Numerical Modelling and Sensitivity Analysis of Tunnel Deformations in London Clay

Sammanfattning: In dense cities, the interactions between all structures, from tall skyscrapers to complex underground tunnel systems, need to be carefully analysed as soon as a new project is considered. This is necessary because of the stress changes in the soil induced by each new construction. Demolishing a building could cause heave at the base of the excavation, deflections in supporting structures and settlements of the surroundings. The behaviour can be modelled in order to predict how large the deformations will be. This thesis investigates the effectiveness of such models. This is done through the application of a parameter sensitivity analysis on models created in Plaxis. The purpose of the analysis is to identify which factors cause discrepancies between the models and the actual displacements monitored on site. The project being examined is located in central London. The analysis focuses on the displacements of existing tunnels below the site caused by the demolition of two buildings. An analysis was carried out to investigate the significance of different parameters, of different material models and methods of analysis, of 3D effects and of inaccurate groundwater data. Ground investigations, laboratory tests and published data were the main sources used to collect reliable initial input parameters for the material models. A model was created in Plaxis 2D using the Mohr-Coulomb and the Hardening Soil with small-strain stiffness material models, using two types of undrained analysis. A model using the Mohr-Coulomb material model was created in Plaxis 3D as well. A sensitivity analysis was then carried out on the 2D models to identify which input parameters were most significant to the tunnel displacements. The results were compared to monitoring data and a back-analysis was carried out to produce more accurate results. The initial and adjusted input parameters were also tested on the 3D model. Finally, the groundwater level was altered. The results indicate that soil stiffness and effective cohesion are the most significant. Small-strain stiffness is shown to be especially important when analysing small tunnel deformation. The 3D model generally yielded more accurate results than the 2D model, while the groundwater level did not appear to affect the deformations.