Internal Erosion in Glacial Tills: Mechanisms, Assessment, and the Potential of Transparent Soils—A Review

Internal erosion is a major threat to the long-term performance of embankment dams, yet its initiation and progression remain difficult to predict, particularly in broadly graded glacial tills. This review examines factors governing internal erosion susceptibility and integrates findings from empirical stability criteria, numerical modeling, hydro-mechanical investigations, and emerging transparent soil techniques. The review indicates that internal stability cannot be characterized solely by particle size distribution, but depends on the combined influence of soil structure, hydraulic loading, and stress conditions. Furthermore, suffusion emerges as a progressive hydro-mechanical process in which fine particle migration continuously modifies the soil structure and its hydraulic and mechanical response, thereby influencing the subsequent progression of erosion. Empirical criteria remain valuable screening tools, whereas numerical and experimental approaches provide complementary information on particle-scale and hydro-mechanical mechanisms. Transparent soils, formed by combining synthetic soil surrogates with refractive-index-matched pore fluids, provide optical access to internal processes and enable direct visualization of particle migration, clogging, and preferential flow. However, their application to glacial tills requires sufficient comparability of the physical, hydraulic, and mechanical characteristics governing internal erosion. Integrating these approaches may provide a more comprehensive framework for assessing suffusion in glacial till embankment dam cores.