Suffusion, also called internal instability, is an internal erosion mechanism that occurs in embankment dams when fine-grained particles are washed out of the core soil matrix by seepage. Initiation of internal erosion depends mainly on three major factors: grain size distribution of the soil, stress conditions and hydraulic load; whilst its continuation depends of the filter properties. Broadly graded moraines, as glacial tills, are more susceptible to internal erosion by suffusion than other types of soils used in dams.

Most embankment dams in Sweden consist of a central core of glacial till built more than 50 years ago. At the time of its construction, the available guidelines did not include specific grain size boundaries for the core and the filter related to internal erosion susceptibility. Today, several Swedish embankment dams have experienced incident of internal erosion such as leakages and sinkholes, making internal erosion an important safety issue. This circumstance allows raising the questions: How safe are the Swedish embankment dams and what conditions are needed for internal erosion by suffusion to initiate?

This research aims to contribute to the assessment of dam safety by giving inputs regarding the characterization of internal erosion by suffusion and the relation among the main factors involved on its occurrence (geotechnical characteristics of soil material, degree of compaction and hydraulic load). This in order to increase the knowledge regarding the critical hydraulic gradient needed to develop suffusion in a given till material with a known degree of compaction.

The research includes a laboratory program consistent on suffusion tests, which is an extension of the standard permeability test, and considers post-test examination and diagnosis of the samples. Two main groups of tests were performed: small and large suffusion tests. The small tests serve as a reference of the expected behaviour of soil samples under different boundary and test conditions, which allows optimizing the number of test to be performed in large tests.

Results show that suffusion mechanism can be classified as internal suffusion (or filtration) and external suffusion (loss of soil particles from the soil matrix). The influence of compaction degree on the initiation of suffusion is limited in internally stable soils. However, poorly compacted specimens exposed to high hydraulic gradients could develop both internal and external suffusion if the filter is not capable to retain the eroded particles. The hydraulic conductivity of specimens with internal suffusion tends to decrease with a step wise increase of the hydraulic gradient. Such tendency is the result of the matrix of soil reaching equilibrium with the new seepage stresses. The hydraulic conductivity of specimens with external suffusion tends to increase with the increase of the hydraulic gradient.