Practical risk assessment for embankments, dams, and slopes - Risk and Reliability in Geotechnical Engineering - page 457

only the natural uncertainty. Estimating the value of the epistemic uncertainty distribution
for the APF requires repeating the process for the full range of fragility curves obtained in
the previous section.

11.6 SuMMarY oF MaIn PoIntS

1. Much of the benefit from conducting a risk assessment stems from engaging the engi-
neers to thoroughly evaluate the fundamentals controlling the performance of a con-
structed facility.
2. Carrying out correct engineering analyses helps engineers identify the fundamentals
involved and the key parameters affecting performance.
3. Understanding the applicable fundamentals and key parameters facilitates preparation
of accurate event trees and probability estimation.
4. Empirical correlations of safety factor with annual failure probability provide a practi-
cal means of estimating probabilities for a risk assessment.
5. Evaluation of uncertainty should play an important role in the analysis of the behavior
of a constructed facility.
6. Fragility curves represent a relationship between failure probability and the magnitude
of the loads that produce the failure.
7. An event tree is a graphical representation of a logical model that includes all the
conceivable chains of events resulting from an initiating event that can produce the
structural failure.
8. Probabilities in event trees, except for the initiating event, are always conditional.
9. When performing Monte Carlo simulations, using a low number of simulations (e.g.,
100 or even 10,000), a single pair of sampled values resulting in failure. Always increase
the number of calculations until the calculated annual probability value stabilizes.
10. Small differences in the average fragility curve can have significant influence on the
calculated level of risk.

aCknoWleDgMentS

The authors gratefully acknowledge the contribution of the many colleagues that contrib-
uted with the application of risk-based decision tools for the earth dam presented in Example
11.2.2, including Donald A. Bruce, David S. Bowles, T. William Lambe, W. Allen Marr,
and Steve J. Poulos.
The work presented in this chapter has been supported by the Spanish Ministry of Science
and Innovation (MICINN) through the grant of the iPRESARA project (BIA 2010-17852).

lISt oF MaIn SYMbolS anD aCronYMS

AEP annual exceedance probability

ALARP as low as reasonably practicable principle

ALL annualized life loss

ANCOLD Australian National Committee on Large Dams

APF annual probability of failure

ASM advanced second moment

F probability of life loss