Lessons learned | reassess dam safety A need to
Climate change is only one of many natural and man-made hazards which are changing with time, and must be taken into account in dam safety assessments. Martin Wieland explains why there is an urgent need to evaluate the seismic safety of existing dams
Above: IWP&DC would like to congratulate Martin Wieland for receiving an Honorary Member Award from the International Commission on Large Dams in 2022
IN CONNECTION WITH THE discussion of climate change, it can be concluded that dams are structures, which are directly affected by these changes. If nothing is done, then the safety of dams may decrease. However, if current safety practice of the dam industry is followed, this is not necessarily the case. Climate change is only one of the many natural and man-made hazards which are changing with time, and must be taken into account in the safety assessment of dams, and which dams must be able to withstand safely. It must be kept in mind that the ultimate goal of dam safety is that people living downstream of both a new and an old dam should feel equally safe. This implies that old and new dams must both satisfy the same minimum safety criteria. This is particularly a problem for the earthquake safety of large dams, as the seismic design criteria and seismic safety criteria have undergone important changes since many of the existing dams were built. This is, for example, true for dams built or designed against earthquakes before 1989 when ICOLD published its first guideline on the Selection of Seismic Parameters for Large Dams. Because of these developments, it is not known if older dams comply with today’s seismic safety criteria. As all dams should satisfy the current seismic safety criteria at all times, there is an urgent need for the seismic safety evaluation of dams that were not designed according to today’s safety standards. The minimum design and safety criteria, which must be satisfied, are those published by ICOLD. They represent the state-of- the-art practice. From the observation of dams damaged by strong earthquakes the following conclusions may be drawn: ● Dams are not inherently safe against earthquakes. Because of the satisfactory behaviour of dams during earthquakes, several engineers and owners may be of the opinion that a dam, which has survived for say over 50 years without any earthquake damage, is safe against earthquakes. This is a misconception, especially in areas of low to moderate seismicity, where strong earthquakes occur rarely.
● The simple pseudo-static analysis and design method is not a safe method for dams with a large damage potential. This has already been known since the 9 February 1971 San Fernando earthquake in the US. Although the pseudo-static method is outdated, or even wrong, it is still used today.
● All dams, both new and existing ones, must satisfy the present seismic design and safety criteria, which are different from those used during the design of most dams.
12 | February 2023 |
www.waterpowermagazine.com
Today’s seismic safety criteria not only apply to the dam body but also to safety-critical elements like gated spillways and low-level outlets, which must be operable after strong earthquakes in order to keep the water level in the reservoir at a safe limit, or to lower the reservoir level to increase the safety of the dam. Moreover, the stability of slopes, whose failure may create impulse waves in the reservoir that could overtop the dam crest, or block the intakes of spillways and low-level outlets, must be checked for the ground motions of the safety evaluation earthquake.
Risk classification An important issue is the risk classification of dams,
which may vary for different countries or organisations. Risk classification is the main factor that governs the seismic design and safety criteria. As a result, the seismic safety criteria of similar dams may differ in different countries. There are also new safety requirements that concern electro-mechanical and hydro-mechanical engineers, as well as geologists and geotechnical engineers involved in slope stability analyses, who may not be familiar with the current seismic safety concepts for dams. There are many dams that have been built without taking into account earthquakes or which were designed against earthquakes using the obsolete pseudo-static analysis method. Therefore, it is not known if these dams satisfy today’s seismic design and safety criteria. There are many reasons why the seismic safety of both water storage and tailings dams needs to be reassessed: ● New information on seismic hazard (ground shaking, mass movements) and/or seismotectonics is available.
● A dam has been subjected to strong earthquake shaking.
● New seismic design criteria are introduced. ● New seismic performance and safety criteria are introduced.
● New dynamic methods of analysis are introduced, such as nonlinear dynamic analysis methods.
● Certain dam types and poorly designed, constructed and maintained dams are vulnerable to earthquakes.
● The seismic vulnerability of a dam has increased due to dam modifications, ageing, etc.
● Changes in the risk classification of dams. ● The seismic risk has increased, e.g., due to the increased number of people living downstream of a dam and/or due to economic development, etc.
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