The use of exposed geomembranes for the rehabilitation of concrete and masonry dams was introduced in Italy in 1976, when the entire upstream face of a 11 m high masonry dam was lined with a 2 mm thick PVC (polyvinyl chloride) geomembrane as waterproofing element (Lago Miller).
In the following applications (among which Lago Nero, 1980; Piano Barbellino, 1987; Cignana, 1988), the system progressively evolved. In fact, instead of using only one component, i.e. a PVC geomembrane, the watertight function was performed by a bi-component geocomposite, obtained by thermal coupling of the PVC geomembrane with a PET (polyester) nonwoven needle-punched geotextile directly during the manufacturing process in factory, thus obtaining an anti-puncturing function, reducing the installation time and increasing the dimensional stability of the geomembrane itself. The geotextile in-plane drainage capacity also allowed to prevent the accumulation of water between the geomembrane and the dam itself, solving one of the major concerns for a good long-term performance of the whole system.
From 1988 to present, this technique underwent several other changes, mainly thanks to the use of additional geosynthetics, such as geonets (Publino, 1988) and thick nonwoven geotextiles which allow to avoid the surface regularisation before the geomembrane installation (Camposecco, 1993).
In all these applications, the geomembranes were left exposed on the upstream face, without external protection, to environmental factors and weather conditions, especially to UV rays, very intense also because many of these dams are at high elevation (typically greater than 2000 m).
On the other hand, the fact that geomembranes are exposed makes it easier to control and monitor their behaviour vs. time. Therefore, in order to verify the performance of the waterproofing system over time, several geomembrane samples have been taken from a considerable number of dams. These samples have then been subjected to physical and mechanical tests, in order to observe the evolution of the properties of the geosynthetics over the years.
In a lot of cases, particularly when the results of tests on virgin samples were available, it was possible to evaluate the residual life of exposed geomembranes on the different types of dams.