Microtopography promoting Benggang erosion: Formation and stability of niche on collapsing wall

Benggang erosion occurs in granite red soil regions of southern China, where headward erosion acts as a key driver of its development. The development of niches on collapsing walls marks the initiation of Benggang headward erosion. The morphological evolution and stability mechanisms of niches remain relatively understudied. In this study, the morphological characteristics and mechanical properties of natural niches were analyzed based on field investigations and representative case studies to identify the dominant controlling factors. The results revealed that significant morphological variability among niches was observed. Moreover, the long axis, short axis and depth of concavity were highly significantly positively correlated (p < 0.01), collectively influencing the other parameters. Characterized by downcutting erosion, niches exhibited a primarily elliptical planar morphology. Soil properties within collapsing wall profiles characterized by niche development exhibited pronounced spatial heterogeneity. Vertical heterogeneity of shear strength was fundamental to niche development, demonstrating a positive linear relationship with the factor of safety (FS). A decrease in clay content and an increase in moisture reduced niche stability, establishing feedback mechanisms between niche development and soil destabilization. Organic matter, clay content and free iron oxide were critical soil properties influencing niche stability, with favorable predictive capacities (R² = 0.876). Specifically, organic matter had the highest importance score (0.402), followed closely by clay content (0.312) and free iron oxide (0.312). Morphological parameters were key influencers of niche stability. Random forest and structural equation modeling revealed that the contribution of the length-depth ratio (LDR) was the highest for FS, followed by the short-depth ratio (SDR). These findings offered significant insights for monitoring and modeling the morphological evolution of niches, enhancing the understanding of the headward erosion of collapsing walls.