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Saturday, 16 Dec 2017

Hällby Dam

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A Rockfill Embankment Dam in Sweden

Lessons Learnt from Ten Years of Resistivity Monitoring

Hällby Dam and Installations for Resistivity Monitoring

The embankment dam at Hällby is divided into a left and a right part by the centrally placed power plant and spillway. The left dam is 120m and the right dam is 200m long. Both dams have a maximum height of around 30m and are constructed as a zoned rockfill embankment dam with a vertical central core of glacial till. Hällby was the first Swedish embankment to get a permanently installed monitoring system intended for resistivity measurements. Daily measurements started to take place in 1996, which make these long-term monitoring data unique.

The monitoring temporary halted as the dams were upgraded and the electrodes re-installed. Measurements with the new system started in 2006.

Indications of Internal Erosion and Increased Seepage

The figure demonstrates plots of resistivity over time for a few selected depths in a certain area on the left dam, adjacent to an area with reported sinkholes. Generally, the highest resistivity variation is close to the crest, which is explained by the ground freezing in the winter. The amplitude of the variation decreases with depth, as a consequence of the decreasing temperature with depth. Contrary to the expectations, there are significant variations also at large depths at this area, which have not been seen elsewhere. Furthermore, there are signs of increasing variations and increasing absolute resistivities for the deepest levels. These increasing variations may be a sign of increasing seepage flow, and the increased resistivity may be a sign of material change due to washout of fines and internal erosion. The observations thus, can be explained by the occurrence of an ongoing internal erosion in the dam.