# Modelling the Structure and Anisotropy of London Clay Using the SA_BRICK Model
## 6 Discussion  
An overview of the development and validation of a constitutive model SA_BRICK is presented in the paper. The SA_BRICK model is a continuation of the development of the BRICK model which was initially developed by [PERSON] [8] for overconsolidated clays. The SA_BRICK model was created on the basis of the extended S_BRICK model [16] by including a variation in the modelling of soil structure. It was further improved by adding the ability to model anisotropy, as developed by [PERSON] et al. [44] for the basic BRICK model. All the above-mentioned features of the mechanical behaviour of natural soils are found in London clay, which was used as a prototype material in this research to the check applicability of the SA_BRICK model to simulate tunnel excavation in London  
Figure 17: Normalized diagrams of the results of modelling of the excavation of the east tube compared with the Gaussian distribution and field data. clay. By using high-quality laboratory data [47; 48], the model was calibrated to define the unique selection of input parameters. The calibration considered the geological history of the location from which the samples were taken, and the full modelled stress path in the laboratory to include unloading caused by sampling, and shearing in compression or extension, as appropriate. The validation of the model was carried out for samples originating from three different London clay units and a good agreement was achieved. Generally, the data on small strains were in better agreement with the laboratory results in comparison to the behaviour at large strains. This was attributed to the fact that several samples failed in the laboratory on pre-existing fissures, the modelling of which was beyond the scope of the research. This feature does not affect the overall conclusions of the numerical analyses as the field measurements of the displacements of the tunnel excavation were in the small-strain region. Calibration of the SA_BRICK model was carried out by using a purpose-developed single finite element program NapGeo. Once the model was checked and calibrated in NapGeo, it was automatically set to be used for numerical analyses of tunnel excavation which were carried out in finite element program PLAXIS. This approach enables the development and calibration of numerical models in single-element form and easy transition to the validation procedure on real size boundary value problems using already developed finite element software. By applying the above-described methodology, the SA_BRICK model was used to carry out \"Class C1\" [62] analyses to predict ground behaviour caused by the excavation of the Jubilee Line tunnels at St. James's Park in London. The prediction of the model was checked against the predictions of other kinematic hardening models, denoted M2-SKH [7] and M3-SKH [13; 51], and the basic BRICK model [8]. The analyses using M2-SKH and M3-SKH models were carried out using the finite element program ICFEP [50], while the analyses using the BRICK and SA_BRICK models were carried out using PLAXIS [19]. Particular care was taken to apply equivalent numerical procedures for modelling in ICFEP and PLAXIS to achieve comparable results. In particular, the initial stress state and the modelling of geological history of the site were carefully adjusted so that the numerical analyses of the tunnel excavation could be carried out using matching boundary conditions, including the \(K_{0}\) distribution. The thorough approach was used to compare two different calculation engines (i.e., ICFEP and PLAXIS) in analysing the same boundary value problem. Sound agreement between the two sets of results confirmed the suitability of the used methodology and the high reliability of both software packages.