Dam Safety Aspects of Reservoir-Triggered Seismicity

Transcription

1 Dam Safety Aspects of Reservoir-Triggered Seismicity Dr. Martin Wieland Chairman, Committee on Seismic Aspects of Dam Design, International Commission on Large Dams (ICOLD) Poyry Energy Ltd., Zurich, Switzerland

2 Official ICOLD Terminology Old (misleading) term: Reservoir-induced seismicity (RIS) New (correct) term: Reservoir-triggered seismicity (RTS)

3 ICOLD Bulletin 137

4 Table of Contents (Bulletin 137) 1. INTRODUCTION 2. RESERVOIR TRIGGERED SEISMICITY PHENOMENA AND DEVELOPMENT OF THEIR EVALUATION AND INTERPRETATION 3. FREQUENCY OF RESERVOIR TRIGGERED SEISMICITY 4. CHARACTERISTICS OF RESERVOIR TRIGGERED SEISMICITY 5. MECHANISM OF RESERVOIR TRIGGERED SEISMICITY AND RHEOLOGY OF EARTH CRUST MATERIALS 6. PORE PRESSURES DIFFUSION TIME 7. GENERAL STATEMENT ON UNDERSTANDING RTS PHENOMENA 8. RISK MANAGEMENT 9. CASE HISTORIES 9.1. Hsingfengkiang Dam Case History 9.2. Mratinje Dam Case History 9.3. Kurobe Dam Case History 9.4. Takase Dam Case History 9.5. Poechos Dam Case History 10. ASSESSING THE POTENTIAL AND MONITORING RTS 11. CLOSING CONSIDERATIONS 12. REFERENCES

5 Is RTS a safety concern for large dam projects?

6 Overview Introduction RTS Dams and RTS Seismic design criteria and RTS Effects of RTS Monitoring of RTS Conclusions

7 Basic requirements for RTS existence of active faults and/or existence of faults near failure limit

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10 Main features of RTS Seismic events during and after impounding are more frequent than background seismicity before impounding. With increase of reservoir level and variation of water level, number and magnitude of RTS events increase. Often RTS events decrease towards background activity after peaking.

11 Frequency of RTS Number of cases with M > 5.7: 6 Number of dams with height h > 100 m: > 400 Probability of RTS (M > 5.7 and h > 100 m): 6/400 = This is not a negligible value!

12 Large dams subjected to RTS Hsinfengkiang buttress dam (China), 105 m high 1962, M=6.1, dam damaged and strengthened Koyna gravity dam (India), 103 m high 1967, M=6.3, dam damaged and strengthened Hoover arch dam (USA), 220 m high, 1935, M=5.0 Kremasta embankment dam (Greece), M=6.2 Kariba arch dam (Zambia), M=6.3 Maximum magnitudes suspected of being caused by RTS: M = 6.0 to 6.3

13 Upper bound for RTS The maximum observed/suspected RTS magnitude is about 6.3. It is unlikely to trigger the Safety Evaluation Earthquake (SEE) by a reservoir this has not yet happened.

14 RTS Wenchuan Earthquake 2008 (China)? Was the May 12, 2008 Wenchuan (Magnitude 8.0) earthquake triggered by the reservoir stored behind the 156 m high Zipingpu concrete face rockfill dam? The Wall Street Journal Scientists Link China's Dam to Earthquake, Renewing Debate

15 Observed Seismicity at Zipingpu Dam (China)

16 Water Level in Zipinpu Reservoir (China)

17 Reservoirs and Wenchuan Earthquake There exists no factual evidence that supports the assumption that the devastating Wenchuan earthquake of May 12, 2008 was triggered by the Zipingpu reservoir! (Note: Several earth scientists still believe that this was the case.)

18 Example of RTS: Nurek dam, Tajikistan World s highest dam

19 Damage due to RTS Koyna dam, a 103 m high straight gravity dam, M = 6.3 (1967) Hsinfengkiang dam, a 105 m high buttress dam, M = 6.1 (1962) Earthquakes suspected of being caused by RTS. Both dams developed substantial longitudinal cracking near top. Damage attributed to design or construction details that would be avoided in modern structures. Both dams were strengthened and are still in service.

20 Reservoir water levels at Koyna 1961 to 1995 and M > 4.0 events Three periods of M > 5.0: 1967, 1973 and 1980

21 Increase in water pressure at various depths with time in days

22 Repair of Koyna dam, India

23 Strengthening of Koyna dam, India

24 Seismic hazard due to RTS ground shaking: vibrations in dams, appurtenant structures, equipment and foundations mass movements into reservoir and rockfalls at dam site: impulse waves in reservoir; blockage of intakes; damage of hydro-mechanical and electromechanical equipment, and other damage. fault movements in dam foundation fault displacement in reservoir bottom: water waves in reservoir or loss of freeboard. noise Note: Fault movements will be small due to small RTS magnitudes

25 Rockfall Hazard at Dam Sites

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27 Landslide at tailrace of hydropower plant, 2008 Wenchuan earthquake, Sichuan Province, China

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29 Taipingyi Hydropower station overtopping

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33 Damaged gate by rockfall at power intake, Wenchuan earthquake 2008

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36 Penstock failure at expansion joint

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41 Transmission tower failure due to rockfall, Sefid Rud dam

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43 Micro-seismic networks for RTS monitoring Comprehensive monitoring of RTS before construction, during construction, duringreservoir impounding, and during first years of operation is strongly recommended for large storage dam projects located in areas with faults and high tectonic stresses in order to dispel any doubts about what is actually happening.

