Refining shear strength characteristic value using experience

Determination of characteristic values for soil properties forms a critical step in the foundation design process. The refinement of such values to account not only for site-specific data but also for existing knowledge and previous experience can result in more efficient design and increased confidence. This paper presents a logical, pragmatic approach for the selection of characteristic values of shear strength for the design of piled foundations within the context of Eurocode 7. The process of refining conceptual models of geotechnical properties in a quantitative, objective manner to include previous knowledge and wider experience is described and demonstrated through case study examples. The result of applying the updating methods is to achieve a revised mean that is a weighted average of the site data and the prior knowledge; the weighting is a function of the variability of each set of data. Such refinement relies upon a quantification of previous knowledge: mean values, trends, variations and distributions of data are required, and in this paper a dataset for undrained shear strength of London Clay is presented and applied to case study examples. Application of the techniques described leads to a better estimate of the ground properties and a reduction in the risk attached to a design solution.