The flow of small particles in porous media is of interest in many branches of science. In geotechnical engineering, this phenomenon usually occurs in graded granular materials and is termed as internal erosion. In this work, graded granular materials are simplified as binary mixtures under vertical compression, and water flows perpendicular to the vertical direction. We use the coupled Lattice Boltzmann method (IBM) and discrete element method (DEM) to investigate the initiation of internal erosion under different stress levels and hydraulic gradients. Suffusion was observed as the fine particles are transported by the seepage flow. The change of void ratio due to water flow and vertical compression demonstrates three patterns: erosion, stability, and compaction. Particles with low engagement in force transmission are more prone to erosion, while the hydraulic gradient is an external disturbance factor. The ballistic, caging, and diffusive regimes are observed in the initial process of internal erosion. (C) 2020 Elsevier B.V. All rights reserved.