## 5 Discussion  
### Stability Mechanism  
Based on detailed field investigations, numerical simulations, and comprehensive analysis, this study concludes that the inverse grading structure formed by high-velocity long-runout hard rock landslides is a governing factor in the stability of the Conaxue Co landslide dam. Before analyzing the formation mechanism of the inverse grading structure within the dam, a brief overview of the formation process of the Conaxue Co landslide dam is required. Based on previous studies of high-speed long-runout landslides [26, 27], this study proposes a three-stage formation mechanism for the Conaxue Co landslide dam through field investigations and comprehensive analysis. The formation of the Conaxue Co landslide dam primarily consists of three stages (Figure 17): (1) the kinematic sieving stage; (2) the impact, ejection, and compaction stage; (3) the dam-forming stage. During the initial kinematic sieving stage, the granite slopes on both banks of the river channel were subjected to intense seismic forces, which resulted in failure and subsequent high-speed and long-distance movement (Figure 17A). During this movement, the relatively intact granite rock mass was subjected to an uneven sliding surface. This generated vibrational forces within the rock mass. When these forces exceeded the rock's shear strength, the originally intact rock from the source area was subsequently fragmented into blocky debris. Simultaneously, the intense vibrational forces generated interstitial voids between coarse particles within the landslide mass. This facilitated the downward migration of debris. As a result, vibrationally fragmented and size-heterogeneous mass was mechanically sorted into a coarse-over-fine stratified structure. This process is termed vibrational sieving [28, 29, 30]. During the subsequent impact, ejection, and compaction stage, the landslide masses originating from both banks of the river channel converged and collided near the valley floor. Coarse-grained boulders were ejected into the air by powerful impact forces. These large boulders then fell freely under gravity and subsequently impacted the deposit surface again. The descent of multi-ton massive boulders from considerable heights exerted significant compaction effects on the underlying fine-grained particles, resulting in increased density of the lower fine-grained layer (Figure 17B). Numerous studies on the kinematic characteristics of high-speed and long-distance landslides have also confirmed the presence of impact, ejection, and compaction processes during their formation [31, 32]. Finally, the landslide dam was formed after the impact, ejection, and compaction of coarse boulders on the underlying fine-grained particles were completed. With the continuous inflow of debris from upstream,  
Figure 16: Porosity and porosity variation rate at different locations within the Conaxue Co landslide dam. (**A**)—Porosity at different locations within the Conaxue Co landslide dam; (**B**)—porosity variation rate at different locations within the Conaxue Co landslide dam. the water of the landslide-dammed lake discharged through the relatively low-elevation spillway of the dam (Figure 17C). The formation process of the Conaxue Co landslide dam revealed that the fine-grained layer underlying the inverse grading structure exhibits a higher compaction degree. This resulted in a lower hydraulic conductivity within the lower fine-grained layer of the dam, forming a relatively impermeable aquitard. Consequently, the lake water can only discharge through the surface spillway, thereby effectively preventing seepage failure to the dam. During the dry season, the compacted fine-grained layer in the lower portion of the landslide dam acts as an impermeable aquitard to prevent Conaxue Co Lake from drying out due to seepage. The coarse-grained layer in the upper portion of the landslide dam enhances hydraulic conductivity, which enables rapid saturation following overdland flow.