A case study involving the assessment and re-design of an existing dyke, founded on a layered soil, has compared deterministic analysis based on 5-percentile property values and a reliability-based random finite element analysis consistent with the requirements of Eurocode 7. The results show that a consideration of the spatial nature of soil variability generally leads to higher computed factors of safety and, for those dyke sections requiring remedial action, to more economic designs. Back-figured characteristic values are shown to be considerably higher than the 5percentile soil properties; hence, a reduction in over-conservatism is achieved.

The vertical spatial correlation functions, under and next to a pre-existing dyke on a soft soil, have been estimated using cone penetration test (CPT) data. Distinct differences were found between different locations in reference to the dyke body, i.e. under the crest of the dyke, under the toe and slope of the dyke and in the polder. The results strongly suggest that deformations caused by the dyke construction change the spatial correlation, especially in soft soils. It is hypothesised that the change in spatial correlation due to deformation, within close proximity of the dyke, will impact the calculated reliability and should therefore be considered when using limited CPT data to estimate the vertical correlation at an existing dyke.

Soil spatial variability has a significant impact on the reliability of geotechnical structures. In particular, the horizontal variability is important for linear infrastructure, which has only limited vertical height and width, but extensive length. Due to depositional and geological processes, the variability is often substantially different in the vertical and lateral directions. This variability can be characterised by a spatial correlation length, or scale of fluctuation, which is a measure of how significantly soil properties are correlated in space. An analysis of the reliability of such a measure has been undertaken using synthetic data, leading to a design chart which quantifies the statistical uncertainty in the scale of fluctuation for specific site investigation designs, which can be an important input for probabilistic analyses of the structure response. Moreover, practical guidance for site investigation design is proposed which can reduce the statistical uncertainty. The method has been applied to a real site investigation comprising a row of 29 closely spaced cone penetration tests (CPTs), within a larger site investigation of 100 CPTs, and applied to a simple design calculation for a long embankment to illustrate the impact on slope stability assessment. The site investigation data are made available to add to the limited amount of detailed data in this field.

Measuring soil variability has many challenges. One of the challenges is soil deformation, which can change the spatial correlation structure arising from the original geological processes. In particular, this is important in the assessment of existing structures. In this paper, the effect of existing structures on the horizontal spatial correlation is investigated using a dataset consisting of 100 CPTs in close proximity. These data were collected under and adjacent to a dyke structure which has caused considerable deformations. Using CPTs in zones where deformation has not occurred has allowed a quantification of this effect. The impact of the dyke is shown to dominate the natural scale of fluctuation.

A reliability-based analysis framework, accounting for uncertainty arising from the spatial variability of soil properties, has been validated for the controlled, well-instrumented slope failure of an historic dyke in the Netherlands. Using soil property statistics derived from the results of laboratory and cone penetration test (CPT) data for the different soil layers at the site, the dyke was analysed for the initial (i.e. operating) conditions, as well as for the later stage of the test leading up to failure. The computed probabilities of failure and back-figured factors of safety were consistent with the point at which failure occurred in the test, as was the range of possible failure mechanisms. The uncertainty in the stability assessment was reduced by considering the spatial nature of the soil variability, and by conditioning analyses to CPT measurement data. It is shown that the reliability-based approach enables more informed stability assessments that could make the difference between a dyke being assessed as safe or requiring costly improvement.