Statistical analyses of inherent variability of soil strength and effects on engineering geology design

Clayey soil strata, as all natural deposits, generally show variability of the values of their geotechnical properties. This is mainly due to geological and environmental processes, such as deposition and diagenesis, which introduce heterogeneity, anisotropy and variability in soil properties. Other causes of variability and then uncertainty are the representativeness of samples and errors related to testing procedure, measurement and data processing procedures. To improve the knowledge about the inherent variability of geomechanical properties of clays, this work presents a case study related to the analysis of the strength variability along a log of marine stiff clay deposits, which are apparently quite homogeneous. The analysis is based on pocket penetrometer strength measure and it is performed both punctually and across the whole deposit. The adopted fast and reliable testing procedure provides a really wide dataset of the investigated soil property, more than 800 strength data. These allow for a detailed variability analysis and a reliable estimation of coefficient of variations as well as for the research of the best fitting probability density functions, which are key factors for robust design. The presented case study allows to discuss on inherent variability of soil properties and its influence on the characteristic values of soil strength for geotechnical design.