Establishment of generic transformations for geotechnical design parameters

Geotechnical design parameters are typically estimated based on the transformations from site investigation results. In general, one expects the transformation uncertainty to change depending on the number and type of sites in the database. This study tries to address the following two issues pertaining to the transformation uncertainty: (a) how transformation uncertainties change with the number and type of sites in the database and (b) whether transformation uncertainties will eventually fall within a narrow range when a “generic” transformation is developed from a sufficiently large database. This study also attempts to propose a framework to establish such a generic transformation and quantify its uncertainty. This framework is demonstrated by the transformation between piezocone CPTU data and undrained shear strengths (Su) of clays. It was found that the CPTU–Su transformation and its uncertainty is site or region-dependent, and the “local” transformation equation from one site may not be applicable to another site, both in terms of the mean trend (which is well known) as well as the coefficient of variation (c.o.v.). An approach is proposed to develop the generic CPTU–Su transformation equations that can be applied for downstream reliability analysis or design in the absence of local data. Sensitivity analysis shows that it requires data from at least 15 sites with the accompanying implicit assumption that sufficient geographical coverage typically implies sufficient geologic diversity to reliably build such “generic” transformation equations.