The most common type of retaining structure in Quebec is the embankment dam. Constructed with geological materials, these dams may suffer from the migration of fine particles under seepage flow, a phenomenon known as suffusion. This selective internal erosion depends largely on the internal stability of the materials, which in turn depends on their grain size distribution (GSD). The most reliable approach to assess the internal stability is through laboratory flow tests. As an alternative to these costly and time-consuming tests, geometric criteria based on the GSD are often applied, with the semi-empirical criteria of Kezdi, Sherard, and Kenney & Lau being the most commonly used.
Both experimental and geometrical approaches have limitations that affect the reliability of internal stability assessment. Flow tests exhibit significant variability across laboratory studies, partly due to a lack of standardization. They differ in their experimental procedures, in the parameters measured during testing, and in the thresholds used to distinguish between stable and unstable materials. Although imperfect, these experimental results are used to identify the advantages and limitations of semi-empirical criteria, in particular those of Kezdi, Sherard and Kenney & Lau. These criteria, generally considered conservative, are often modified and/or combined, resulting in a multitude of derived criteria which may diverge in their assessment of internal stability. The main objective of this thesis is therefore to improve the assessment of internal stability in order to enhance the safety of embankment dams. Two sub-objectives are pursued, each based on one of the two existing approaches to assessing internal stability.
The first sub-objective comprises two stages. The first stage evaluates the efficiency of various methods of saturating granular materials. The results show that complete saturation is achieved by combining a method of removing dissolved air from the water with a method of eliminating air from the pores. The most effective approaches combine an upward seepage flow of deaerated water, produced by cavitation under vacuum, with either a purge of carbon dioxide (CO2) or vacuum applied at the outlet. The CO2 purge method, which achieves a complete saturation in a shorter time, is selected for the flow tests of the second stage. This stage aims to establish a rigorous experimental criterion for assessing the internal stability. Coarse granular materials, classified according to their internal stability experimentally established in the literature, were reconstituted with glass beads and tested under seepage flow in a rigid-wall permeameter designed for this study. Parameters sensitive to fine migration were monitored and analyzed according to the published assessment of the internal stability. Four threshold conditions, each associated with two parameters defining the internal structure of the materials, must be satisfied simultaneously for a material to be considered stable.
The second sub-objective is to establish a more reliable geometric criterion for assessing internal stability by comparing various semi-empirical criteria, particularly those of Kezdi, Sherard, and Kenney & Lau, as well as their derivatives. Applied to selected GSDs from the literature, the results of these assessments are compared with the corresponding published experimental results. The comparison indicates that two methods are required to properly assess internal stability, depending on the GSD form: the De Mello criterion for discontinuous GSDs, and a combination of the modified Sherard and Kenney & Lau criteria for continuous GSDs.
Bouin, C. (Author),
Éthier (Supervisor) &
Dubé (Co-supervisor),
17 Dec 2025Student thesis: Doctoral thesis › Doctorate in Engineering: Engineering