Internal Erosion in Colluvial Gravelly Soil Under Different Rainfall Intensities and Its Effects on Water Migration and Soil Structure

Rainfall infiltration commonly triggers shallow landslides by raising pore-water pressure and reducing shear strength. This mechanism, however, presupposes that the soil structure remains stable. In colluvial slopes composed of gravelly soils, rainfall infiltration may induce the migration of fine particles, a phenomenon known as internal erosion that alters the soil structure and consequently affects slope stability. To investigate internal erosion in colluvial gravelly soil under different rainfall intensities and its effects on water migration and soil structural characteristics, a series of laboratory soil column tests were conducted using a self-designed rainfall infiltration apparatus. The wetting front migration, soil moisture evolution, and fine-particle outflow under five rainfall intensities (30, 60, 90, 120, and 150 mm/h), together with the changes in grain-size distribution and pore structure before and after the tests, were systematically analyzed. The results indicate that rainfall intensity exerted a markedly layer-dependent influence on the infiltration process of colluvial gravelly soil: the steady-state moisture content increased most markedly in the upper layer and moderately in the lower layer but remained almost unchanged in the middle layer. Under the present test conditions, a rainfall intensity threshold of 30–60 mm/h was identified for the onset of fine-particle mobilization. Below this threshold, no particle washout occurred. Once it was exceeded, the effluent fine-particle concentration exhibited an initial peak followed by a later-stage decline, and the cumulative eroded mass increased with rainfall intensity. The vertical distribution of particle loss was closely aligned with the moisture content response across layers. The image-estimated porosity change rate at the front face of the soil column increased with rainfall intensity. It was negative under low-intensity rainfall, with values of −5.07% and −4.63% at 30 and 60 mm/h. At higher intensities, it turned positive, reaching 5.4% at 90 mm/h and 19.88% and 33.96% at 120 and 150 mm/h. Under rainfall intensities of 120 and 150 mm/h, coarsening zones and seepage pathways became visible on the column surface. This study demonstrates that rainfall-driven internal erosion can substantially alter the hydraulic and structural properties of colluvial gravelly soils under controlled laboratory conditions. The findings improve our understanding of rainfall-induced internal erosion in colluvial gravelly soils and provide an experimental reference for evaluating its implications for slope stability.