The construction of a slurry shield tunnel produces large amount of excavated soil. Direct discharge of the excavated soil by transportation not only increases the overall construction cost but also raises environmental concerns of pollution. In this study, we explore in-situ recycle of excavated soil in the Nanjing Yangtze River tunnel project as the back-fill grout. A series of laboratory were carried out to determine the synchronous grout parameters, including compression strength and shear strength, initial setting time, fluidity, consistency, and bleeding rate. The mixture ratio of the back-fill grout is also discussed to optimize the grout performance. It is found that: (1) the excavated soil produced from the construction of slurry shield tunnel in this project can be reused as the material of back-fill grout in the sand stratum; (2) Proper selection of binder/sand ratio and fly ash/cement ratio improves the performance of the synchronous grout. The test results demonstrated that the discharged soil can be recycled in-situ as the grouting material in the sand stratum.

As the soil-resistance coefficient in a soft soil area is small, overlarge bending moment may cause exceeding transverse deformation for the shield tunnel and cause structural diseases and waterproof failure at the longitudinal segment joints. Hence, a new idea of cross-section design for a minimum bending moment shield tunnel was proposed. This article has first put forward the concept of a zero bending moment shield tunnel. Then, based on rational and feasible hypotheses, a structural mechanical model and an analytical expression of axis for the cross-section of the zero bending moment shield tunnel was obtained, and computational formulas of internal force and key geometry parameters were given. Based on the case of the metro shield tunnel constructed in the Shanghai soft soil area, the zero bending moment shield tunnel was designed, and its characteristics were analyzed. Considering only one cross-section of shield tunnel can be adopted in one metro line, the design method and procedure of the minimum bending moment shield tunnel were put forward. Finally, taking one of the Shanghai metro lines as an example, a cross-section of a minimum bending moment shield tunnel was designed, and its bending moment was compared with the bending moment of a circular section shield tunnel, which had the same horizontal diameter. The comparison revealed that the cross-section of the minimum bending moment can significantly reduce the bending moment of shield tunnel.