[justify]Until now no people have died from the failure or damage of a large water storage dam due to earth quake. Earthquakes have always been a significant aspect of the design and safety of dams
A large storage dam consists of a concrete or fill dam with a height exceeding 15m, a grout certain or cutoff to minimize leakage of water through the dam foundation, a spillway for the safe release of floods a bottom outlet for lowering the reservoir in emergencies and a water intake structure to take the water from the reservior for commercial use. Depending components such as a power intake, penstock, powerhouse, device for control of environmental flow, fish ladder etc..,
During the Richter manitude 8 wnechnan earthquake of 12 May 2008, 1803 concrete and embankment dams and reserviors and 403 hydropower plants were damaged. Likewise, during the 27 February 2010 Maule earthquake in chile of Richter mangnitude 8.8, several dams were damaged. However, no large dams failed due to either of these two very large earthquakes.
What Earthquake action does a dam have to withstand
In order to prevent the uncontrolled rapid release of water from the reservoir of a storage dam during a strong earthquake the dam must be able to withstand the strong ground shaking from even an extreme earthquake, which is referred to as the safety evaluation earthquake (SEE) or the maximum credible earthquake (MCE). Large storage dams are generally considered sage if they can survive an event with a return period of 10000 years, ie. Having a one percent chance of brng exceeded in 100 years. It is very difficult to predict what can happen during such a rare event as very few earthquakes of this size have actually affected dams. Therefore it is important to refer to the few such observations that are available. The main lessons learnt from the large wenchnan and chile earthquake will have an impact on the sismic safety assessment of existing dams and the design of new dams in the future.
There is a basic difference between the load bearing behavior of buildings and bridges have to carry mainly vertical loads due to the dead load of the structures and some secondary live loads. In the case of dams the main load is the water load, which in the case of concrete dams with a vertical upstream face acts in the horizontal direction. In the case of embankment dams the water load acts normal to the impervious core or the upstream facing. Earth quake damage of buildings and bridges in mainly due to the horizontal earthquake component. Concrete and embankment dams are much better suited to carry horizontal loads than buildings and bridges. Large dams are required to be able to withstand an earthquake with a return period of about 10000 years whereas buildings and bridges are usually designed for an earthquake with a return period of 475 years. This is the typical buildings code requirement which means the event has a 10% chance of being exceeded in 50 years. 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.
Moreover, most of the existing building and bridges have not been designed against earthquakes using modern concepts whereas dams have been designed to resist against earthquakes since the 1930s. Although the design criteria and analysis concepts used in the design of dams built before the 1990s are considered as obsolete today, the reassessment of the earthquake safety of conservatively designed dam shows that in general these dams comply with today’s design and performance criteria and are safe. In many parts of the world the earthquake safety of existing dams is reassessed based on recommendations and guidelines documental in bulletins of the International Commission on Large Dams (ICOLD)
Seismic – Hazard is a Multi Hazard
Earthquakes represent multiple hazards with the following features in the case of a storage dam.
• Ground shaking causes vbibrations and structural distortions in dams, appurtenant structures and equipment, and their foundations
• Fault movement in the dam foundation or discontinuities in dam foundation near major faults can be activated, causing structural distortions
• Fault displacement in the reservoir bottom may cause water waves in the reservoir or loss of freeboard
• Rockfalls and landslides may cause damage to gates, spillway piers (cracks) retaining walls (overturning) surface powerhouses Cracking and puncturing and distortions) Electro – mechanical equipment, penstocks masts of transmission lines, etc
• Mass movements into the reservoir may cause impulse waves in the reservoir
• Mass movements blocking rivers and forming landslide dams and lakes whose failure may lead to overtopping of run – of –river power plants or the inundation of powerhouses with equipment and damage downstream[/justify]