The performance of geotechnical structures is governed by the spatial average soil properties, such as the average shear strength over an appropriate length or area or volume of soil. This is considered in Eurocode by defining the characteristic value as being “a cautious estimate of the value affecting the occurrence of the limit state” and stating that the selection of this value should take account of, among other factors, “the extent of the zone of ground governing the behavior of the geotechnical structure at the limit state being considered”.

To quantify the characteristic values, 3 statistical properties are required: The arithmetic mean, the variance and the scale of fluctuation. The mean and the variance (or equivalently the standard deviation or the coefficient of variation) are known to most geotechnical engineers. The scale of fluctuation may be interpreted as the distance within which soil properties are largely correlated, whereas for greater distances the soil properties are largely uncorrelated or statistically independent. This degree of spatial correlation considerably influences the variance of the soil properties averaged over a certain geometrical zone.

This paper provides guidelines for the assessment of the characteristic soil properties for the use in geotechnical designs, taking into account explicitly soil variability, as well as the soil volume affected by the design.

Examples to illustrate the practical application of the proposed procedures for typical geotechnical routine work are provided.