Shield tunneling inevitably disturbs the surrounding soil, primarily resulting in changes in stress state, stress path, and strain. Modifications to certain parameters, such as shield thrust, shield friction, and soil loss, are made based on the elastic mechanics Mindlin solution and the mirror method, and a calculation expression for additional soil stresses induced by tunneling was derived. Additional soil stresses are calculated using the parameters of the Hangzhou Metro Kanji section. 3D principal stress paths and deviations of the principal stress axes near the tunnel crown, waist, and invert during shield tunneling were obtained by applying a transition matrix orthogonal transformation. These results are compared with experimental data to validate the theoretical solution's accuracy. The stress distribution along the tunneling direction and the 3D principal stress paths and deviations of the principal stress axes in the surrounding soil are determined. The results are as follows: The additional soil stresses along the tunneling direction follow a normal distribution and an S-shape. Under the combined influence of three construction mechanics factors, the shear stress component is approximately 1/3 to 1/2 of the normal stress and should not be neglected. During shield tunneling, the deviation angle of the principal stress axis at the tunnel crown changes from 90 degrees to 180 degrees, with little change in the magnitude of the principal stress. At the invert, the magnitude of the principal stress rapidly increases from 0.25 kPa to 8 kPa, with minimal deviation in the principal stress axis. At the shoulder, the principal stress variation and axis deviation are small. At the foot of the arch, the deviation angle of the major and minor principal stress axes is larger, while the magnitude of the principal stress slightly changes. At the waist, the deviation angle of the major principal stress is larger, and the magnitude of the minor principal stress significantly changes. A strategy for addressing changes in soil stress paths during shield tunnel construction is also proposed.

As metro lines continue to expand rapidly in urban areas, the excavation of twin tunnels in shallow depths using shield tunnelling methods has become widespread. By analysing field data obtained from an actual shield tunnelling project, it has been observed that the post-ground settlement occurring over the preceding tunnel during the excavation of the following tunnel in silty sand is approximately 42% of the green field settlement, which cannot be disregarded. Accurate approximation of the post-ground settlement is useful for preventing any damage due to excessive deformation and to determine the total ground settlement profile during twin tunnel construction stage. And yet, only a few number of studies have focused on investigating and predicting the postground settlement that occurs during twin tunnel construction in soft soils. Therefore, this study develops a transparent model using the multi-gene genetic programming (MGGP) method, enabling the prediction of postground settlement during twin tunnelling. Comparative analysis demonstrates that the proposed model is userfriendly and capable of generalising to unseen data. The reliability of the MGGP-based model has been validated through sensitivity and parametric analyses. Additionally, when estimating post-settlement during twin tunnelling, it is essential to consider the spacing between twin tunnels, soil cohesion, and crucial operational parameters of the shield, such as torque and face pressure.

Natural soil layers often exhibit overconsolidation due to their deposition history, which significantly affects soil mechanical properties. However, traditional analytical methods for determining critical tunnel face pressure are ineffective in considering the overconsolidation effect. This study introduces a nonlinear Hvorslev surface as the strength criterion for overconsolidated soil. The equivalent Mohr-Coulomb strength parameters are derived using the tangent technique and then incorporated into the modified three-dimensional collapse analysis. A new model is established to predict the critical face pressure of tunnel faces in clay layers with varying over consolidation ratios (OCR). The model's validity is confirmed by comparing it with the existing model in its simplified form. The critical tunnel face pressure (sigma(c)) in overconsolidated soil is influenced by the over consolidation ratio (OCR), tunnel diameter (D), the ratio of the swelling line slope to the compression line slope (kappa*/lambda*), pore water pressure coefficient (ru), soil lateral pressure coefficient (K-0), tunnel depth-to-diameter ratio (C/D), and the stress ratio at critical state (M). The findings show that with increasing OCR, the collapse zone at the tunnel face shrinks, leading to a decrease in the critical tunnel face pressure (sigma(c)). When OCR is constant, sigma(c) positively correlates with D, kappa*/lambda*, and r(u), while negatively correlating with K-0, C/D, and M. The impact of kappa*/lambda* on K-c is significant at high OCR values, and C/D and K-0 have a high sensitivity at low OCR values. Therefore, to enhance the design of tunnel face pressure in overconsolidated soil, engineers should consider factors like stress history, OCR, tunnel dimensions, and depth.