Inherent variability of geotechnical properties for Finnish clay soils

Compared to manufactured materials, soil properties often exhibit significant variability even within a seemingly homogeneous soil layer. The uncertainty related to this variability can be dealt in a robust manner by means of reliability-based methods. Hence, effort has been made to collect soil statistics in order to provide approximate guidelines for selecting the value of coefficient of variation (COV) of inherent variability. It has been observed that the COV value for the same physical property tends to vary within a relatively narrow range, meaning that the literature COV ranges could be utilized with some confidence on sites which lack sufficient soil data. However, it is not certain whether these prior COV values can be used in Finland since many Finnish clay soils are soft and sensitive due to their unique geological history shaped by the last post-glacial processes. Hence, this paper evaluates the inherent variability of various geotechnical properties (index, strength and consolidation properties) in four clay soil sites located in Southern Finland. Besides prior ranges of COV, this paper provides prior ranges of the mean soil property, applicable for soft post-glacial clays and clayey gyttjas. Furthermore, the shape of the probability distribution is evaluated for various soil properties at one clay site by means of normality tests and visual assessment. It is concluded that the derived COV values are in accordance with literature ranges, but for more reliable estimates, soil statistics derived for Finnish clay soils should be preferred when possible. Nonetheless, no literature range can replace extensive site-specific soil statistics. Finally, it is confirmed that nearly all the soil properties at the studied Finnish clay site can be modelled as normal or lognormal distribution.