The mechanical properties of soils can never be established with complete certainty. The uncertainty is due to natural variability of soils, imperfect interpretation models, measurement errors, insufficient data, etc. The selection of soil properties for geotechnical design is often based on subjective judgment and experience, and the uncertainties in soil properties are only indirectly accounted for when the characteristic design values are selected. Although statistical methods can quantify uncertainties and account for them in a rational manner, they are rarely used in establishing the design soil parameters. The profession also uses imprecise definitions of ‘characteristic’, ‘best estimate’, ‘upper bound’ and ‘lower bound’ values for design. The profession needs to make a recommendation of which values to use in design. The paper demonstrates how characteristic values of soil parameters can be extracted from available data with statistical methods. Examples of characteristic (design) soil parameters over the past 25 years were re-analysed using statistical methods. Recommendations for choosing characteristic values and expressing variability are given.
Soils are naturally variable because of the way they are formed and the continuous processes of the environment that alter them. The uncertainty in the mechanical properties of offshore soils is due to the natural variability from point to point within a soil volume, insufficient data and imperfect interpretation models, measurement errors and other sources.
The selection of soil parameters for geotechnical assessment is often based on subjective judgment and accumulated experience. The uncertainties in the soil properties are only indirectly accounted for when the characteristic (design) value(s) are chosen.
Statistics and probability are useful tools for the quantification of the mean (most probable, expected) value and the possible range of values of a parameter. Statistical and probabilistic methods can quantify the uncertainties and make it possible to account for them in a rational and consistent manner. They are, however, rarely used in practice to establish the design soil parameters. The reason for this is unclear.
Perhaps it has become a habit that no one questions, or the restricted use of statistical methods maybe a reflection of the fact that often not enough data are available to actually implement statistical methods with confidence. Det Norske Veritas (DNV) and Norsk Hydro1 prepared a guidance note on the statistical representation of soil data.
When describing the design soil profile, expressions such as ‘characteristic’, ‘best estimate’, ‘upper bound’ and ‘lower bound’ values are used in practice. A common understanding or a standard criterion does not exist for the selection of ‘best estimate’, ‘upper bound’ and ‘lower bound’ values, and there is an uncertainty on what is covered by these expressions. The profession now needs to make a recommendation of which values to use in design. The paper attempts to quantify some of these concepts through the reanalysis of case studies in the North Sea and elsewhere, where characteristic (design) values have been selected by experienced engineers. Examples of design soil parameters recommended by the Norwegian Geotechnical Institute (NGI) for offshore sites over the past two decades were re-evaluated using statistical methods