# Modelling the Structure and Anisotropy of London Clay Using the SA_BRICK Model
## 5 Analyses of the Settlement Trough  
### Modelling of the Excavation of the Jubilee Line Tunnels at St. James's Park  
The tunnels under St. James's Park (London, UK) were built in 1996 as part of the Jubilee line metro extension project. This boundary value problem in London clay was chosen with an aim of using high-quality observation data which were unaffected by urban development. South of the lake in St. James's Park, an instrumented greenfield monitoring section had been set up, based on which the analyses of the monitoring data were carried out by [PERSON] et al. [20]. The monitoring system and the precise measurements of the settlement trough caused by the excavation of the Jubilee Line tunnels were comprehensively presented by [PERSON] and [PERSON] [27]. Finite element programs ICFEP [50] and PLAXIS [19] were both used to carry out a series of numerical analyses with an aim of comparing predictions of displacements around the mentioned tunnels. In the program ICFEP, the models M2-SKH and M3-SKH were used, while in PLAXIS, the BRICK and SA_BRICK models were used. The purpose of the use of more kinematic hardening models was to make a comparative analysis of the accuracy of the model predictions. The geological profile with the geometry of the tunnels is shown in Figure 12. The profile consists of a 5.5 m thick made ground and alluvium, under which there is 2.7 m of Terrace gravel underlaid by some 35 m thick layer of London clay, below which is a base of hard Lambeth Group Beds. The groundwater was immediately above the gravel layer. The diameter of the tunnels excavated at 20.0 m (east tube) and 30.5 m (west tube) depths was 4.75 m. The west tube was built first, followed by the east tube some eight months later. The tunnels were excavated using a TBM of open-faced type with a 4.2 m long shield, while face excavation was carried out by a back-hoe machine. The precast concrete linings were installed behind the shield after thrusting ahead off the previously constructed ring. Excavation of the two tunnel tubes under St. James's Park took place in undrained conditions and was modelled accordingly using the \"volume loss\" method. It should be noted that the term \"volume loss\" is not strictly technically correct as there is no change in volume in undrained conditions. It is used to indicate a part of the volume change caused by the convergence movement of tunnel excavation, which is compensated for by the subsidence of the terrain on the surface. Thus, the volume of material converging to the tunnel cavity (\(V_{l}\)) is equal to the volume of deformation that appears as settlement on the terrain surface (\(V_{s}\)). The excavation of the tunnel is modelled in small excavation increments, while the installation of the tunnel lining is simulated at the point in which the prescribed value of the volume loss is achieved. The use of the \"volume loss\" method in finite element analyses is detailly described in [50]. The presence of groundwater was also considered at a depth of 5.5 m, as indicated in Figure 12. The hydraulic boundary conditions were determined as drained conditions for two upper layers (made ground and gravel), while the pore pressures were assumed to be constantly increasing along the boundaries of London clay. In the ICFEP program, the initial stresses, the pore pressures, and the coefficient of horizontal pressure at rest \(K_{0}\) values were defined at the beginning of the analyses, as a separate input file. Models with kinematic hardening M2-SKH and M3-SKH enable the modelling of geological history, but geological history was not included in the analyses. Instead, the focus was on the adequate \(K_{0}\) profile, which means that there were kinematic surfaces centred prior to the modelling of the excavation of the tunnel. Figure 12: Geological profile and layout of twin tunnels beneath St. James Park (adapted after [27]). The input data for the M2-SKH model and the M3-SKH model were determined based on simulated undrained triaxial laboratory tests.