Background Collapsing gully is a special kind of soil erosion phenomenon in southern China, and may destroy the farmland by depleting soil nutrients, water and fertility, thus damaging the ecological environment and threatening public safety. Studying the weathering characteristics of collapsing gully sections can facilitate a better understanding of the formation mechanism of collapsing gully and provide a theoretical basis for collapsing gully prevention.

Methods Soil samples were collected from the granite collapsing gully in Tongcheng county, Hubei province, and gully profile was divided into 5 layers from top to down based on soil color and texture, named TC1, TC2, TC3, TC4, and TC5. The bulk density was measured, and the basic physicochemical properties of soil samples were determined. The particle composition was determined by the pipette method and the cation exchange capacity by the ammonium acetate exchange method. Soil cohesion was measured under natural water content using a straight ring knife. The total element content was measured with a plasma emission spectrometer and the result was converted to the oxide percentage. The oxide content in different layers was investigated by a quantitative analysis. The weathering intensity was calculated with a formula. The weathering characteristics of granite collapse lands and the formation mechanism of collapsing gully were explored by analyzing the relationship of the weathering intensity with particle composition, cation exchange capacity, cohesion and other factors.

Results The weathering crust of granite is in desiliconization by Fe-Al-infiltration and salt-based leaching. As the depth of soil decreases, the content of Al₂O₃ and Fe₂O₃ increases while the content of SiO₂ decreases. Additionally, the content of K₂O, CaO, Na₂O and MgO is low in the soil surface layer, but higher in the soil middle and deep layers. The amount of cation exchange increases gradually from bottom to top. Soil cohesion under natural water content also increases from bottom to top, with the maximum in TC2. The degree of weathering follows the order of TC2 > TC1 > TC3 > TC4 > TC5, with the overall trend that weathering increases with the decrease of soil depth.

Conclusions  Weathering intensity is positively correlated with clay content, cation exchange capacity, and soil surface weathering intensity, but negatively correlated with sand content. A higher weathering degree shows a higher content of clay and free iron oxide, a stronger structure and higher cohesion. Meanwhile, poor drainage during precipitation leads to an increase in the weight of the soil layer. The ratio of the soil macrospores increases from TC1 to TC5 while the capillary porosity decreases, making it easier for water to invade the underlying soil. In contrast, a low weathering degree indicates a decrease of clay content, an increase of sand content, lack of cementing material, easy water saturation, and low cohesion, as well as easy moisture dispersion, large gaps between particles, and faster formation of dry soil cracks due to coarse particles. These factors are responsible for weathered crust soil erosion, poor corrosion resistance, instability, vulnerability to collapse, and the formation of collapsing gully.