THEME BPROBABILITY OF FAILURE OF AN
EMBANKMENT DAM DUE TO SLOPE INSTABILITY AND OVERTOPPING
Adrián Morales-Torres [email protected] Escuder-Bueno [email protected] Politècnica de València and iPresas, Spain
INTRODUCTION• Dam engineers can no longer ignore the techniques of performing risk
assessments, which are more and more required to sanction appropriatefunds for major rehabilitations.
• The present theme follows the tradition initiated in Valencia 2011, where theaim was analyzing the probability of failure of a concrete dam due to slidingfailure mode, and Graz 2013, where the purpose was comparing theestimation of consequences due to dam failure
• The main focus of this theme consists in calculating fragility curves for slopeinstability and overtopping and use them to calculate annualized failureprobability, accounting for both natural and epistemic uncertainty.
PREVIOUS CONCEPTSTYPES OF UNCERTAINTY:
• Natural uncertainty: Produced by the inherent variability in the naturalprocesses. It cannot be reduced. In the risk analysis context, it is usuallyrelated to the occurrence of loading events that can produce the structuralfailure and the randomness of the structure’s resistant behavior for the loadproduced by the events.
• Epistemic uncertainty: Resulting from not having enough knowledge orinformation about the analyzed system. In the risk analysis context, is usuallyfocused on the lack of knowledge of the failure mechanisms, the structure’sresistance parameters and the consequences produced by the failure.
PREVIOUS CONCEPTSFRAGILITY CURVES:
When risk is analyzed quantitatively for dam safety, three main concepts areusually combined:
Fragility curves address the second term, since they represent a relationshipbetween failure probability and the magnitude of the loads that produce thefailure.
Probability of loads Dam response Failure
consequences
PREVIOUS CONCEPTSFRAGILITY CURVES AND UNCERTAINTY:
Natural and epistemic uncertainty in the system response can be addressed witha family of fragility curves:
PREVIOUS CONCEPTSFRAGILITY CURVES AND QUANTITATIVE RISK ANALYSYS:
PREVIOUS CONCEPTSFRAGILITY CURVES AND QUANTITATIVE RISK ANALYSYS:
Quantitative risk profiles can be used to compare the effect of risk reduction and uncertaintyreduction measures for a better risk-informed dam safety management:
CASE STUDY• Homogeneous earth fill dam.
• Inspired in a real Spanish dam but with non-real resistant and hydrologicaldata.
• Two failure modes are analyzed: overtopping and dam instability.
PHASES AND EXPECTED RESULTSPhase 1. Analysis of the information for the instability failure mode.
Phase 2. Calculation of reference fragility curves (only natural uncertainty).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
12 14 16 18 20
Cond
ition
al p
roba
bilit
y of
failu
re
Water pool level (m)
Overtopping
Sliding
PHASES AND EXPECTED RESULTSPhase 3. Computation of water pool level probabilities based on flood routinganalysis in the reservoir.
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
11 12 13 14 15 16 17
Annu
al E
xcee
danc
e Pr
obab
ility
Water pool level (m)
Flood routing analysis
PHASES AND EXPECTED RESULTSPhase 4. Computing failure probability and sensitivity analysis for the referencefragility curve.
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
11 12 13 14 15 16 17
Annu
al E
xcee
danc
e Pr
obab
ility
Water pool level (m)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
12 14 16 18 20
Cond
ition
al p
roba
bilit
y of
failu
re
Water pool level (m)
Overtopping
Sliding
Failure probability
PHASES AND EXPECTED RESULTSPhase 5. Assessing epistemic uncertainty and obtaining a profile of failureprobability (Optional).
Sliding and overtopping
REFERENCES• L. Altarejos-García, F. Silva-Tulla, I. Escuder-Bueno, A. Morales-Torres, Practical
risk assessment for embankments, dams, and slopes. Risk and Reliability inGeotechnical Engineering, CRC Press, 2015, Ch. 11, pp. 437–470.
• EPRI, Methodology for Developing Seismic Fragilities, Electric Power ResearchInstitute, 1994.
• SPANCOLD, Risk analysis applied to management of dam safety – TechnicalGuide 8 (Vol. 1), Spanish National Committee on Large Dams, 2012.
