Dam failures and landslides can be caused by erosional processes. Suffusion, which is a special type of internal erosion, is the initial stage of the backward erosion process, which leads to a creeping degradation of the dam’s stability and, ultimately, catastrophic outcomes. A new model based on the Theory of Porous Media represents the mechanical aspects of particle transport through the pore space. Resistivities are defined to quantify the interaction forces between the three soil constituents: grain skeleton, water, and particles. The particles can be eroded and deposited. These three phases can move independently, and their motion is described by balance equations and constitutive laws. The constitutive laws quantify the mass exchange between the grain skeleton and the particles and the momentum exchange between all three constituents. An analytical result for a boundary value problem is presented. The results are plausible for different flow regimes and different particle sizes.