44 RTS Karkeh dam, Iran , M=5.1

45 Effect of small magnitude earthquakes on poorly built dams: Sharredushk Dam, Albania, after 2009 earthquake, M=4.1, PGA = 0.07 g

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47 Is RTS a safety concern for large dams?

48 Integral Dam Safety Concept Structural Safety Design of dam according to state-of-practice (codes, regulations, guidelines, etc.) (earthquake design criteria, methods of seismic analysis etc.) Dam Safety Monitoring Dam instrumentation, visual inspections, data analysis and interpretation, etc. Operational Safety Guidelines for reservoir operation, qualified staff, safe software, maintenance, etc. Emergency Planning Emergency action plans, water alarm systems, dam breach analysis, evacuation plans, Engineering back-up, etc.

49 Seismic design criteria Dam and safety-relevant elements (spillway, bottom outlet): Operating basis earthquake, OBE (145 years) (negotiable with owner) Safety evaluation earthquake, SEE (ca. 10,000 years) (non-negotiable) Appurtenant structures (powerhouse etc.): Design basis earthquake, DBE (ca. 475 years) Temporary structures (coffer dams) and critical construction stages: Construction level earthquake, CE (> 50 years)

50 Seismic performance criteria for dam and safety-relevant elements (i) Dam body: OBE: fully functional, minor nonstructural damage accepted SEE: reservoir can be stored safely, structural damage (cracks, deformations) accepted, stability of dam must be ensured (ii) Safety-relevant elements (spillway, bottom outlet): OBE: fully functional SEE: functional so that reservoir can be operated/controlled safely and moderate (200 year return period) flood can be released after the earthquake

51 Ground shaking Earthquakes affect all components of a dam project at the same time: dam foundation safety devices pressure system underground works appurtenant structures hydro-mechanical equipment electro-mechanical equipment etc.

52 Title Element / Component Design Earthquake CE DBE OBE/ SEE Diversion Facilities - Civil Intake/outlet structures X Tunnel, tunnel liner - Geotechnical Rock slopes X Underground facilities Cofferdams - Electrical/Mechanical Gate equipment X Dam: Dam Body Dam body X - Individual Blocks OBE Crest bridge Crest spillway cantilevers X X Bottom Outlet cantilevers Foundation/Abutments Abutment wedges X X Bottom Outlet Main gates, Valves X X Guard gate Operating equipment X X Dam: Electrical/Mechanical Essential parts X X X X X X X

53 Comparison of RTS ground motion with seismic design criteria for dams and buildings Dam and safety-relevant elements: RTS < SEE Appurtenant structures and buildings in reservoir area: RTS > DBE or RTS < DBE

54 Assessing the Potential and Monitoring of RTS Any large dam of greater height (h > 100 m) is a candidate for RTS. To assess its potential, the following data are needed: tectonic conditions and data on structural geology, supported by study of aerial photographs. macroseismic data for reservoir under study. information on active faults and all data on recent fault activity in dam and reservoir region. assessment of seismic capability of all faults in dam and reservoir region. regimes of underground water.

55 Case studies (ICOLD Bulletin 137) Hsingfengkiang Buttress Dam in China, as a representative of large triggered seismicity, causing a strong local earthquake, which significantly damaged the dam. Mratinje Arch Dam in Yugoslavia as a representative of moderate RTS. It is of particular interest as seismic monitoring was introduced prior and after impounding, witnessing re-appearance of RTS after 17 years of service.

56 Case studies Kurobe Arch Dam was monitored prior and after impounding. The dam was reported as an RTS case on the basis of microseismic monitoring after impounding. But later analyses led to the conclusion that Kurobe dam was not an RTS case. Takase Rockfill Dam is a large dam monitored prior and after impounding, where similar microseismic activity was present before and after impounding.

57 Case studies Poechos Embankment Dam in Northern Peru as a case of seismically active environment where RTS was absent or was masked by basic background activity.

58 Summary on RTS Probability of RTS increases with dam height and reservoir size. RTS potential needs to be considered for dams with h >100 m. Triggered earthquakes are seismic events, which needed effects of reservoir load and pore pressure build up, to be manifested. Maximum magnitude and maximum surface intensity of seismic events cannot be increased through effects of reservoir impounding. Earthquake safety of dam is covered by SEE. Earthquake safety of appurtenant structures and buildings in dam vicinity should be checked for RTS. In cases of low historical seismicity, the neotectonic studies undertaken to assess the RTS potential might lead to determination of controlling earthquake.

59 Conclusions There is no need to treat RTS separately as ground motion due to SEE is larger than that of RTS. Maximum observed magnitude for RTS is about 6.3. Impossible to prove that strong earthquake is caused by reservoir as focal depth is several kilometres. RTS may cause mass movements into reservoir, resulting in water waves. overtopping or blockage of intakes. RTS may cause rockfalls damaging appurtenant structures and hydro-mechanical and electromechanical equipment.

60 Conclusions Seismic safety of dams, where RTS has taken place, has to be re-assessed. Buildings and structures in reservoir region are designed for smaller seismic forces than SEE, thus RTS may cause damage and loss of lives in these regions. RTS, which can be felt or heard, creates safety concerns in the population.

61 Conclusions Monitoring of seismic activity Prior to construction, During impounding of reservoir, and During first years of reservoir operation is highly recommended for (i) large storage dams, and (ii) dams located in tectonically stressed regions.