# Modelling the Structure and Anisotropy of London Clay Using the SA_BRICK Model
## 1 Introduction  
Advances in the laboratory testing have contributed to a better understanding of the non-linear behaviour of natural stiff clays at small and large strains. Constitutive models, which are widely applied to simulate soil-structure interaction using numerical analyses, have been developed in parallel with advances in laboratory testing. This development started some fifty years ago with simple nonlinear elastic models [1, 2] and early works on elastic-plastic models using kinematic hardening [3] and bounding surface concepts [4]. These were further improved by more advanced kinematic hardening models, which can describe both the nonlinearity and plastic behaviour in the initial stages of loading using a single kinematic surface (e.g., [5]). Further development included the capability of modelling the recent stress history effect [6] using two kinematic surfaces inside the state boundary surface (e.g., [7]). The BRICK model, initially developed by [PERSON] [8] for overconsolidated clays, has the ability to model the non-linear stress-strain response of soil and the recent stress history effect, both of which are the main manifestations of the mechanical behaviour characterised by kinematic hardening. With the development of the fundamental understanding of the behaviour of structural soils [9, 10] and similarities in the behaviour of hard soils and soft rocks [11], there were further advances in kinematic hardening models to include other aspects of natural materials behaviour, such as structure and anisotropy [12, 13, 14]. The BRICK model was further developed to include the structural behaviour of hard soils and soft rocks [15, 16] and was successfully validated on three natural materials: Pappadai clay, North Sea clay andCorinth Marl, thus covering a wide range of natural structured materials. [PERSON] et al. [17] introduced anisotropy into the BRICK model, which resulted in an evident improvement in the prediction of the behaviour of London clay. This paper presents further development of the BRICK model to include both anisotropy and structure in modelling the behaviour of a natural stiff clay, namely the SA_BRICK model. The SA_BRICK model was firstly validated using single element predictions of computed stress paths and stiffnesses with laboratory test results for undrained shearing in compression and extension. The high-quality laboratory triaxial test results on London clay [18] were used and compared with the numerical predictions of the SA_BRICK model. After the validation in single element form, the model was developed for a generalised three-dimensional (3D) stress state and integrated in the finite element program PLAXIS [19]. The predictions of a full-sized boundary value problem were carried out in the study of the excavation of the Jubilee Line tunnels at St. James's Park in London, UK [20]. A considerable improvement in the prediction of the settlement trough above the tunnel was calculated, proving that the BRICK model can be successfully upgraded to include anisotropy and structure in modelling the behaviour of London clay. Recent Developments in the Modelling of the Behaviour of London Clay  
The development of sophisticated measuring devices in the laboratory and in situ resulted in a better understanding of the clay composition and structure (e.g., [21; 22; 23; 24]) and clay shear strength behaviour [25]. A comprehensive overview of the geological history and geotechnical and geomechanical characteristics of London clay is given by [PERSON] et al. [26]. Their work was based on the data obtained from ground investigations for Terminal 5 at London's Heathrow Airport, which was also the lithological unit used in this research. London clay is a typical heavily overconsolidated marine clay of mineral composition comprising illite, chlorite, kaolinite, smectite and montmorillonite.