Effects of critical tunnelling parameters on consolidation-associated long-term greenfield settlement due to shield tunnel in soft ground

This study investigates the long-term settlement behaviour induced by shield tunnelling in soft ground, employing a case study and numerical modelling to achieve a comprehensive understanding. Soil consolidation plays a critical role in long-term surface settlement, necessitating precise calibration of essential tunnelling parameters such as face pressure and grouting pressure. The findings indicate that soil settlement progressively increases with increasing face pressure P_f and grouting pressure P_g during the excavation of a shield tunnel. Furthermore, for long-term consolidation settlement, it has been established that setting P_f at 90% of the lateral earth pressure σ_xx consistently minimizes settlement across all cover depths. This phenomenon is attributed to the soil arching mechanism, which also reduces the height of the loosened zone at this specific P_f level. Similarly, the optimal P_g is identified to be within the range of 130% to 150% of the vertical earth pressure σ_zz. For cover depths within the loosened zone, the smallest consolidation settlement and loosened zone height are observed at P_g of 150% of σ_zz. Conversely, for shallower cover depths, beyond the loosened zone but within the arching zone, the smallest consolidation settlement and loosened zone height occur at P_g of 130% of σ_zz. This study reveals that adjusting the cover depth significantly influences the reduction in vertical stress and the resulting settlement, demonstrating the importance of tailored grouting pressure calibration for varying depths to limit consolidation settlement.