dam safety aspects of reservoir-triggered seismicity€¦ · dam safety aspects of...
TRANSCRIPT
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
Official ICOLD Terminology
Old (misleading) term:
Reservoir-induced seismicity (RIS)
New (correct) term:
Reservoir-triggered seismicity (RTS)
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
Overview
• Introduction RTS
•Dams and RTS
•Seismic design criteria and RTS
•Effects of RTS
•Monitoring of RTS
•Conclusions
Main features of RTS
•Seismic events during and after impounding are more frequent than background seismicitybefore 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.
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 = 0.015
This is not a negligible value!
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
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.
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 9.2.2009
Scientists Link China's Dam to Earthquake, Renewing
Debate
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.)
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.
Reservoir water levels at Koyna 1961 to 1995 and M > 4.0 events
Three periods of M > 5.0: 1967, 1973 and 1980
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 electro-mechanical 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
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.
Effect of small magnitude earthquakes on poorly built dams: Sharredushk Dam, Albania, after 2009 earthquake,
M=4.1, PGA = 0.07 g
Integral Dam Safety Concept
Structural SafetyDesign of dam according to state-of-practice (codes,
regulations, guidelines, etc.) (earthquake design criteria, methods of seismic analysis etc.)
Dam Safety MonitoringDam instrumentation, visual inspections, data
analysis and interpretation, etc.
Operational SafetyGuidelines for reservoir operation, qualified staff,
safe software, maintenance, etc.
Emergency PlanningEmergency action plans, water alarm systems, dam breach
analysis, evacuation plans, Engineering back-up, etc.
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)
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
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.
Design Earthquake
Title Element / Component
CE DBE OBE/
SEE
Diversion Facilities
- Civil Intake/outlet structures X
Tunnel, tunnel liner X
- Geotechnical Rock slopes X
Underground facilities X
Cofferdams X
- Electrical/Mechanical Gate equipment X
Dam: Dam Body Dam body X
- Individual Blocks OBE
Crest bridge X
Crest spillway cantilevers X X
Bottom Outlet cantilevers X
Foundation/Abutments Abutment wedges X X
Bottom Outlet Main gates, Valves X X
Guard gate X
Operating equipment X X
Dam: Electrical/Mechanical Essential parts X
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
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.
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.
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.
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.
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.
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 electro-
mechanical equipment.
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.