Dam safety |
practices for temporary works As indicated earlier, there are no consistent international
3. Review of seismic design
standards regulating the seismic design of cofferdams and other temporary works in dam projects. As a result, the seismic design of temporary works is typically addressed on a project-by-project basis. In the following, the commonly adopted design practices are briefly summarised.
3.1 Non-consideration of seismic action In many cases, seismic actions are not considered in the design due to the short service life of temporary works. In the most extreme instances, earthquake effects are entirely disregarded4
. However, this approach is inconsistent with ICOLD guidelines1 and
not unanimously accepted within the professional community. From a practical standpoint, the omission of seismic effects may be accepted for cofferdams and other temporary works located in regions of low to moderate seismicity. However, this approach is not suitable for projects situated in areas of high seismicity. In such cases, earthquake actions must not be ignored under any circumstances. For temporary works located in high seismicity regions, especially cofferdams, seismic risk should be addressed in the design, because these temporary structures are not only associated with potential property loss but, more importantly, pose a risk to public safety. Accordingly, structural measures should be taken to enhance their stability and resilience against seismic actions. To this end, it is advisable to establish a threshold that defines when seismic actions should be considered. This proposal will be presented in Chapter 4.
3.2 Seismic design using the same criteria as for flood design Martin Wieland5, 6, 7
values of operating basis earthquake (OBE), design basis earthquake (DBE) and safety evaluation earthquake (SEE), while PGA values for other return periods are usually not available. To overcome this limitation, the interpolation method presented in Eurocode 8 (EN 1998)8
may be used to calculate the
required PGA at the prescribed return period from the given PGA values. The details of the method are presented and discussed in Chapter 5 of this paper. Furthermore, the risk associated with return periods of 25 and 50 years for main concrete and embankment dams, respectively, can be calculated using Eq. (A-1) in Appendix-1, expressed as probability of exceedance, as listed in Table 1 below. For the construction of large concrete dams, a
probability of exceedance below 20% is considered acceptable, given that concrete dams are generally perceived to be more resistant to overtopping failure. For embankment dams, a probability of exceedance up to 10% is considered reasonable due to their great susceptibility to erosion during overtopping flows, which can lead to the loss of completed portion of the dam body.
3.3 Seismic design using 0.5 times the PGA value for OBE In design practice, it is frequently observed that some designers adopt a PGA value equal to 0.5 times the PGA value for OBE as the construction earthquake. To our knowledge, this method is not founded on a sound engineering principles but rather serves as a rule-of-thumb. According to ICOLD recommendations, the return period of the OBE is 145 years. Using Eq. (3) the corresponding return period associated with 0.5 . PGA145
can be calculated (let k=3) as follows: suggested that “the CE is to be
used for the design of temporary structures and river diversion facilities such as cofferdams, diversion tunnels and intake. The return period of the ground motion parameters of the CE of diversion facilities may be taken as that of the design flood of the river diversion”. In engineering practice, floods with return periods of 25 and 50 years are typically used for the hydrological design of cofferdams and other diversion facilities during the construction of large concrete and embankment dams (main dams), respectively. These return periods can also be applied to seismic design. In the author’s opinion, using the same return periods for both the hydrological and seismic design of temporary works is reasonable and logical. Therefore, we support this approach. It should be noted that project-specific Seismic Hazard Assessments (SHA) typically provide PGA
Table 1: Risk analysis
Return period (TR
, year) 25 50
Service life (tL
, year) 3 5 3 5
26 | January 2026 |
www.waterpowermagazine.com Probability of
exceedance (p, %) 11.53 18.46 5.88 9.61
Solving this equation gives a return period of TL =18
years. If the service life of the temporary facilities is three years or less, the probability of exceedance using this method is p=15.66%. This level of risk may be acceptable for the construction of main concrete dams. However, for embankment dams, this criterion may be overly optimistic and could lead to inappropriate decision-making due to the higher vulnerability of embankment dams to overtopping damage.
3.4 Seismic design using 0.5 times the PGA value for DBE As one more way, some designers consider using a PGA equal to 0.5 times the PGA for DBE as the construction earthquake. This method is an approximate treatment in design and generally considered as a rule-of-thumb, but more conservative than the method described in Section 3 above. According to ICOLD recommendations, the return period of the DBE is 475 years. Using Eq. (3) and setting k=3, the corresponding return period for 0.5 . PGA475
can be determined as follows:
Thus, the return period is TL
=59 year. Even with a
service life of 5 years, the corresponding probability of exceedance using this method is p=8.14%.
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