Mechanical behavior and construction control measures for shield launching assisted by a steel sleeve in water-rich soft ground

To investigate the stress and deformation characteristics of steel-sleeve-assisted shield launching in water-rich soft ground when shield launching is immediately by undercrossing an existing line, this study takes the shield tunnel section from Yongmao West Road Station to Jiulong Avenue Station of the Ningbo-Cixi Suburban Railway as the engineering background. A three-dimensional finite element model of steel-sleeve-assisted shield launching was established using ABAQUS, and the model was validated against field measurements. The stress and displacement responses of key sleeve sections during the stages of sealing and pressure maintenance, shield launching, and shield-tail grouting were extracted to analyze the stress and deformation characteristics of the sleeve in different construction stages. Different cases with different face pressures and synchronous grouting pressures were further investigated. The results show that the circumferential stress of the steel sleeve is dominated by tensile stress, with peak values mainly distributed at the segmental connection interfaces as well as the top and side regions of the sleeve. The peak longitudinal stress occurs near the excavation face, migrates forward with shield advance, and gradually decreases after entering the grouting stage. Compared with face pressure, synchronous grouting pressure has a more pronounced effect on the local deformation of the steel sleeve. The peak value of the top longitudinal section can increase by up to about 1.2 times, and the maximum radial displacement at the side region reaches 0.31 mm. The overall deformation of the steel sleeve remains within the elastic range, and the unfavorable positions of the transverse section are mainly located at the side regions of the sleeve. The zone within 2–3 m before and after the grouting ring is the key section for deformation control. Based on the deformation characteristics of the sleeve, a coordinated control method involving end reinforcement, steel-sleeve sealing, and key-parameter control is proposed. It is recommended that the face pressure and synchronous grouting pressure be controlled within 0.20 MPa and 0.30 MPa, respectively, and that the interfaces between sleeve blocks and the sections adjacent to the grouting ring be taken as key targets for structural verification and monitoring.