Internal erosion in the pervious foundation of an embankment dam : A case study on the Lossen dam

Abstract: The Lossen dam is an embankment dam in the Swedish river Ljusnan. The dam is founded on thick layers of stratified glaciofluvial sediments and till. Ever since construction, there have been problems with high pore pressures, large seepage flows and springs downstream of the right part of the dam. After the first filling of the reservoir, a large drainage trench was constructed downstream of the dam to lower pore pressures. Sinkholes and settlements downstream of the dam have occurred repeatedly over the lifetime of the dam, particularly in the area surrounding the large drainage trench. This study aims to investigate the causes of the sinkholes and assess the risks of internal erosion in the foundation and in the soil downstream of the dam. A model of the groundwater flow has been created in Visual MODFLOW. Calculations for assessing the soils susceptibility for contact erosion and suffusion have been performed, using soil gradation curves (both from the 1960s and from new samples). The results from the erosion calculations show that the probability of contact erosion being the sole cause of the internal erosion is minor. However, a majority of the soil samples tested are internally unstable (susceptible for suffusion). The possibility of backward erosion piping can not be dismissed but would need further investigation in order to be properly assessed. Due to the very heterogeneous structure of the soil in the area downstream of the dam, it is hard to predict the extent of the eroding soil layers and, thus, the future development of the erosion. A conclusion is that the soil downstream of the dam is subject to internal erosion, which is also the cause of the observed sinkholes and settlements. The internal erosion is probably not a threat to dam safety at the moment, but can possibly evolve as a problem in the future. Therefore close monitoring of the changes in seepage patterns and sediment transport is recommended. Possible solutions to stop the erosion include extending the drainage system with more pressure relief wells, and placing a filter blanket in the seepage exit area.