Two mechanisms of leakage are identified, through the geomembrane, i.e. through holes in the geomembrane, and around it, i.e. at attachments with rigid structures. Measures to reduce leakage concern construction and design. Construction quality assurance should be supplemented with electrical leak detection, which can detect holes as small as 1mm, if performed on the geomembrane, or 5mm, if performed on a 0.5m soil layer covering the geomembrane. The design measures to minimize the number of holes and their impact stem from the general idea of associating materials. A geotextile associated with a geomembrane protects it from being punctured. When a geomembrane is associated with clay to form a composite liner, leakage through the geomembrane holes is impeded by the clay, provided the two materials are in intimate contact. To ensure intimate contact wrinkles should be eliminated during construction. In addition, liquid should be prevented from accumulating between geomembrane and clay to avoid uplift of the geomembrane. This requirement is more easily met at landfills, where hydraulic head is low and waste weight acts as ballast, compared to water reservoirs, where the use of two independent liners directly on top of each other should be avoided unless they are ballasted. Instead, for reservoir leakage control, it is recommended to use a double liner with two geomembranes and a drainage layer in between for leakage collection and detection. The concept is explained with the example of the first reservoir with a double liner with two geomembranes, constructed in 1974 and still (2024) in service. In this case, leakage control measures were implemented to mitigate the geotechnical risk of instability of the sloping area where the reservoir is located. For the second mechanism of leakage at attachments, two key parameters are identified, the tension-strain curve of the geomembrane and the geometry of the connection, and recommendations are given. Monitoring results are presented for landfills and reservoirs and an extrapolation from landfills to reservoirs is proposed as guidance to obtain target values for acceptable leakage rate as a function of the reservoir depth.

The final part of the lecture concerns geomembranes used in dams with the dual goal of minimizing water loss and preventing deterioration by water of the body of the dam. The relative importance of each goal depends on the type of dam. Water loss is the main concern for rockfill dams, while the priority for earth dams is to prevent internal erosion and instability. Similarly for concrete dams, mechanisms considered are deterioration of dam material (alkali-aggregate reaction, leaching of cement, freeze-thaw) and instability of the dam (caused by water pressure in cracks and lift joints). For all these reasons, the body of a concrete dam should remain dry, which can be achieved by combining a geomembrane and a drainage system on the dam upstream face. Case studies of dams rehabilitated with geomembrane and a drainage system show that leakage rate is typically reduced by a factor ranging from 10 to 100, in addition to the benefit of allowing the body of the dam to dry and remain dry. The lecture ends with a summary of the main points and conclusions for geotechnical engineers, one of which cautions against “copying” leakage control designs from landfills and “pasting” them to reservoirs.