Effects of tunneling-induced soil disturbance on the post-construction settlement in structured soft soils

Earth pressure balanced shield machine (EPBS) is widely adopted for metro tunnel construction in urban areas. Support pressure on excavation face and grouting at the EPBS tail are the main measures to control the ground disturbance and settlement. The post-construction settlement is of significance to the serviceability of metro tunnel. By weakening the compression parameters of underlying soils related to soil disturbance degree (SDD) and using layer-wise summation method, this study developed a method to predict the post-construction ground and tunnel settlement. The SDD is quantitatively evaluated by the shear-strain state obtained from a series of isotropically consolidated undrained triaxial compression (CIU) tests. Based on a case history of metro tunnel constructed in structured soft soils, a suite of three dimensional finite-element analyses are performed to investigate soil disturbance during tunneling construction. The numerical simulation and the applicability of the proposed method are validated by comparing the numerical results with the field measurements. In addition, influences of support pressure and grout ratio on the tunneling-induced soil disturbance, ground and tunnel settlement are revealed. The SDD shows remarkable falloff within 0.32D above the tunnel crown, while 0.24D below the tunnel invert. Furthermore, within the above distance range, the linear relation between the SDD and distance from the tunnel crown or invert can be built. The analysis results show that the excessively high or low support pressure and grout ratio can both cause severe SDD which results in the significant post-construction settlement. Grout ratio of about 200% and support pressure of 2.2-2.4P0 are effective for reducing both the post-construction ground and tunnel settlement. The research presented in this paper provides a reference for controlling the post-construction ground and tunnel settlement constructed in structured soft soils.