Many of the concrete dams in the western world today have reached a considerable age. With an ageing population of dams, re-assessment of their structural safety is becoming increasingly important to ensure that failure does not occur because of material deterioration in the dam body or foundation. But re-assessing the current safety of an existing dam is many times more difficult than ensuring that a dam-to-be-built satisfies all safety criteria.
In sliding stability re-assessment of concrete dams, sliding along potential weakness planes in the rock foundation is an important failure mode that needs to be addressed. However, the parameters governing the shear strength of this failure mode are associated with large uncertainties and are difficult to assess due to the existing dam.
This keynote paper presents a summary of the work performed at the Division of Soil and Rock Mechanics, KTH Royal Institute of Technology on this topic during the last decade by the author and colleagues. The main massage is that a combination of reliability-based design, statistical decision theory and observations through investigations and testing are key tools to obtain optimal decisions for acceptable dam safety in the re-assessment of existing concrete dams.
Hydropower is a clean, renewable source of energy. The emissions are low and the hydropower’s effect on the climate is small. Therefore, hydropower is today a keystone in the energy supply to our modern society. Moreover, the expansion of wind and solar power in the electricity generation will make the role of hydropower even more important in the future, as storing energy in the reservoirs provide an efficient way to balance the fluctuating nature of wind and solar power.
In Sweden, we have roughly 10 000 dams of varying size and age that are used to regulate our rivers and store water for hydropower generation. Of these dams, approximately 200 dams are higher than 15 m (see for example Fig. 1). Most of these dams were built around the 1920s and also in the 1950s, 60s and 70s, but some are 100 years or older and the average age is now 83 years (Bernstone 2006) – a situation that is shared with many other countries in the western world. With an ageing population of dams, re-assessment of their structural safety is becoming increasingly important to ensure that failure does not occur because of material deterioration in the dam body or foundation. Furthermore, the climate change implies that the probable maximum flood (PMF) can be expected to increase, which puts higher demands on the dam to sustain loads. In addition, assessing the current safety of an existing dam is many times more difficult than ensuring that a dam-to-be-built satisfies all safety criteria – the presence of the existing dam and significant load from the water reservoir make testing of relevant parameters difficult and expensive. Retaining a high structural standard in the dam population by sustainable and costeffective stability-enhancing modifications is therefore a challenge for the hydropower industry.