Large-deformation finite-element modelling of face instability during tunnelling in clayey soils: Incorporating dynamic excavation process

Earth pressure balance (EPB) shield tunnelling plays a crucial role in urban infrastructure development but faces challenges due to potential face instability. Previous studies often overlook the impact of dynamic excavation processes on face stability, particularly in clayey soils. This study proposes a three-dimensional coupled Eulerian-Lagrangian (CEL) large-deformation approach to explore the failure process of tunnel face in clayey soils during both EPB shutdown and excavation conditions by incorporating the influence of dynamic excavation process. The findings reveal that the face stability during dynamic excavation conditions is consistently lower than that during EPB shutdown, indicating that neglecting dynamic excavation effects could overestimate face stability, potentially compromising construction safety. Moreover, when considering the dynamic excavation process, the opening ratio ξ (the ratio between the cross-section area of cutterhead opening to the total area of tunnel face) is a critical factor affecting face stability. As expected, the face is more stable for a small opening ratio (e.g., ξ = 15 %) while face stability is reduced for a large opening ratio (e.g., ξ = 30 % and 50 %). This is because for large opening ratio, the soil disturbance caused by the cutterhead excavation process exceeds the supportive effect that the cutterhead panels can provide to the face. Finally, the obtained numerical results were used to calculate support pressure in a real tunnel project considering the dynamic excavation process, which matches well with the measured data in the project, validating the effectiveness and superiority of the current CEL approach in modelling face instability and estimating face support pressure. This study offers a significant advancement over the traditional methods for the design of support pressure in clayey soils, providing new insight into the dynamic cutterhead-soil interaction and valuable guidance for reducing the risk of face collapse.