Formation mechanism of collapsing gully in southern China and the relationship with granite residual soil: A geotechnical perspective

•Cross-disciplinary work that explains a collapsing gully’s formation mechanism.•Collapsing gully’s material source is systematically studied from a geotechnical perspective.•Soil disintegration process is quantified.•Weakening of cementation is shown to be responsible for disintegration and collapsing gully. Recently, there has been an increase in collapsing gullies in the south of China as one of the most destructive types of soil erosion. Most collapsing gullies are formed on a well-developed granite crust; thus, granite residual soil plays a critical role. However, the extent to which the geotechnical features of residual soil, especially soil disintegration, affects collapsing gully formation is poorly understood. This study performed laboratory disintegration tests on granite residual soil taken from the red soil, sandy soil, and detritus layers of a collapsing gully. The disintegration behaviour was quantified by defining the disintegration ratio, Rd, and three equivalent disintegration rates, vI, vII, and vIII, corresponding to Rd = 10%, 30%, and 50%, respectively. The results revealed that the red soil layer (depth