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2000_factors of safety and reliability in geotechnical engineering_duncan

2000_factors of safety and reliability in geotechnical engineering_duncan

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JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING / APRIL 2000 / 307 F ACTORS OF S AFETY AND R ELIABILITY IN G EOTECHNICAL E NGINEERING By J. Michael Duncan, 1 Honorary Member, ASCE A BSTRACT : Simple reliability analyses, involving neither complex theory nor unfamiliar terms, can be used in routine geotechnical engineering practice. These simple reliability analyses require little effort beyond that involved in conventional geotechnical analyses. They provide a means of evaluating the combined effects of uncertainties in the parameters involved in the calculations, and they offer a useful supplement to conventional analyses. The additional parameters needed for the reliability analyses—standard deviations of the parameters —can be evaluated using the same amount of data and types of correlations that are widely used in geotechnical engineering practice. Example applications to stability and settlement problems illustrate the simplicity and practical usefulness of the method. FIG. 1. Cantilever Retaining Wall with Silty Sand Backfill INTRODUCTION The factors of safety used in conventional geotechnical practice are based on experience, which is logical. However, it is common to use the same value of factor of safety for a given type of application, such as long-term slope stability, without regard to the degree of uncertainty involved in its calculation. Through regulation or tradition, the same value of safety factor is often applied to conditions that involve widely varying degrees of uncertainty. This is not logical. Reliability calculations provide a means of evaluating the combined effects of uncertainties, and a means of distinguish- ing between conditions where uncertainties are particularly high or low. In spite of the fact that it has potential value, reliability theory has not been much used in routine geotech- nical practice. There are two reasons for this. First, reliability theory involves terms and concepts that are not familiar to most geotechnical engineers. Second, it is commonly per- ceived that using reliability theory would require more data, time, and effort than are available in most circumstances. Christian et al. (1994), Tang et al. (1999), and others have described excellent examples of use of reliability in geotech- nical engineering, and clear expositions of the underlying the- ories. The purpose of this paper is to show that reliability concepts can be applied in simple ways, without more data, time, or effort than are commonly available in geotechnical engineering practice. Working with the same quantity and types of data, and the same types of engineering judgments that are used in conventional analyses, it is possible to make approximate but useful evaluations of reliability. The results of simple reliability analyses, of the type de- scribed in this paper, will be neither more nor less accurate than conventional deterministic analyses that use the same types of data, judgments, and approximations. While neither deterministic nor reliability analyses are precise, they both have value and each enhances the value of the other.
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