Internal erosion is one of the major reasons for embankment dam failures. Despite this, the knowledge of the temporal development of internal erosion in full scale structures is limited. Detection of internal erosion is complicated using conventional methods, and new or improved methods are appreciated. Hällby was the first Swedish embankment to get a permanently installed monitoring system intended for resistivity measurements. Daily measurements started to take place already in late 1996, which make these long term monitoring data unique. This paper includes examples of long term time series from Hällby along with some evaluation and interpretation techniques used when analysing such monitoring data. Time-lapse inversion was used to focus the variation over time and suppress artefacts due to the resistivity structure. Seasonal resistivity variations inside the dam are obvious. Increasing long term resistivity has been noticed in a particular zone in the left embankment. This zone also exhibits larger seasonal variations relative to other parts of the dam, and the variations are increasing. The observations may relate to an ongoing internal erosion process in the dam. The long term change may be indicative of a change in material properties, and the large and increasing variations may reflect higher and increasing seepage levels. In the years 2004 to 2006 the dam was upgraded and the resistivity system reinstalled. The results indicate that resistivity monitoring may have a chance of detecting development of internal erosion at an early stage.
Methods for monitoring seepage are important for the safety of embankment dams. Increased seepage may be associated with internal erosion in the dam and internal erosion is one of the main reasons for dam failures. Internal erosion progresses inside the dam and is difficult to detect by conventional methods. Therefore there is a need for new or improved methods. The resistivity method is a nondestructive method that may accomplish this task. It has been tried in an on‐going research programme in Sweden. Daily resistivity measurements are carried out on permanent installations on two Swedish embankment dams. In this paper the installations on the Sädva embankment dam are described and selected parts of the results are presented. In addition, a method for evaluating the seepage from resistivity monitoring data is theoretically described and tested for four selected areas in the foundation of the Sädva dam. Seasonal resistivity variations are apparent in the reservoir as well as inside the dam. Most parts of the dam have a homogeneous resistivity distribution with consistent variations. The overall status of the dam is satisfactory. However part of the foundation demonstrates a slightly different behaviour pattern with regard to the seasonal variation. The four selected areas represent localities with low, intermediate and high variations in seasonal resistivity. The areas are compared qualitatively and thereby permeable zones within the dam may be identified. Quantitative assessment of the seepage flow is also carried out as an initial test of the described method. It is concluded that the experiences from the Sädva dam are valuable with regard to the use of the resistivity method on embankment dams. Resistivity monitoring data may be used to qualitatively assess the seepage through the dam. For quantitative assessment, the method is promising and the data from the Sädva dam constitute an interesting initial approach. However, many assumptions and simplifications are made and more work on refining the method is needed.
Internal erosion is a cause of embankment dam failure, thus it is important to develop methods for seepage monitoring and internal erosion detection. In order to evaluate the potential of resistivity monitoring to give an early warning of such leakage/erosion, a test was undertaken on a rockfill embankment dam with a central glacial till core at the Røssvatn test facility in Norway. Three defects, consisting of permeable material, were built into the dam at various depths and locations unknown to the personnel carrying out the measurements and data interpretation. A numerical modelling pre-study was carried out, showing that all the actually constructed defects were too small to be detected by single time investigation. In the final test, repeated measurements were undertaken with different reservoir levels, i.e. a limited monitoring approach. This increased the detection capability, confirming the value of the geophysical approach and that monitoring is superior to single time investigations.Résumé L'érosion interne est une cause de rupture des barrages en terre, de sorte qu'il est important de développer des méthodes pour surveiller les écoulements et détecter l'érosion interne. Afin d'évaluer le potentiel des méthodes de résistivité électrique pour donner une alerte précoce de ces phénomènes, un test a été entrepris sur un barrage en enrochement comportant un noyau central de moraine glaciaire sur la station d'essai de Rossvatn en Norvège. Trois défauts, réalisés à partir de matériaux perméables, ont été mis en place dans le barrage à différentes profondeurs et en des endroits non connus des personnes réalisant les mesures et l'interprétation des données. Une pré-étude par simulation numérique a été réalisée, montrant que tous les défauts mis en place étaient trop petits pour être détectés par des investigations isolées. Dans le test final, des mesures répétées ont été réalisées avec différents niveaux de réservoir, i.e. suivant une approche par surveillance. Ceci a augmenté les possibilités de détection, confirmant la valeur d'une approche géophysique et l'intérêt d'une surveillance par rapport à des investigation isolées.Mots clés Résistivité Á Modélisation Á Barrage en terre Á Erosion interne Á Fuite Á Détection d'écoulement
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