While the presence of geological uncertainty is often recognized by the engineer, it is seldom considered explicitly in current geotechnical reliability analyses. Indeed, it is largely a missing element in a geotechnical reliability analysis and its effect is generally not known or examined. In this paper, we present a case study to illustrate the characterization and consideration of the geological uncertainty in a geotechnical analysis. We illustrate that when combined with the random field theory, the soil behavior type index (Ic) derived from the cone penetration test (CPT) may be used to determine the composition of the ground at any given point, which forms a basis for characterization of the geological uncertainty at a site. Once the uncertainty associated with Ic is quantified, it may be used to assess the uncertainty associated with the soil stratigraphy and the impact of the geological uncertainty on the predicted geotechnical performance. Further, in the random field modeling the scale of fluctuation is an important parameter that defines the distance within which the information gathered through CPT sounding at a given location can affect the neighboring area. When the number of CPT soundings at a site is relatively small, the estimated scale of fluctuation may not be robust. The results obtained through the random field modeling can help guide decision on how to conduct additional site investigation work. Suggestions are also made for future studies regarding the identification and characterization of the geological uncertainty for use in the geotechnical analysis.

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