## 4 Discussion  
### Factors Influencing Retrogressive Loess Landslides  
Landslides are often caused by the combined effects of multiple factors [44]. In this study, the factors influencing the retrogressive landslides in the study area can be categorized into several aspects: triggering factors, predisposing factors, and precursors. First, the rise in groundwater levels is a triggering factor for landslide occurrences. The groundwater level in the platform has significantly increased due to long-term irrigation (i.e., about 50 years). As shown in Figure 6a, the groundwater level in Jiangliu, Zhaitou, and Miaodian villages increased by 24.5 m, 38.2 m, and 24.0 m from 1976 to 2016, respectively. This increase in groundwater level is a key factor in triggering retrogressive landslides. As the groundwater rises, it saturates the loess soil, which is highly sensitive to moisture, weakening its internal structure and reducing the slope's shear strength. We observed that the fluctuation in groundwater levels before 2016 was relatively large, and the rise in groundwater contributed to higher soil moisture content at the slope base and surface, further decreasing slope stability. Water seepage is commonly observed at the slope toe or slope body, particularly near the terrace surface, as evidenced by the field investigation in Figure 6b. In some areas, the groundwater level in the slope has even risen above the platform surface, resulting in an increased soil moisture content. Our field measurements confirm that areas of higher soil moisture correlate with observed landslides, indicating that saturation could be a critical factor in slope failure. This dynamic is consistent with the findings of previous studies, which identified the saturation of loess as a crucial precondition for retrogressive landslides. For instance, when the groundwater level rises, it reduces the internal cohesion of loess soils, thus triggering the slope failure. Cracks serve as precursors to the development of retrogressive landslides; the initiation and progression of these cracks play a significant role in the destabilization of the slope. As shown in Figure 7, the cracks are predominantly located near the scrap edge of the retrogressive landslide group at Zhaitou village. These cracks, with horizontal displacements of 26 cm, 23 cm, and 15 cm (Figure 7a,b), and vertical displacements of 50 cm, 35 cm, and 20 cm (Figure 7c), are indicative of the deformation caused by rising groundwater levels. Our analysis indicates that these cracks primarily develop due to the combined effects of moisture infiltration and mechanical stress on the loess soil. The long-term effect of repeated irrigation and groundwater rise seems to promote the expansion of these cracks, which reduces the slope's ability to withstand shear forces. Further investigation of the crack distribution (Figure 8) shows that 36 cracks with lengths exceeding 5 m were observed along the slope edge, of which six were more than 180 m in length. These large cracks, more than 90% of which are within 30 m of the slope edge, indicate the critical role of moisture penetration in weakening the soil structure. As groundwater rises, these cracks are progressively widened by water infiltration, further destabilizing the slope. In this context, the relationship between crack expansion and landslide occurrence is clear: as the cracks expand, they serve as pathways for increased water infiltration, which reduces the internal strength of the loess, making the slope more susceptible to failure. Figure 6: Groundwater effects on the landslide in the study area. (**a**) Comparison of groundwater level between 1976 and 2016; (**b**) groundwater seepage on slopes and terraces. Figure 7: Distribution of loess cracks near the scarp edge of the ZT landslide group at the Zhaitou village. (**a**) Field pictures; (**b**) Landslide sidewall; (**c**) Distribution of loess cracks.