Recent studies on London Clay have identified a number of different units in the geological profile, and have highlighted the role of soil structure in mechanical behaviour. In fact, structure is the dominant factor determining the differences in the mechanical response of different units. In the paper, numerical analyses simulating the undrained excavation of a tunnel in St James's Park are presented. London Clay behaviour is characterised by a kinematic-hardening structured soil model incorporating structure and stiffness degradation. The parameters and initial conditions are based on a careful calibration that takes into account the presence of different units within the London Clay formation and the different degrees of soil structure. The analyses performed result in a very satisfactory reproduction of the magnitude and patterns of short-term surface and subsurface displacements, as well as pore pressures. The paper concludes with a discussion of the results in the context of other analyses performed previously, and puts forward some considerations concerning design issues.