Granite residual soil properties in collapsing gullies of south China: spatial variations and effects on collapsing gully erosion

Collapsing gully erosion, a special erosion phenomenon in the hilly granitic regions, is an important factor responsible for the deterioration of the ecological environment in southern China. The granite residual soil properties tend to vary with soil depth and their spatial variations have an important impact on the formation and development of collapsing gullies. Here, two typical granitic soil profiles in the north boundary of collapsing gully erosion in south China were selected to investigated the spatial variations of soil physicochemical, hydraulic, and mechanical properties along the profile and their influences on collapsing gully erosion. The upper two soil layers (surface soil layer and red soil layer) were significantly different from the lower two soil layers (sandy soil layer and detritus layer) in the investigated properties. The soil texture performed a large variation along the profile, which might be a positive factor for the formation of secondary vertical joints, faults, and weak structural planes. Meanwhile, the lower soil layers had a lower value of PL, LL, organic matter, and iron-aluminum oxides, demonstrating their poor structure and vulnerability to erosion under rainfall conditions. Additionally, the upper soil layers had a higher water-holding capacity, but a lower decreasing rate in the unsaturated coefficient of permeability, leading to the overburdening of the lower soil layers under rainfall conditions. And the low soil layers had a low shear strength, which could also lead to the sliding of the gully head and sidewall, especially under saturated conditions. Collectively, the special geological structure of the granite residual soil layers plays an important role in the occurrence and development of collapsing gully erosion. The data reported in this paper may facilitate a better understanding of soil mass failure on the gully head and the collapsing gullies.