| Innovations & dam safety
Determination of the downstream inundation zone in case of (partial) failure (e.g. breach mechanisms, remobilisation of retained debris, vulnerability of downstream objects). Specific monitoring and maintenance concepts. Emergency planning. A risk-based approach is considered to be the most appropriate and reliable option to develop a proportionate safety assessment for these natural hazard protection dams.
AAR
One of the other challenges facing Swiss dams as they age includes alkali aggregate reaction (AAR) when certain aggregates in concrete react with the alkali pore solution, causing expansion and cracking. Schwager et al claim the majority of large Swiss concrete dams were built without much knowledge about the reactivity of such aggregates, and the Swiss Committee on Dams estimates that 35-45% of the country’s concrete dams are affected. However, recent findings indicate that this could be even higher. Although previous research has focused on understanding and diagnosing the chemical and micromechanical processes of AAR and possible modelling approaches have been published, no comprehensive framework has yet been established to reliably determine the effects of AAR on dam safety. Schwager et al say that in order to build such a framework, the following questions need to be answered:
How does concrete with AAR behave under environmental conditions (such as under the influence of temperature, moisture, alkali content in concrete, stresses and creep in concrete)? How can the behaviour of a dam with AAR be predicted on the basis of model calculations? How does the evolution of damage due to AAR affect the safety of the entire dam (eg how is the resistance of a dam against static and seismic loading affected by AAR)?
To answer these questions, the Swiss Federal Office of Energy (SFOE) is initiating in-depth research and is also planning to evaluate the above in a few well instrumented case studies of dams, establishing a database on the subject.
Seismic Switzerland has one of the oldest portfolios of dams
in the world which, at an average age of 72 years old, were mostly not designed for earthquake loading – or at least not for modern day seismic standards. Various reviews have taken place in recent years and the dam safety directive was revised in 2021 to include more seismic hazard analysis. These revisions aim to follow the regulatory philosophy that reduced uncertainty due to in-depth investigations may allow for less conservative model assumptions. Since the publication of the new directive, a comprehensive review of seismic hazards for all federal dams confirmed the expected increase of the hazard (in terms of spectral acceleration) is about 75% of dams. SFOE is also working to update safety assessments
to the state-of-the-art, such as accounting for foundation, mass and radiation of energy at the model boundaries; plus consideration of nonlinear behaviour of material, block and dam-foundation joints, etc. It is also working on a manual on state-of-the-art seismic modelling of dams, in particular regarding the assumptions for non-linear FEM analyses, which will support dam owners and their engineers.
Above: The Contra Dam supports a 105MW power station on the Verzasca River in the Val Verzasca, Switzerland
References
Swiss dam safety regulation: Framework, recent changes and future perspectives by M.V. Schwager, A. Askarinejad, B. Friedli, P.W. Oberender, A.J. Pachoud & L. Pfister. Role of Dams and Reservoirs in a Successful Energy Transition – Boes, Droz & Leroy (Eds). Proceedings of the 12th ICOLD European Club Symposium 2023 (ECS 2023, Interlaken, Switzerland, 5-8 September 2023)
https://doi.org/10.1201/ 9781003440420
Left: The 280MW Mapragg hydropower project on the Tamina river in Switzerland. . In 2021, the country generated over 61% of its electricity from hydropower © Luciavonu /
Shutterstock.com
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