The objective of this paper is to verify the suitability of a geosynthetic when applied as a drainage tool in hydraulic works. The geosynthetic that has been chosen is formed by a polyester geomat located between two polypropylene geotextiles. The analyzed specimens have been extracted from three different zones of the reservoir, which have been in direct contact with the concrete. Additionally, samples from stockpile material from the construction site have also been taken for analysis. All the specimens have undergone a series of identical experiments that have confirmed an outstanding degradation of the geomat primarily due to the hydrolysis of the ester function in the basic environment.