Background The special composition and structure of the soil in the collapsing gully development area create its unique soil-water characteristic properties, which is also the main internal cause for the soil in this area to be prone to disintegration and thus form gully erosion. Currently, there are relatively few systematic theoretical research results on the influence of soil composition and structure on the soil-water characteristics and the subsequent triggering of collapsing gully.

Methods The residual soil and fully weathered soil in the high-incidence area and low-incidence area of collapsing gully were selected, and the soil-water characteristic curve and ψ-k curve of each sample were compared to obtain the soil-water characteristic specificity of fully weathered soil in the high-incidence area of collapsing gully, and the correlation and sensitivity between soil-water characteristics and soil composition were analyzed. Accordingly, the specificity of pore structure of fully weathered soil in the high-incidence area was deduced. The results were verified by SEM images.

Results Ultimately, it was concluded that compared to the reticular pore channels formed by residual soil with fine-grained platy minerals, the fully weathered soil in the high-incidence area has a soil skeleton composed of coarse particles, with fine-grained minerals filling the interior. As a result, its pores form a binary structure with "dominant pores" and fine pores. This pore structure leads to the soil water characteristic curve parameters of the fully weathered soil in the high-incidence area presenting characteristics of a smaller a value, larger n and m, and a steeper slope in the steepening section of the ψ-k curve. Macroscopically, it manifests as large air intake values, poor water retention, and large variations in permeability coefficients. It can be inferred that the formation of collapsing gully is mainly controlled by three factors: 1）The poor water retention properties of fully weathered soil in the high-incidence area make it more prone to repeated water entry and loss, resulting in erosion and deterioration of the cohesive substances under the cycle of dry and wet conditions, and a decrease in strength. 2）Under natural conditions, the low water content, high suction, and high permeability characteristics of fully weathered soil in the high-incidence area cause rapid infiltration of pore water during rainfall, a sharp increase in pore pressure, and soil failure. 3）The particle and pore characteristics of fully weathered soil in the high-incidence area lead to a slow increase in wet suction and a relatively low cohesive force provided by pore water.

Conclusions To sum up, the composition and structure of all weathered soil in the collapsing gully have obvious control over soil-water characteristics of soil, and the specificity of the soil-water-force coupling is the main reason for the formation of the collapsing gully.