# Modelling the Structure and Anisotropy of London Clay Using the SA_BRICK Model
## 4 Validation of the SA_BRICK Model  
### Verification of the Modelling of Anisotropy  
A series of numerical calculations were carried out using the single finite element program NAPGeo and the SA_BRICK model to investigate the behaviour of London clay under different stresses and types of loading conditions. These predictions were directly compared against the results of high-quality laboratory triaxial tests, which were undertaken as part of several doctorate research projects [47; 48] and also [18; 28]. In order to check the validity of the introduction of anisotropy in the SA_BRICK model, the first comparison was made with the results of the validation of the anisotropic model A_BRICK. In his research, [PERSON] [45] used samples tested by [PERSON] [48] from the units C and B2, that is, samples labelled 7.2 UC and 7.2 UE, and by [PERSON] [47], i.e., samples 10.1 UC and 10.2 DE, taken from the Terminal 5 site at London's Heathrow Airport. The number indicates the depth from which the sample was taken, and the symbol indicates whether it was a triaxial drained (D) or undrained (U) investigation under compression (C) or tension (E) conditions. The modelling considered the geological history of the location from which the samples were taken. This initial stress path was followed by the unloading caused by sampling, and after the establishment of the initial stress state through isotropic compression, a shearing in compression or extension was modelled. A detailed explanation of the validation modelling procedure will be given in the next section. For the sake of comparability, the same geological history and the same sample stress paths were used in the laboratory in this research as in the analysis provided by [PERSON] [45]. It should be observed that when discretizing the S-curve, [PERSON] adapted string lengths to fit the stiffness degradation curves determined for London clay by [PERSON] [47]. In this type of verification of SA_BRICK, identical input parameters as in [45] were used, along with the anisotropy, which was taken into account from the very beginning, i.e., already during the course of sedimentation. The SA_BRICK model results are given in Figure 7, together with the prediction of the A_BRICK model and the laboratory results (labelled LAB). The results are shown for samples 10.1 UC (undrained shear in compression; upper part in all diagrams except  
Figure 6: Scheme of the SA_BRICK stress integration procedure in the form of the PLAXIS user-defined soil model. 7d) and 10.2 DE (drained shear in extension; lower part in all diagrams) showing the \(q\)-\(p^{\prime}\) diagram, \(q\)-\(e_{y}\) diagram, \(u\)-\(e_{y}\) diagram, and \(\varepsilon_{y}\)-\(\varepsilon_{\text{vol}}\) diagram (only for drained sample 10.2 DE). Verification data of the A_BRICK model on London clay samples do not allow a comparison of stiffness, as they are not given in [PERSON]'s work. It can be seen in the figure that the modelling of anisotropy in SA_BRICK is fully comparable with the results in [45] for both cases, as SA_BRICK gives practically the same results as the A_BRICK model. A partial deviation is visible only in the case of volume deformation prediction (Figure 7d). It should be noted that the laboratory test 10.1 UC (upper part in diagrams) was undrained so that the effective stress paths in the laboratory were driven by the changes in pore pressures. As it can be seen in Figure 7c, this change was underestimated by both the A_BRICK and SA_BRICK model, so it was impossible to model the stress path exactly. In the drained test 10.2 DE, this was not the issue, so the stress path was fully matched. Certain minor deviations can also be attributed to the different steps for the load steps, the position or placement of the initial bricks and strings, and the accuracy of the given geological history and sampling.