The hydrological and mechanical properties of granitic residual soils can be significantly altered by periodical wetting and drying (W-D) cycles. The soil structure degradation induced by W-D cycles can lead to soil mass failure and collapsing gully erosion in granitic hilly slopes in south China. However, limited attempts have been made at a comprehensive investigation of the effects of W-D cycles on the structure degradation of granitic residual soils, especially at the pedon scale. The purpose of this paper is to investigate the structural degradation of granite soils induced by W-D cycles and explore its potential influence on the development of collapsing gully erosion. The granitic soil properties, including hydraulic properties, shear strength, and disintegration characteristics, were performed after W-D cycles. The results indicated that the W-D cycles altered the soil pore structure, leading to variations in soil hydraulic properties. Specifically, with increasing alternate W-D cycles, the initial saturated water content and residual water content decreased, while the saturated hydraulic conductivity increased. Meanwhile, increasing W-D cycles contributed significantly to variations in cohesion and internal friction strength by decreasing the shear strength variables, especially the soil cohesion strength. Correspondingly, soil disintegration was increased during W-D cycles. Furthermore, most degradation of soil structure was recorded within the first two cycles of W-D. The obtained results indicate that the W-D cycles weaken soil structure, increase rainwater infiltration, decrease soil shear strength and disintegration resistance, and accelerate soil erosion. A vicious cycle of granitic slope failure induced by W-D cycles is eventually formed. This study provides useful information about the mechanism of soil mass failure and collapsing gully erosion in granitic hilly slopes.

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