<p>Seepage-induced internal erosion in sandy clay soils can result in internal instability, which may cause disasters in coastal hydraulic structures. Owing to the difficulty in quantifying eroded clay minerals during seepage tests, the erosion process in sandy clay soils has not been elucidated. In this study, we apply a novel triaxial apparatus to simulate the three-dimensional stress state such that the soil erosion process can be investigated. Tests are performed on artificial sandy soil with different kaolinite contents and under different hydraulic gradients. Our results show that soil erosion in samples with a higher clay fraction tends to result in the loss of clay particles at a higher rate. Hydraulic conductivities show a decreasing trend with time because the eroded clay particles concentrate locally, block some seepage conduits, and create seepage-induced heterogeneity. With the same clay fraction, the hydraulic conductivity of samples increases gradually with the applied hydraulic gradient. This difference is more significant in samples with lower clay content than in samples with higher clay content. In this paper, a structural state model is adopted to quantitatively explain the erosion differences in samples with different clay content.</p>