The influence of man-made structures on spatial variability of soft soils

Abstract

The subsurface is often modelled as multiple homogeneous soil layers. In reality, the soil properties of these layers are heterogeneous and spatially variable, making it difficult to properly deduct the engineering properties that are required in the design of geotechnical structures. This leads to uncertainty in the design. The probabilistic framework known as the random finite element method (RFEM) can take the spatial variability of soil properties into account in geotechnical designs in an effort to reduce the uncertainty. The spatial variability is represented by the mean, the standard deviation and the scale of fluctuation. The scale of fluctuation is defined as the distance in which a significant correlation is found within the soil properties and is related to the geological deposition of the soil layer. The calculated reliability of man-made structures, such as dykes, is the output of the RFEM and is influenced by the scales of fluctuation of the subsurface. Since many man-made structures in the Netherlands are constructed on soft soils, they cause deformations in the subsurface. These deformations change the geological layering of the soil. It is therefore expected that the deformations caused by man-made structures influence the scales of fluctuation and that the changed scales of fluctuation influence the calculated reliability of the man-made structures. The objective of this thesis is to research how the spatial variability of soft soils is influenced by man-made structures. To research this, a case study is undertaken at the Leendert de Boerspolder in the Netherlands. One-hundred cone penetration tests (CPTs) have been taken in an area of 15 by 50 meters in the polder as well as in the dyke as part of the research programme Reliable Dykes. From the cone resistance data of the CPTs the spatial variability of the area can be estimated. The case study is accompanied by two theoretical studies. The first study aims to estimate the effect of deformations on the spatial variability of generated random field data for different types of deformations. The second study of this thesis will provide the deformations that have occurred in the area of the case study from a plane-strain finite element analysis of the dyke and polder.
The first part of the research focussed on four deformations that are often encountered in the proximity of a dyke: vertical compression, horizontal compression, simple shear and rotation. The effect of these deformations on the spatial variability has been studied by applying the deformations to three resolutions of random fields and then estimating the change in scales of fluctuation. Since no soil properties were assigned to the random fields, the results can only be used qualitatively. The research is then continued with the estimation of the spatial variability of the Leendert de Boerspolder. It was necessary to identify the soil layers first. These are a top peat layer, a clay layer which shows a transition from organic at the top to silty at the bottom, and a bottom peat layer. For each of the CPT lines parallel to the dyke the outliers of the cone resistance data were removed for the different soil layers and the depth trends were removed. This resulted in normalised cone resistance data from which the spatial variability in different directions could be estimated. Finally, a two-dimensional plane strain analysis of the soil layers at the Leendert de Boerspolder was made. In the first step of the analysis the depths of the soil layers are at the level before construction of the dyke. Then the dyke was constructed step-wise. This allowed the initial soil layers to deform as close as possible to the actual deformations under influence of the dyke. Finally, the following conclusions are made by combining the resulting deformations with the results of the other research. At the Leendert de Boerspolder, the spatial variability of the polder next to the dyke is not influenced by the dyke and can be used as the baseline spatial variability to which to compare the spatial variability in the proximity of the dyke. Underneath the crest a significant vertical compression causes both the vertical and horizontal scales of fluctuation to decrease, while a smaller amount causes a change in just the vertical scale of fluctuation. Underneath the slope and toe of the dyke, rotation increases the larger vertical scale of fluctuation and decreases the larger horizontal scale of fluctuation. Horizontal extension of the soft soil underneath the slope results in a larger horizontal scale of fluctuation. Horizontal compression underneath the toe results in a smaller horizontal scale of fluctuation.