The stability of a slope is governed by the spatial average of the shear strength over the extent of the failure surface. In Eurocode 7 the average soil properties are taken into account by defining the characteristic soil parameter as being "a cautious estimate of the value affecting the occurrence of the limit state” and further stating that this value should be based on, among other factors, "the extent of the zone of ground governing the behavior of the geotechnical structure at the limit state being considered”. To completely quantify the characteristic shear strength along a failure surface, three statistical values are required: the arithmetic mean, the variance and the spatial correlation. The mean soil properties and to a lesser degree the variance (or equivalently the standard deviation or the coefficient of variation) are known and used by most geotechnical engineers for the selection of characteristic soil properties. The scale of fluctuation, however, is not generally used. The scale of fluctuation is a measure of the soil spatial variability and can be understood as the range within which soil properties are correlated and beyond which they are statistically uncorrelated. This paper investigates the influence of the variability of shear strength on the reliability of slopes based on simulated autocorrelated random fields created by the turning bands method. In particular, the influence of the length of the failure surface on the characteristic value is investigated. Numerical Monte Carlo analyses verify the validity of a simplified practical approach presented to determine the characteristic soil properties according to Eurocode 7
