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

Abstract

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 < 1.3 m) and the soil at the shallower depth of the sandy soil layer (depth < 3.0 m) showed similar disintegration behaviours, which were complete (Rd = 100%) and rapid, with vI values in the range of 66.7–266.7%/min. The soil disintegration in the sandy soil layer was characterised by an incremental increase in Rd to 100% within 100 min. The residual soil at the bottom of the sandy soil layer and the top of the detritus layer (depths of 4.0–8.0 m) disintegrated consistently at the first, after which the disintegration rate gradually decreased with vIII lower than 1%/min. The detritus layer soil at a depth greater than 10.0 m showed incomplete disintegration, and the ultimate Rd was approximately 60%. The formation mechanism for the soil disintegration and gully collapse was also proposed. The weakening of cementation triggered the breakup of soil aggregates and led to soil disintegration and the occurrence of a gully collapse. This study provided new insights on gully erosion.