Environmental Engineering Reference
In-Depth Information
not always an easy task, using a total stress analysis to avoid dealing with pore pres-
sures does not bypass this difficulty. When using total stresses, the engineer implicitly
accepts that the pore pressures developed during the tests used to determine strength
apply to the field problem at hand. This is not always the case and the impact of any
differences on the computed safety factor should be evaluated by the individual per-
forming stability analyses.
Baecher and Christian (2003) present a comprehensive treatise on the various methods
to estimate event probabilities. Even when engineers recognize the benefits of probabilistic
risk analyses, estimating event probabilities can seem like an unsurmountable obstacle for
most projects without large budgets and long durations. Empirical correlations between
commonly performed stability analyses and probability of failure can make the estimation
of failure probabilities less daunting. Figure 11.1 shows one correlation we have used suc-
cessfully for decades with some minor adjustments to the four curves to reflect new data or
improved representation of the data.
Figure 11.1 classifies earth structures into the four categories described in Table 11.1 .
The category of the earth structure, based on the level of engineering, ranges from Best
(Category I) to Poor (Category IV). We establish the level of engineering by examining the
practices followed for design, investigation, testing, analyses and documentation, construc-
tion, and operation and monitoring. The four categories correspond to the following types
of facilities:
• Category I—Facilities designed, built, and operated with state-of-the-practice engi-
neering. Generally, these facilities have high failure consequences.
• Category II—Facilities designed, built, and operated using standard engineering prac-
tice. Many ordinary facilities fall in this category.
• Category III—Facilities without site-speciic design and substandard construction or
operation. Temporary facilities and those with low failure consequences often fall in
this categor y.
• Category IV—Facilities with little or no engineering.
10 -8
I
10 -7
II
10 -6
10 -5
10 -4
III
10 -3
IV
10 -2
10 -1
1 0.5
1
1.5
2
2.5
Factor of safety
Figure 11.1 Factor of safety versus annual probability of failure.
 
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