Suffusion of Glacial Till Dam Cores : An Experimental Investigation

Sammanfattning: Suffusion, also called internal instability, is an internal erosion mechanism that occurs in zoned 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 side distribution of the soil, stress conditions and hydraulic load; whilst its continuation depends of the filter. Broadly graded moraines (glacial tills) are, to a large extent, more susceptible to suffusion than other types of materials.In Sweden, most embankment dams 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 related to internal erosion susceptibility for the core and the filter. Today, several Swedish embankment dams has experienced recurring incident of internal erosion such as leakages and sinkholes, making internal erosion an important safety issue. This circumstance allows raising the question: How safe are the old 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 status by providing a screen plot that gives a better idea understanding regarding the sensitivity to internal erosion by suffusion in glacial tills. The screen plot intents to integrate the three major factors involved in the initiation of internal erosion. This in order to define what the critical hydraulic load for a given till material is, with a known compaction degree, to get internal erosion by suffusion.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 setups were performed: small and large scale suffusion tests. The first group serves 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 scale.Results show that the influence of compaction degree on the initiation of suffusion of is limited in internally stable soils. However, poorly compacted specimens under high hydraulic gradients could present internal erosion by suffusion if the filter is not capable of retain the eroded particles. Furthermore, as the hydraulic gradient increases, hydraulic conductivity tends to decreases due to an internal suffusion process that initiates as a result of the matrix of soil reaching equilibrium with the new seepage stresses. The rate of variation of hydraulic conductivity depends on the rate of increment of hydraulic gradient.