This article outlines the merits of the proposed synchronous propulsion and assembly method in the construction of a long-distance tunnel. In traditional shield construction, the stress of the unlooped segment during the assembly process is systematically overlooked. However, in the novel tunnel construction method, the advancing force of the shield directly acts on the unlooped segment, so the safety of the unlooped segment is unknown. In this paper, focusing on the safety of the segment assembly process, the segment interactions and stress concentrations under asymmetric force effects are analysed in detail via a suite of finite element models. The results show that in the synchronous propulsion and assembly mode the segments will rotate inward. A clamping effect will gradually appear during the assembly process, which makes segment deflection decrease and the stress distribution more uniform. Under asymmetrical stress, the damage to longitudinal segments is highly correlated with the types of assembly errors. The damage position that is deflected radially inward will change with the deflection angle, and the outer joint of the segment is the largest. Based on the numerical outputs, guidelines for the application of synchronous propulsion and assembly technology in practical engineering are provided.

With the improvement of the urban rail transit network, crossing construction has become an inevitable situation in the construction of subway tunnels. The special-shaped shield has great advantages in space utilization and the ability to cross narrow roads. However, due to its section characteristics, the special-shaped shield under construction often has a greater impact on the existing operating subway. Based on the background of quasi-rectangular shield tunneling through multiple existing tunnels, this paper proposes a two-stage safety pre-control (prevention before construction and control during construction) treatment method for large crosssection special-shaped shield construction. With three protection or auxiliary measures, the pre-reinforcement and process treatment of the crossing construction section are carried out. The effectiveness of these three measures is evaluated by establishing a 3D finite element model. Moreover, the reasonable range of construction parameters of the quasi-rectangular shield is optimized by numerical calculation. This study exposes the strata displacement law and tunnel deformation characteristics under corresponding protection or auxiliary measures and optimized construction parameters. The analysis proves that the proposed two-stage method can effectively reduce the impact of shield tunneling on the surrounding environment and existing tunnels. After applying the corresponding safety precautions, the surface uplift and tunnel uplift are reduced by about 30% on average, and the surface settlement is reduced by about 80%. Parameter optimization analysis shows that the thrust (face) pressure and grouting volume (filling rate) are more sensitive to environmental impact. It is recommended to select 100-160 kPa thrust pressure, 200% grouting filling rate, moderate grouting pressure, and slow propulsion speed in construction.

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Large-section special-shaped shield tunnels feature many advantages, such as versatility and a large space utilization rate in energy transmission and public transport; however, guaranteeing the quality of segments’ assembly is difficult. Based on the quasi-rectangular shield tunnel project of Hangzhou Line 9 in China, this study investigated the formation mechanism and control measures of lining segment assembly defects. By quantifying the manufacturing error and positioning error, a simulation program for segment assembly is developed to calculate the error. Furthermore, considering the relative accumulative error between the upper and lower T blocks, the finite element model of key blocks (T-LZ block) is established to perform mechanical analysis, based on which the relative error control standard of the key block under the corresponding working conditions is proposed. The results show that the assembly quality can be effectively improved by assembling the LZ block first and applying corresponding error control measures, and the displacement of the segment along the rZ direction should be carefully controlled during the construction. The error caused by normal assembly will not damage the LZ block, and the corresponding control standard under the action of multi-degree-of-freedom error (extreme case) is 9.8 mm. Using this method to predict the assembly quality of segments can provide a basis for actual construction control of segment assembly.

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