To simplify risk assessments, assumptions must be made about different types of uncertainty. By far the most important of these is separating uncertainty between (1) natural variations over space and time, and (2) lack of knowledge. This separation is an analytical convenience, not a fact of the world. This paper considers the meaning of uncertainty in relation to risk analysis in geotechnical engineering and discusses an increasingly common vocabulary for describing uncertainties in such analyses. The paper finishes by suggesting implications of the division between these two types of uncertainty on the results and use of risk analysis in geotechnical engineering.
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