166
11 Dam Stability Failures
finally also a 1/100 earthquake, which, by the way has only a PGA of 0.012 (g). The
codes do not ask for the probability of failure and do not impose a FoS value for this
type of event.
With a code-compliant dam we proceeded to calculate the probability of failure
for the three cases above (p fstat , p f1/2500 , p f1/100 ). The probability that the FoS will
be smaller than 1 (see Sect. 11.1) is used as the probability of failure. This is a
simplification, as the mathematically correct formulation is a bit more complicated,
as noted in the referenced section.
We Used Two Different Approaches for the Evaluation
Now comes the big point of this discussion. Actually, the static case occurs “every year”, right? However, in the first approximation the 1/2500 does occur “only”
1/2500. Please forgive us for this gross approximation and remember that two 1/2500
quakes are always possible in the same year. This is also true for the 1/100 quake,
and so forth. What that means is that the probability of failure under seismic event is
actually a conditional probability equal, for example, respectively to p f1/2500 divided
by 2500 and p f1/100 divided by 100. It is possible that the yearly probability of failure
for a minor quake be higher than for a stronger phenomenon.
Thus, after reading the Expert review report we can summarize our interpretation of Cadia tailings facility failure (http://www.newcrest.com.au/media/market_
releases/2019/Report_on_NTSF_Embankment_Slump.pdf) and risk considerations
as follows:
• It is time to stop using misleading factors of safety to characterize dams “safety.”
FoS has the unfortunate tendency to foster excessive audacity, especially under
seismic conditions of various intensity.
• Sometimes there are simple explanations to apparently complex problems, which
make it possible to avoid invoking “complexity” and clarify issues.
• Those simple explanations lead to reasonable prioritization of dams portfolios and
to sustainable, ethical mitigative roadmaps.
References
Altarejos-García L, Silva-Tulla F, Escuder-Bueno I, Morales-Torres A (2015) Practical risk assessment for embankments, dams, and slopes, Risk and Reliability in Geotechnical Engineering, ed.
Kok-Kwang Phoon & Jianye Ching, Chap. 11, Taylor & Francis
Oboni C, Oboni F (2013) Factual and Foreseeable Reliability of Tailings Dams and Nuclear Reactors
-a Societal Acceptability Perspective, Tailings and Mine Waste 2013, Banff, AB, November 6 to
9, 2013
Oboni F, Bourdeau PL (1983) Determination of the Critical Slip Surface in Stability Problems - Proc.
of IVth Int. Conf. on Application of Statistics and Probability in Soil and Structural Engineering,
Florence. Università di Firenze (Italy) 1983, Pitagora Editrice, pp 1413–1424
Rosenblueth E (1975) Point Estimates for Probability Moments. Proceedings of the National
Academy of Sciences of the United States of America PNAS 72(10): 3812–3814
Salmon G, Hartford D (1995) Risk analysis for dam safety. Part I of II. Int. Water Power & Dam
Construction 46(3): 42–47
11 Dam Stability Failures
finally also a 1/100 earthquake, which, by the way has only a PGA of 0.012 (g). The
codes do not ask for the probability of failure and do not impose a FoS value for this
type of event.
With a code-compliant dam we proceeded to calculate the probability of failure
for the three cases above (p fstat , p f1/2500 , p f1/100 ). The probability that the FoS will
be smaller than 1 (see Sect. 11.1) is used as the probability of failure. This is a
simplification, as the mathematically correct formulation is a bit more complicated,
as noted in the referenced section.
We Used Two Different Approaches for the Evaluation
Now comes the big point of this discussion. Actually, the static case occurs “every year”, right? However, in the first approximation the 1/2500 does occur “only”
1/2500. Please forgive us for this gross approximation and remember that two 1/2500
quakes are always possible in the same year. This is also true for the 1/100 quake,
and so forth. What that means is that the probability of failure under seismic event is
actually a conditional probability equal, for example, respectively to p f1/2500 divided
by 2500 and p f1/100 divided by 100. It is possible that the yearly probability of failure
for a minor quake be higher than for a stronger phenomenon.
Thus, after reading the Expert review report we can summarize our interpretation of Cadia tailings facility failure (http://www.newcrest.com.au/media/market_
releases/2019/Report_on_NTSF_Embankment_Slump.pdf) and risk considerations
as follows:
• It is time to stop using misleading factors of safety to characterize dams “safety.”
FoS has the unfortunate tendency to foster excessive audacity, especially under
seismic conditions of various intensity.
• Sometimes there are simple explanations to apparently complex problems, which
make it possible to avoid invoking “complexity” and clarify issues.
• Those simple explanations lead to reasonable prioritization of dams portfolios and
to sustainable, ethical mitigative roadmaps.
References
Altarejos-García L, Silva-Tulla F, Escuder-Bueno I, Morales-Torres A (2015) Practical risk assessment for embankments, dams, and slopes, Risk and Reliability in Geotechnical Engineering, ed.
Kok-Kwang Phoon & Jianye Ching, Chap. 11, Taylor & Francis
Oboni C, Oboni F (2013) Factual and Foreseeable Reliability of Tailings Dams and Nuclear Reactors
-a Societal Acceptability Perspective, Tailings and Mine Waste 2013, Banff, AB, November 6 to
9, 2013
Oboni F, Bourdeau PL (1983) Determination of the Critical Slip Surface in Stability Problems - Proc.
of IVth Int. Conf. on Application of Statistics and Probability in Soil and Structural Engineering,
Florence. Università di Firenze (Italy) 1983, Pitagora Editrice, pp 1413–1424
Rosenblueth E (1975) Point Estimates for Probability Moments. Proceedings of the National
Academy of Sciences of the United States of America PNAS 72(10): 3812–3814
Salmon G, Hartford D (1995) Risk analysis for dam safety. Part I of II. Int. Water Power & Dam
Construction 46(3): 42–47