An experimental study on influence of grain-size composition on collapsing erosion of granite residual soil

Abstract

Severe collapsing erosion frequently occurs in South China, where granite residual soil is widely distributed and causes a serious loss of water and soil. The grain-size composition of the granite residual soil varied significantly, resulting in a remarkable variation in the collapsing erosion. The effect of grain-size composition on the morphological characteristics of collapsing erosion is rarely studied. Therefore, in this study, laboratory-scale experiments were conducted on the slopes of granite residual soil with three different grain-size compositions using self-designed rainfall simulation equipment to investigate the influence of the grain-size composition on the evolution of collapsing erosion. The erosion process, morphological characteristics, moisture content, and soil temperature of the collapsing erosion were simultaneously monitored during the experiments. The results show that the main erosion factors and their effects vary significantly for slopes with different grain-size compositions. The damage induced by ephemeral gully erosion and gully erosion is slighter on slopes with more gravel because the gravel can effectively resist the shear stress of runoff. However, on a slope with fine soil, the depth of gullies increases rapidly, causing significant gully erosion and serious soil collapse. Moreover, both the average width and width-to-depth ratio of the rills or gullies increased with gravel content. The infiltration rate is lower and the moisture content increases slower for slopes with higher gravel content, because the existence of gravel prolongs the infiltration path of rainfall and results in earlier runoff and a higher runoff rate. However, in a slope with fine soil, a high infiltration rate induces a sharp increase in the moisture content, accompanied by severe collapsing erosion. A deeper temperature diffusion range of the slope with more gravel was observed because the preferential flow through larger pores in the soil with higher gravel content induces a deeper wetting front.