| Siesmic analysis
Several strong earthquakes in Japan during the last 20 years. Strong earthquakes in China such as the 2008 Mw 7.9 Wenchuan earthquake and the 2022 Mw 6.7 Luding earthquake.
The effects of the 2016 Mw 7.8 Kaikoura earthquake in New Zealand, which resulted in an estimated 80,000 to 100,000 landslides (Figure 5), was also focused upon, plus developments in the seismic analysis, design and safety evaluation of large concrete dams in China during the last 60 years, and the effects of strong earthquakes on large tailings dams in Chile, including the 2015 Mw 8.4 Illapel and the 2010 Mw 8.8 El Maule intraplate earthquakes. During these major earthquakes several dams were damaged and during the magnitude 9.0 Tohoku earthquake in Japan in 2011 the 18.5m high Fujinuma embankment dam failed. The flood wave created by the release of the reservoir caused the loss of eight lives.
Seismic safety The seismic safety of dams is important because
earthquakes can result in catastrophic effects on people, property, infrastructure, historical and cultural sites, and the environment if they fail. To prevent uncontrolled release of water a dam must be able to withstand the seismic hazards, including ground shaking, associated with an extreme earthquake which is referred to as the Safety Evaluation Earthquake. The subject of the seismic safety of dams and levees continues to evolve. The awareness of the importance of seismic safety and the technical knowledge base has expanded considerably since a question on seismic aspects of dams was discussed at the ICOLD Congress in Montreal in 2003 where the first author was the General Reporter. The main recommendations of the General Report
are as follows: 1. Large dams should be instrumented with strong motion recorders (free-field, base, mid-crest and abutments) to measure accelerations. Deformation monitoring is also important. Measurements of response during earthquake events allows calibrating estimates of seismic hazard (ground shaking) as well as models that predict response of the dam to earthquakes. The information can also add to the body of knowledge for all dams.
2. Monitoring and observations of dam and safety- critical equipment behaviour during earthquakes should continue and be carefully interpreted. Observations should be shared to allow benefit to all stakeholders. People with responsibility for dam safety should study the lessons learnt and apply them to their work.
3. There are multiple features of earthquake hazards (i.e., ground shaking, surface fault movement, mass movements, waves in reservoirs, etc.). They all need to be considered.
4. The seismic safety of dams needs regular review because of the evolving understanding of seismic hazards, changes in the risks associated with potential failure of a dam, deterioration of the dam, the understanding of potential failure modes and the measures available to mitigate risks.
5. There is continuing evolvement of the dynamic properties and constitutive models for dam materials and foundations and advances
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in numerical modelling of the stress-strain characteristics of materials and estimation of the deformation of dams when subject to earthquake ground motion. However, there are still significant uncertainties associated with numerical analyses. Sensitivity studies are always recommended to understand the effects of various assumptions and to properly consider uncertainty.
Final comment
It is obvious that the earthquake safety of dams will remain an important issue for years to come and, therefore, future congress questions on the earthquake safety of dams can be expected. Finally, it should be pointed out that the seismic design criteria that are based on the seismic hazard, are not related to climate change. The uncertainties in the seismic hazard are orders of magnitude larger than any theoretical effect of climate change. However, climate change effects (increased snow melt and rainfall) may affect the seismic triggering of mass movements in the reservoir region and the catchment as well as glacial lake outburst floods. The latter is of main concern for dams located in the Himalayan region.
Above and below – Figure 5: Landslide dam shortly before failure (above) and longitudinal cracks along crest of embankment dam (below) caused by the 2016 Mw 7.8 Kaikoura earthquake in New Zealand
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