The surface layer is the Quaternary Holocene loess, with well-developed joints (Figure 2b). The structure was relatively loose, and the wormhole, large void structure, belonging to low-plastic silt. The silt (0.075–0.005 mm) content of the loess in the Piliqinghe area is high, reaching 69.8–86.0%; the fine sand (0.25–0.075 mm) content is 3.7–18.0%; and the clay (<0.005 mm) content is 10.3–12.2%. As a result, the loess expands and collapses after encountering water, and it is prone to motion liquefaction under certain static or dynamic water conditions, which provide good source conditions for landslides.

## 3.2.2. Topography

The Panjinbulake loess landslide was located on the south bank of the river and had long been subjected to the lateral erosion of the river, resulting in good conditions for the landslide front to be in the air. The hillslope was steep, with a slope of 40◦ (Figure 3a). Corresponding tensile stress condition occurred near the top of the mountain, and the cracks at the trailing edge of the landslide were gradually enlarged (Figure 3b). According to the Google Earth remote sensing image map from 18 May 2013, the front edge of the landslide had slipped. The sliding volume was about 9000 m3 (Figure 3a), and several tensile cracks appeared on the trailing edge. Consequently, a steep ridge of up to 1 m was formed (Figure 3c,d), which provided good topographic conditions for loess deformation and stress relief.

#### 3.2.3. Hydrogeological Condition

The landslide area belongs to the temperate continental semi-arid climate. The average annual precipitation (for the period of 2011–2016) is 330.6 mm. Precipitation is highest from March to July, during which the monthly rainfall exceeds 30 mm, accounting for 52.5% of the annual rainfall (Figure 2c). Snowfall mainly occurs from October to the following March. The snowfall thickness can reach 94 mm per month. The fissure water inside the slope is frozen, and the vertical joints and cracks become enlarged due to the frost heaving action. From mid-March, the temperature rises, the snow that covered the surface begins to melt, infiltrating the cracks and joints and forming a certain transient water pressure and transient saturation zone in the surface layer of the slope. This results in a decrease in the anti-sliding force of the slope, thus inducing landslides. The rising water level of the river also causes the hydraulic gradient inside the slope to drop. Ice and snow meltwater can also be stored in the mountain for a long time and can continue to increase the slope sliding force and accelerate the formation of the potential slip surface. In addition, fissures are relatively developed and accumulated in the bedrock, which is exposed in the landslide. There is a large amount of ice and snow meltwater, which readily forms a "pipeline" channel that is in contact with the surface of the Quaternary aeolian loess and is discharged outward in the form of a spring. The flow volume of a spring was measured to be 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> m3/s. The mineralization of water is less than 1.0 g/L. Based on the soil test, the saturation of the soil is 85.7–91.2%, which shows that it is a very wet sliding body. The natural moisture content of the soil was 17.2–20.4%, the plastic limit was 15.8%, and the liquid limit was 27.6%. According to the measured data of groundwater level, the groundwater in the hill is shallow and buried in the range

of 0–15 m. These provide good hydrological conditions for inducing landslides and also provide good groundwater conditions for the rapid conversion of loess landslides into high speed and long runout sliding landslides.

**Figure 3.** The multi-temporal sensing images of the landslide.

Qualitative analysis of the disaster-formation mechanism of the Panjinbulake landslide was conducted based on a field geological survey and remote sensing satellite images, but this was far from adequate for geological disaster prevention and control. Instead, quantitative analysis methods are required to fully investigate landslide movement and predict the secondary disaster, which can be explored using the landslide dynamic analysis software DAN-W (see Section 5).
