Ground Improvement Techniques for Tunnel Excavation in Saturated Clay Medium: A Hydro-Mechanical Coupled Analysis of Drains and Capsule Grouting

This thesis investigates ground improvement techniques, focusing on the application of drains and the innovative capsule grouting technology (CGT) for tunnel excavation in saturated clay mediums. The excavation of an underground metro systems frequently encounters geo-materials of poor geotechnical properties, making it essential to improve the ground's strength and stiffness under certain conditions to support the excavation process. Pre-excavation drainage of clayey layers is typically considered effective. It involves drilling boreholes and activating them for a specific duration. Special attention is directed towards the innovative capsule grouting technology (CGT), which involves injecting grout into capsules (or inflating pockets) made from a special membrane to create an artificial volumetric increase, ensuring controlled grout distribution within heterogeneous ground conditions. A comprehensive numerical modelling approach is employed using Plaxis 2D to simulate different scenarios and evaluate the performance of drains and capsule grouting in improving ground conditions. The advanced elasto-plastic constitutive model, specifically the Hardening Soil Model, was implemented to simulate realistic and accurate ground behavior. The study introduces a transient hydro-mechanical coupled analysis that describes the interaction between the hydraulic and mechanical processes. Additionally, the proposed numerical modelling approach for capsule grouting involves a prescribed volumetric expansion of elements representing the grouted body. The results show significant improvements in isotropic effective stress, active pore water pressure, cohesion, and stiffness. Moreover, sensitivity analyses on permeability and volumetric expansion provide further insight into the conditions that maximize the effectiveness of these ground improvement techniques.