Concrete lining of the core-foundation interface to prevent internal erosion: An efficiency assessment

Abstract

The core-foundation interface of the embankment dams requires particular attention during design and construction, and specific surveillance during operation. A deficient treatment of this area may facilitate seepage along the interface and erosion of the core-base which, in turn, could lead to a major internal erosion process. Several dam incidents and failures were initiated following this mechanism. For instance, an imperfect core-foundation treatment was one of the causes of the Teton Dam collapse in 1976. Therefore, it is clear that this surface requires specific study and treatment. Stripping, cleaning, removing unsuitable materials and shaping, are considered the minimum operations for preparing it. In addition to this, a gallery beneath the core-base could be used. This gallery is an excellent safety element, as it provides a direct access to foundation, facilitating inspection and grouting during operation. Hence, despite its construction cost, is a usual arrange on impervious core dams. However, depending on the specific lithology and stratification, this option could be replaced by a concrete lining of the core-foundation interface. An option that could be advantageous when the following circumstances concur: the foundation is not erodible and no additional grouting treatment is predictable; and the excavation of the trench which houses the gallery would damage or decompress the rock, deteriorating the foundation conditions. The risk of contact erosion at the core-foundation interface increases in those sites where the bed?s strike is orthogonal to the dam axis. In such case seepage may flow along the joints eroding the clay particles of the core-base . Internal erosion through the contact when these adverse geological conditions appear could be prevented by placing a concrete lining . This concrete protection works as a barrier, separating the core particles from the beds and joints. This measure, that could be a cost-effective alternative to galleries, was used to prevent internal erosion on Andévalo dam. It is a rockfill dam with a thin clay core, 80 m high and 1862 m long, with one of the largest Spanish reservoirs (1025 hm3). Its construction was completed in 2002 and its first filling has been finished in 2013. This paper presents that experience, describing the foundation characteristics, the design features and the performance assessment of the lining during the impoundment.