Depending on the risk category of buildings and bridges, importance factors are specified in earthquake codes, which translate into longer return periods, but they do not reach those used for large dams.
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Moreover, most of the existing buildings and bridges have not been designed against earthquakes using modern concepts, whereas dams have been designed to resist against earthquakes since the s. In many parts of the world the earthquake safety of existing dams is reassessed based on recommendations and guidelines documented in bulletins of the international-commission-on-large-dams icold.
Effects such as water waves and reservoir oscillations seiches are of lesser importance for the earthquake safety of a dam. Usually the main hazard, which is addressed in codes and regulations, is the earthquake ground shaking. It causes stresses, deformations, cracking, sliding, overturning, etc. However, some of the other hazards, which are normally not covered by codes and regulations, are also important. Therefore, in the earthquake design of dams all seismic hazard aspects must be considered and depend on the local conditions of a storage dam project.
The experience with the seismic behaviour of large dams is still limited. Four dams — two concrete, an earth core rockfill and a concrete face rockfill dam — with heights exceeding m were damaged during the Wenchuan earthquake. Elsewhere, there are another five concrete dams over m in height, which were damaged due to strong ground shaking.
In concrete dams the damage was mainly in the form of cracks but also joints can open up leading to the release of water from the reservoir. In modern embankment dams the damage is mainly by deformations and cracks along the crest that can eventually lead to internal erosion and piping through the dam.
However, we have to be aware that each dam is a prototype located at a site with special site conditions and hazards. Therefore, based on the observation of the earthquake behaviour of other dams it is still very difficult to make a prediction of the damage that could occur in a particular dam. At this time we are still in a learning phase as very few large modern dams have been exposed to strong earthquakes. In the case of the Wenchuan earthquake, a large number of rockfalls took place, which caused significant damage to dams and appurtenant structures.
Surface powerhouses were particularly vulnerable to rockfalls in the steep valleys in the epicentral region of the Wenchuan earthquake. If earthquakes could be predicted, one could attempt to lower the reservoir prior to the occurrence of a large earthquake. There are two problem areas related to this concept. First, despite some 40 years of research on earthquake prediction, it is still not possible to predict the time, location and size of a large earthquake reliably.
Small earthquakes may be predicted but not large ones. The prediction is usually given in terms of the probability of occurrence, e. Such predictions are basically useless for warning purposes and for lowering a reservoir. Even if a large earthquake could be predicted reliably, there would not be sufficient time to lower large reservoirs. Lowering of a reservoir would have to happen by low level outlets bottom outlets or the power waterways if the intake is at low elevation.
Unfortunately, bottom outlets are not available everywhere. As a conclusion, earthquake prediction, which is a slowly developing science, is not a viable option to improve the earthquake safety of dams. The only real option is to have a dam which can withstand the strongest earthquake effects to be expected at the dam site.
This is the current practice in dam design. The greatest hazard of a dam is the water in the reservoir. Therefore, in the seismic design of dams, we have to ensure the safety of the dam under full reservoir condition. Although an arch dam may be more vulnerable to the effect of ground shaking when the reservoir is empty, this case is not critical for the safety of the people living downstream of the dam but it is, of course, an important economical issue for the dam owner if the dam should fail.sis.wt.com.mx/prego/have/the-leader-on-the-couch-a-clinical-approach-to-changing-people-and-organizations.php
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