| Safety
analyses have become very sophisticated and much effort can be given to such analyses, it should not be forgotten that with advanced seismic hazard analyses the dam safety does not improve. Therefore, in view of the remaining large uncertainties in the seismic hazard, it is better to spend the resources in the actual design and construction of dams that are least vulnerable to seismic actions. In addition, the ground motions used for seismic safety evaluations are not real earthquake ground motions but instead models of the ground shaking that ensure the safe design of a dam. This is an old and proven concept in the civil engineering profession, where, for example, different models of the live load on bridges are used to calculate the maximum stresses and deformations, although these models have little in common with the actual live loads on these bridges. Until recently, it was thought that the seismic safety of dams was assured when the dam body could resist the effect of ground shaking. This is still correct but, in addition, it must be realised that during strong earthquakes, besides ground shaking, landslides or rockfalls may occur. These can block the intakes of spillways or low-level outlets that have to function after the earthquake or they may lead to impulse waves in the reservoir that could overtop the dam crest as, for example, in the case of the Vajont dam in Italy on 9 October 1963.
In addition, the possibility of movement along faults or other discontinuities in the footprint of a dam must be investigated, which, for concrete dams would have more severe effects than ground shaking. Besides the dam body, the following structures must also be safe and have to withstand the same seismic actions as the dam body: 1. Spillways and low-level outlets must function after strong earthquakes.
2. The stability of wedges in the dam abutments must be ensured, which is of main concern for arch dams.
3. The stability of slopes, whose failure may block the intakes of spillways or low-level outlets, must be checked.
4. The effects of potential landslides in the reservoir region or even in the catchment area, which may create large impulse waves or extreme floods or debris flows have to be checked.
These are safety aspects that have hardly been considered systematically up to now. These new requirements are a direct consequence of the comprehensive seismic safety concept, which includes the following: 1. Retain the reservoir and protect people from a catastrophic release of water from the reservoir. 2. Control the reservoir level after an earthquake as
a dam could be overtopped and destroyed if the inflowing water into the reservoir cannot be released through damaged spillways or low-level outlets.
3. Lower the reservoir level after an earthquake (i) for repair works, (ii) for increasing the safety of a damaged dam or (iii) when there are doubts about the safety of a dam.
Thus, low-level outlets are essential for improving the earthquake safety of dams. Well designed, built and maintained dams should be able to safely withstand – without damage or easily repairable damage – the effects of an earthquake with a moderate magnitude of about 5. However, there have been recent reports on the failure of tailings dams, where earthquakes with magnitudes of less than 3 were considered as a possible trigger of the failure. If this would be the case for any dam, then such dams must have major safety deficiencies and would require urgent remedial action.
Reservoir-triggered seismicity (RTS) Finally, the issue of reservoir-triggered seismicity (RTS)
always comes up in connection with the construction of large dams and the impounding of large reservoirs. Unfortunately, as mentioned at the beginning, the prediction of the largest RTS events is not possible. But RTS is not a dam safety concern as mentioned above, a dam must be able to withstand the strongest ground motion at the dam site, which includes the strongest RTS events. However, RTS may be a problem for the buildings and infrastructure at the dam site and in the reservoir region, which are not designed for seismic actions like the dam and such buildings and structures could be damaged. Moreover, slope failures might be triggered. Therefore, it should be in the interest of the dam
owner to install a micro-seismic monitoring system in the dam and reservoir region to distinguish between natural seismicity and RTS. This means that such systems must be installed before the impoundment of the reservoir in order to determine the natural seismicity in the project region. It is also recommended to establish a photographic database of the structures, which might be affected by RTS, in order to determine if any observed damage was actually caused by RTS. Such risks could be covered by insurance or the vulnerable structures could be upgraded. ●
Figure 2: Emptying of the reservoir of the 220m high Contra arch dam near Locarno in Switzerland: View of downstream face of the dam with the ungated spillways near the abutments (left) and view of emptied reservoir for maintenance work after about 55 years of operation (hardly any sediments are visible) (right) (Photos taken by author on 7.2.2022)
Author information Dr. Martin Wieland, Chairman of the Committee on Seismic Aspects of Dam Design, International Commission on Large Dams, c/o AFRY Switzerland Ltd.
E-mail: martin.wieland48@
gmail.com
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