Settlement control during shield tunneling through soft strata utilizing pre-reinforcement

The rapid expansion of underground transportation systems has given rise to a corresponding increase in the possibility of encountering soft strata, which usually causes substantial ground settlement during shield tunneling. Cement treatment is typically utilized to alleviate ground settlement induced by excavation. However, the implementation of an appropriate reinforcement approach and associated system parameters can be challenging due to the complexity of on-site geological conditions. For long-distance reinforcement projects, this study employs numerical simulations to evaluate the effectiveness of various reinforcement methods and associated system parameters on ground settlement reduction. The simulation results indicate that the triaxial mixing pile (TMP) outperforms the umbrella arch (UA) in settlement reduction and applicability, making it the preferred option for long-distance pre-reinforcement projects. Compared to the scenario without reinforcement, the ground settlement reduction using the UA is limited to 43.7%, while the TMP achieves a reduction of 67.5% with optimal parameters. (i.e. 0.5D (tunnel diameter) reinforcement width, 0.5D depth, 0.5 pile length ratio, 3.0D tunnel buried depth, 1Pref ~ 2Pref face pressure, and 22% cement incorporation ratios). These optimal TMP system parameters are also validated through on-site monitoring. Additionally, a new model in which a disturbance correction coefficient s(x) is introduced is proposed to predict ground settlement of reinforced strata. The findings presented in this study offer a detailed reference for similar tunnelling projects where ground settlement is a non-negligible concern.