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.

With the widespread application of Earth Pressure Balance (EPB) shield technology, the generation of shield muck has been increasing yearly. This paper aims to investigate the effectiveness of bentonite-silty clay modified slurry (BSC) as a soil conditioner for enhancing the workability of sands during EPB shield tunneling, thus enabling the recycling and reuse of the discarded muck (waste silty clay). Standard slump tests were conducted on three typical sand specimens from Shenyang Metro Line 6. The influence of the types of conditioners and slurry injection ratio (SIR) on slump values were examined to determine an optimal conditioning scheme tailored to the specific formation conditions. Furthermore, the study explored the combined use of BSC and foam to improve workability, employing a three-factor four-level orthogonal experiment. Finally, the rheological parameters (yield stress) derived from the slump tests provide valuable insights for assessing material flow within the tunneling system. The results show that comparative analyses with pure bentonite slurries reveal that BSC is a suitable, economical, and effective alternative for soil conditioning. The particle size distribution of sand specimens significantly influences the conditioning process, necessitating adjustments to SIR and slurry viscosity for optimal results. When the slump value of slurry-conditioned soil falls within the range of 150-250 mm, the slump test can be effectively used to estimate its yield stress under atmospheric conditions. This study contributes to the development of sustainable and economical solutions for soil conditioning in urban tunnel projects, particularly by utilizing excavated materials effectively.