| Dam safety


Viewed this way, this conservative criterion is acceptable for concrete and embankment dams.


4. Threshold for starting


consideration of seismic action As described in section 1, completely disregarding seismic action is not appropriate. To provide a more structured and quantifiable basis for this, we propose establishing a threshold value to guide when seismic action should be considered. It is recognised that, even in the absence of explicit seismic design, structures including cofferdams and other temporary works designed and constructed with modern technology are typically capable of withstanding earthquake shaking up to a level of Richter Magnitude 5.0 or Modified Mercalli Intensity MMI=VI with no damage or with only negligible damage. This level of seismic intensity approximately


corresponds to a PGA=0.10g, where g is the gravity of Earth. Therefore, we propose PGA=0.10g as a threshold for temporary structures, in alignment with the respective return periods described in Section 2. The threshold essentially represents a non-damage or slight-damage criterion. That is, if the expected PGA is below this threshold, no damage or only insignificant acceptable damage is anticipated for cofferdams and other temporary works. In such cases, seismic action may reasonably be omitted from the design consideration. We believe this threshold is just and reasonable, and we anticipate broad support from professionals in this field. Conversely, in high seismicity regions, omitting seismic action from the design of river diversion works may compromise structural safety. Earthquake shaking exceeding the proposed threshold could damage temporary works, undermine their resilience, pose risk to human life, and lead to severe economic losses. Under such circumstances, seismic actions must be duly accounted for in the design.


5. Interpolation of peak ground


exceedance can be expressed via the importance factor I as follows:


(2)


From this, the desired PGA for return period TL calculated


is (3)


in which k denotes the seismic exponent. Equation (3) is the fundamental expression used to determine the target PGATL


References


[1] ICOLD, (2016), “Selecting Seismic Parameters for Large Dams Guidelines” Bulletin 148, Committee on Seismic Aspects of Dam Design, International Commission on Large Dams


. Nevertheless, its application


requires careful consideration of the assumptions and site-specific conditions as discussed in the following subsections.


5.2 Discusions (1) Seismic exponent k The seismic exponent k describes the relationship between the annual probability of exceedance (or return period) and the reference ground acceleration. It depends on seismicity and ground conditions at a specific location. A medium value of k=3 is recommended by EN 1998-1 for most sites if a more precise value cannot be determined. However, EN 1998-1 does not provide instructions on how to determine this exponent. Bisch et al. 9


suggested a


range of k=2.5 to 4, with k=3 being typical for regions of high seismicity. Lower values of k correspond to regions of low seismicity. Dragojevic et al.10


indicated


that k may be as low as 2 or less in low seismicity regions. In our opinion, using k=3 is appropriate for determining the construction earthquakes (CE), especially since CE is primarily considered in high seismicity regions. Nevertheless, if two credible PGA values corresponding to distinct return periods are available (which are supported by a seismic hazard assessment - SHA), the k-value can be calibrated in an engineering-based approach. As an example, a SHA for a large dam project yielded the following seismic parameters:


accelerations 5.1 Formula from Eurocode 8 (EN 1998) It is frequently requested to derive a specific level of the peak ground acceleration PGATL


at a site in a specified


return period TL, based on a known reference peak ground acceleration PGATLR with return period TLR


. For


this purpose, the formula provided in Clause 2.1(4) of EN 1998-1 and in Annex A of EN 1998-28


can be used


for both interpolation and extrapolation of PGA values. This section highlights several specific considerations in applying this formula. At most sites, the annual rate of exceedance, H(PGATLR


acceleration PGATLR may vary with PGATLR


), of the reference peak ground as:


(1)


In accordance with EN 1998, when seismic action is defined in terms of the reference peak ground acceleration PGATLR


, the importance factor I


multiplying the reference seismic action achieves the same probability of exceedance over TL


years. The relationship between PGA and the respective return periods or annual probabilities of


years as in the same probability of exceedance in TL years as over TLR In this context, the reference seismic action corresponds to an important factor =1.0.


● Design Basis Earthquake (DBE) with a 475- year return period: PGADBE


=0.177 g


● Safety Evaluation Earthquake (SEE) with a 9975- year return period: PGASEE


=0.503 g Substituting the parameters into Eq. (3), it comes out:


[2] X.S. Liu, M. Li, L.J. Zhang, D.C. Chen, X. Yang and Y. Li (2021), “Seismic Response of the Rock- filled Cofferdam Slope under Earthquake Conditions Considering Hydraulic-mechanical Coupling Effect”, 11th Conference of Asian Rock Mechanics Society, Earth and Environmental Science, Vol. 861, 21-25. 10. 2021, Beijing, China, doi:10.1088/1755- 1315/861/7/072096


[3] Y. Huang and J. L. Liu (2017), “Seismic performance of hydropower plant and highway system during the 2015 Gorkha Earthquake in Nepal”, 16th World Conference on Earthquake, 16WCEE 2017, Santiago Chile, January 9th to 13th 2017, Paper No.2925


[4] IS 10084-1 (1982): Criteria for design of diversion works, Part 1: Cofferdams


[5] M. Wieland (2007), “Seismic aspects of safety relevant hydro- mechanical and electro-mechanical elements of large storage dams”, The Annual Meeting of International Commission on Large Dams, Prague, Czech Republic, 3-7 July 2017


[6] M. Wieland (2004), „Design criteria“, International Water Power & Dam Construction, Vol 56, No. 6, pp 26-29


[7] M. Wieland (2014), “Seismic Hazard and Seismic Design and Safety Aspects of Large Dam Projects”, Second European Conference on Earthquake Engineering and Seismology, Istanbul, Turkey, 25-29 August 2014


Appendix-1 Probability of Exceedance


Mathematically, a risk analysis can be conducted in terms of the probability of exceedance (p) based on the adopted return period (TR event and the service life (tL


& Ormsbee12 and Annex A of EN 1998-28 expressed using a Poisson probability distribution as: (A-1) ) for the earthquake


) of the diversion structures. According to Zhang , the probability of exceedance can be


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