At present, there is a lack of theoretical and quantitative analysis on the influence of shield tunnel posture on the upward floating phenomenon of shield tunnel segments under synchronous grouting, which makes it difficult to provide a reasonable shield tunnel posture adjustment plan for construction sites. By considering the influence of shield machine attitude on the shield-tail segments, based on the Euler beam theory, the Pasternak two-parameter foundation model, and other related mechanical theories, an analysis model for the influence of shield tunnel segment uplift under synchronous grouting is established, and the uplift phenomenon, deformation, and internal force responses of shield tunnel segments under different shield attitudes are analyzed. The results show that: Reasonable adjustment of the shield attitude can control the uplift phenomenon of segments during construction to a certain extent. The vertical position of the shield machine can be appropriately adjusted downward, and the pitch angle can be adjusted so that the tail of the shield machine rotates downward, which can alleviate the uplift phenomenon of the segments and reduce their internal forces. Unreasonable adjustment of the shield attitude will not only aggravate the uplift phenomenon of the segments but also significantly increase their internal forces. In actual projects, in order to control the uplift phenomenon, the vertical position of the shield machine can be appropriately adjusted downward, and the pitch angle of the shield machine can be appropriately adjusted to keep the tail of the shield machine rotating downward. After adjusting the shield attitude according to the research conclusions of this paper, the uplift phenomenon of the segments in the referenced project was alleviated. The research results can be used to guide the quantitative adjustment of shield tunneling posture when controlling the upward movement of construction pipe segments.

At present, the construction of large-span and super-large-span subway tunnels is increasing. Temporary support is often designed to reduce the span in the construction of those tunnels. However, temporary support needs to be removed before secondary lining is constructed due to the spatial overlap between them. The removal of temporary support is the weak situation, because the original stress balance of the structure will be broken. Improper construction can easily cause safety accidents such as tunnel collapse. In view of the lack of systematic research on the structural mechanical response and safety of the super-large-span tunnel constructed by the double-layer initial support arch-cover method at home and abroad, this paper takes the four-line parallel tunnel of Huahua section of Guangzhou Metro Line 11 as the basis project. Through the combination of theoretical analysis and numerical calculation, the mechanical response and safety of the structure during the dismantling of the tunnel are analyzed from both horizontal and vertical aspects. Finally, a scientific, reasonable, safe, efficient and rapid dismantling scheme was determined, and successfully passed the inspection of on-site construction. The results show that it is feasible to dismantle the super-large-span tunnel in urban soft stratum by using the scheme of ‘first edge and then middle, and symmetrical demolition in cross direction, first three demolition and one demolition, and then one demolition and one demolition in longitudinal direction', which can provide reference and guidance for subsequent projects.

Currently, there is a little research on the impact of simultaneous slurry initial setting time on the buoyancy of tunnel lining during shield tunneling construction. This makes it difficult to provide reasonable recommendations for slurry initial setting time under various construction conditions. Therefore, a quantitative longitudinal analytical model considering the influence of tunnel depth on shield tunnel construction buoyancy has been established. Based on the relationship between simultaneous slurry initial setting time and the length of the section where the shield tunnel lining is affected by buoyancy, the proper selection of initial setting time for simultaneous slurry during the construction period in moderately weathered clayey formations was systematically studied. The results show that the initial setting time of the slurry has a significant impact on the final cumulative longitudinal behavior of the shield tunnel during the construction period. A longer initial setting time leads to greater cumulative uplift load, longitudinal bending moment, and longitudinal shear caused by buoyancy. Moreover, with an equidistant increase in the initial setting time, the magnitudes of these behavior indicators also increase to a certain extent. From the perspective of controlling buoyancy in shield tunnel construction, there should be a matching relationship between the simultaneous slurry initial setting time and the average excavation speed of the shield. Specifically, as the average excavation speed increases, the reasonable initial setting time should be shorter. By adjusting the simultaneous slurry initial setting time to approximately 5 hours through mix design, a significant improvement in the buoyancy problem during construction has been achieved.