The effect of strength anisotropy on the behaviour of embankments on soft ground

The need to build embankments on soft clay deposits is widespread, including flood defences, rail and motorway embankments. The difficulties in constructing embankments on these deposits are well known. The permeability of the ground is usually low, so that construction is generally performed under undrained conditions and, as a consequence of the low undrained shear strength of the clay, these structures are of a limited height, usually only 3–4 m. The classical design of an embankment is often based on a limit equilibrium approach, in which the soil is considered as an isotropic material. However, soft clays are almost always anisotropic, having both stiffness and strength dependent on the orientation of principal stresses. This complicates any design analysis, and as sufficient data are not usually available to define the magnitude of anisotropy, empirical factors are introduced into conventional design procedures. This paper attempts to demonstrate the effect of soil strength anisotropy on the behaviour of soft clay embankments through finite element analysis using an anisotropic soil model. The finite element predictions are compared with data from a full-scale test embankment brought to failure. The dangers of ignoring anisotropy, or treating it empirically, are highlighted.