Rehabilitation Works in the Dry and Underwater

Construction of large dams in the present century has been so far not comparable to the construction activity in the 20th century. Climate change and the increasing need for water supply and hydropower in a growing population and economy may reverse this trend, but the ageing of existing dams is anyway becoming a crucial issue, as tens of thousands of large dams have reached or exceeded an age threshold of 50 years, and many others will soon approach 100 years. Maintaining such old dams so that they can operate safely is essential for the future of mankind. In dams and in their appurtenant structures, one of the aspects of ageing is the loss of watertightness; depending on the type and conditions of the dam, persisting leakage can lead to increasing water content in the dam body, increasing uplifts, instability of slopes, internal erosion. Overall, it can ultimately trigger a safety issue. Restoring watertightness by local repairs is generally not a long-lasting solution and, as it sometimes requires lowering the reservoir's level, it may also become expensive. Among long-term solutions, refacing with a durable flexible geomembrane as new upstream water barrier, which can resist opening of fissures and joints, settlements and differential displacements, is the most reliable and safest option. As owners are often reluctant to lose water to carry out rehabilitation works, since the 1990s geomembrane systems have been developed to adapt to underwater working environments, regardless of the depth. In dry rehabilitation, geomembranes have been used on all types of dams and appurtenant structures. India has extensive experience, with several geomembrane projects accomplished, two of which under Phase I of the DRIP - Dam Rehabilitation and Improvement Project, with funding by the World Bank. The paper presents three projects completed in India. The most recent one on a dam is Upper Bhavani, an 80 m high masonry dam where an exposed geomembrane was installed in stages, to meet the demands of the owner and have the lowest possible impact on operation. The project, completed in 2021 under the DRIP, incorporates an Optical Fibre Cable monitoring system. The oldest project, Kadamparai 67 m high masonry dam, the precursor of Upper Bhavani, is now approaching 20 years of successful operation, testifying the longevity of the system. The third case history, Bajoli Holi, is the first pressure tunnel in India that adopted a geomembrane to stop water losses. In all projects the results exceeded by far the expectations of the owners. Underwater installation to restore watertightness at the upstream face of dams started in 1997. The paper presents the oldest project, Lost Creek arch dam in USA, now in operation for 27 years, with performance data, and Studena buttress dam in Bulgaria, where a new improved design was adopted and underwater installation completed in 2018, also under World Bank funding. The dam, 55 m high, required designing different anchorage systems to adapt to a very complicated geometry. Design and installation issues are discussed for all projects.