Internal erosion by suffusion occurs in the core of an embankment dam when the ability of the soil to resist seepage forces is exceeded and voids are large enough to allow the transport of fine particles through the pores. Soils susceptible to suffusion are described as internally unstable (ICOLD, 2015). As recognized by Sherard (1979), dams with core of broadly graded glacial moraines (tills) exhibit signs of internal erosion to a larger extent than dams constructed with other types of materials.

This contribution presents a description of the laboratory program and set-up defined as part of a research on internal erosion in embankment dams currently in progress at Luleå University of Technology (LTU). The aim of the research is to determine the hydraulic gradient to initiate internal erosion by suffusion in a given moraine used as core fill.

The testing program includes three categories of till soil: i) internally stable, ii) internally unstable; and iii) soils in the transition zone between the two first categories. The categories are defined based on the soil grain size distribution and according to the methods developed by Kenney & Lau (1985, 1986), the modified Burenkova (1993) method proposed by Wan & Fell (2008) and the unified plot approach proposed by Rönnqvist & Viklander (2015).

Samples of each category are prepared at four degree of compaction defined respect to the modified Proctor test. The degrees of compaction considered are: a) 95%, representing a well compacted material based on the recommendation of the current Swedish dam safety guidelines (Svensk Energy, 2012); b) 90% representing a material on the borderline of acceptance; c) 85% representing low compacted material; and d) 80% representing poorly compacted material.

Each sample of 200 mm thickness is compacted in four layers of 50 mm in a steel permeameter with a diameter of 300 mm and a maximum height of 450 mm. Tests are performed with a downward flow after upward saturation of the samples. A layer of coarse material of 150 mm thickness is placed over the sample in order to allow a uniform distribution of the downward flow. To obtain a quicker saturation of the samples, the air content in the gaseous phase is replaced by upward incorporation of CO2 (carbon dioxide) before water saturation. A filter of 50 mm thickness is located at the bottom of the sample; the grain size distribution of this layer is defined according to the criteria given in the Swedish guidelines (Svensk Energi, 2012).

The hydraulic gradient will be increased stepwise until the onset of internal instability, which can be established on the basis of three conditions: end of visual observation of outflow turbidity, the hydraulic pressure head at various depths of the sample, and the rate of water flow through the sample attained steady values (Wan & Fell, 2008). Piezometer measurements are carried out at the interface between different materials and between the layers of 50 mm that compound the specimen.

Post-test examination and diagnosis of the samples will be performed following similar criteria to those applied by Rönnqvist et al. (2017), including vertical displacement measurements and post-test grain size distribution analysis by layer. Layers are identified with suffusion if the post-test gradation curve exhibit changes in distribution compared to the initial condition, without appreciable volume change.

Results will show the effect of grain size distribution and relative degree of compaction on the internal erosion susceptibility of glacial till soils at different hydraulic gradients.