The earthfill dam Białobrzegi is one of eight side dams of Zalew Zegrzyński. Initially, the dam was drained by a drainaged pipe and discharge into a ditch. Difficult hydrogeological conditions of foundation structures resulted in suffusion and hydraulic break in a sand layer. After the renovation, made in the mid-90s, polypropylene and polyethylene nonwoven geotextiles were used in the drainage system. The nonwoven geotextiles used in drainage applications are vulnerable to physical and chemical clogging process. Clogging reduces effectiveness of the synthetic filter layers. The paper presents the research methodology and the results of analyses obtained from the laboratory tests of water permeability characteristic normal to the plane of nonwoven geotextiles after 22 years of exploitation in the earthfill dam Białobrzegi. Also, the parameters of unworn and after 7 years of exploitation obtained earlier were compared with the research results.

119-126