Rehabilitation of Dams with Watertight Geomembranes, in the Dry and Underwater

Geomembranes are watertight synthetic materials, factory-manufactured under computer-controlled procedures that guarantee constant properties and quality, and supplied as flexible sheets a few millimetres thick. Geomembranes are used to waterproof all types of hydraulic structures, for rehabilitation and for new construction, in the dry and underwater. In dams, geomembranes were first used at the end of the 1950ies. This paper focuses on rehabilitation, discussing the available options for concrete and embankment dams, and for RCC dams. The state-of-the-art systems place the geomembrane liner at the upstream face of the dam, generally in exposed position, and with a drainage system behind, to allow discharging possible backpressure, and monitoring the performance of the system. This flexible technology can be adopted to line the entire upstream face of the dam, or only the area/s causing most seepage, or only cracks/failing joints. Projects include all types of dams and subgrades (concrete, bituminous concrete, granular subgrades or RCC). Geomembrane systems do not require heavy equipment and site organization. Large use other types of geosynthetics is made to perform functions that with traditional materials would require complicated and time-consuming operations: thick unwoven geotextiles to provide anti-puncture protection avoiding surface preparation, geogrids or high-performance technical woven geotextiles to provide support over cavities in the subgrade, geonets to enhance drainage capacity. The face anchorage and peripheral anchorage are calculated to resist acting loads such as uplift by wind and waves. Face anchorage on solid subgrades is generally linear, with a tensioning system, while on granular subgrade it is made at points, with deep anchors. Ballast anchorage is rarely used. The paper presents case histories of repairs in the dry and underwater.