In order to secure the water-tightness of the foundation of the 67m high Papadia rockfill dam, a cut off wall up to 35.50m deep, formed by secant piles of plastic concrete was constructed in the Pleio-Pleistocene fluvio-glacial deposits at the river bed. A rigorous Quality Assurance Program was implemented during construction. A great number of tests, both in the field as well as in the laboratory were performed in order to validate the quality of the structure. Two years after filling the reservoir, monitoring data confirm the satisfactory performance of the seepage control measures