Suffusion is the process whereby fine particles are moved locally an transported through a coarse layer. This is a bulk erosion, which corresponds to a source term in the mass balance equations. This bulk erosion is counter intuitive, as erosion is basicaly an interfacial process. We suggest a framework within suffusion in a clayey sand can be viewed as a clay/water interface erosion at the microscopic scale. The coefficient of surface erosion of the clay matrix can therefore be used to quantify the suffusion rate. The proposed erosion law depends on the clay volume fraction, and accounts for the influence of the hydraulic gradient. Comparisons with published experimental data show that this approach gives good results over the full range of hydraulic gradient explored.