Investigation of Suffusion in Glacial Till Dam Cores : Testing methods and critical hydraulic gradients

Abstract: There are 10 000 dams in Sweden of varying size, age and type, being water retention and tailing dams the two major type of dams. About 2000 of the water retention dams belong to hydropower operations, out of which 190 are classified internationally as large dams (higher than 15m). Most of the large Swedish dams were built between 1950 and 1970, and typically consist on earth and rockfill embankment with a central core of glacial till flanked by a filter of sandy gravel. About sixty percent of the large Swedish hydropower dams built before 1997 experienced some kind of incidents named “deterioration events”. Leakage in the foundation or in the dam body combined with sinkholes in the dam crest or at the dam shoulders is the second most frequent type of incident also observed in Swedish tailings dams and typically related to internal erosion. Internal erosion is a deterioration process that occurs when the forces imposed by seepage exceed the ability of the soil particles within an embankment dam or its foundation to resist them. Internal erosion in dams is classified in four mechanisms depending on the cause triggering its initiation. Suffusion, also called internal instability, is one of the four internal erosion mechanisms that occurs either in the foundation or in the core soil of embankment dams when fine-grained particles are washed-out from the core soil matrix by seepage. The initiation of suffusion depends on three major factors: grain size distribution of the soil exposed to suffusion, confining stresses and the hydraulic gradient; whilst its continuation depends of the properties of the downstream filter. Broadly graded moraines, as glacial tills, are more susceptible to internal erosion by suffusion than other types of soils used in dams. Therefore, suffusion is an important dam safety issue in Swedish dams to be address and further studied. Up to date several studies on suffusion have contributed to a better understanding of the geotechnical properties that define the soil’s susceptibility to suffusion. Nevertheless, the influence of the hydraulic conditions on the initiation of suffusion is not understood completely yet. This circumstance motivates further research on suffusion taking into account the hydraulic perspective. In this sense, this study aims to contribute to the assessment of dam safety by defining: i) the optimum empirical method to evaluate the susceptibility to suffusion of glacial till soils; ii) the effects of boundary and testing conditions in the experimental evaluation of the hydraulic gradient inducing the initiation of suffusion in glacial till soils; and iii) reference values of the hydraulic gradients that may induce, under certain conditions, the initiation of suffusion in glacial till soils.To fulfil the research aim the study includes a literature review on the existing empirical methods to evaluate soil’s susceptibility to suffusion, a comparison among the difference testing and boundary conditions applied in the experimental assessment of soil’s susceptibility to suffusion, and an experimental study aiming to determine the critical hydraulic gradient for suffusion to initiate in glacial till soils. The experimental study includes tills with different boundary conditions such as: type of filter, degree of compaction and specimen scale, as well as different particle size distributions representing different initial conditions in the core material, e.g.: internally stable, internally unstable and critically internally unstable. Results show that the Kenney and Lau (1985, 1986) method adapted by Li and Fannin (2008) and Rönnqvist (2015) is the optimum empirical method to evaluate the susceptibility of till soils to suffusion. Furthermore, the critical hydraulic gradient triggering suffusion in a soil is not a unique value but depends on the boundary condition. The boundary conditions playing a major role in the definition of critical hydraulic gradient are: initial and final consolidation stage and filter capability to arrest the eroded soil. Nevertheless, it is also important to highlight that the critical hydraulic gradient of internally unstable soils (susceptible to suffusion) tends to be lower than 5 when tested in the absence of confining stress. This value is considerable higher when the soil is under confining stress. The higher the confining stress, the higher the critical hydraulic gradient as long as the filter is suitable for the core soil exposed to suffusion.