Analytical solutions for performance of existing shield tunnel subjected to overlying excavation under anti-uplift portal frame

The heave of the existing shield tunnels will be induced due to the above excavation, and then structural damages, including leakage and cracking, easily happen. The combination of the shaft excavation and anti-uplift portal frame is an effective technique to limit the heave of the existing tunnels, while the theoretical method is absent presently for the design of the anti-portal frame. Thus, the analytical solutions for the performance of the existing tunnels are proposed for both the longitudinal and cross-section deformations of the existing tunnels during the shaft excavation and foundation pit excavation, respectively. The flow chart is also provided. The results predicted by the analytical solutions are in good agreement with those of the field measurement and 3D numerical simulation, which proves the reliability of the analytical solutions. The mechanism of the anti-uplift portal frame is carefully investigated, and the sensitivity analysis of the buried depth of the existing tunnels and the excavation size of the shaft is performed. The results show that the anti-uplift portal frame can effectively control the deformation of the existing tunnels caused by the above excavation, strongly depending on the thickness of the soil above the existing tunnels. The efficiency of the anti-uplift portal frame is stronger with the decrease of the thickness of the soil above the existing tunnels. The effects of the anti-uplift portal frame mainly depends on the force between the anti-floating slab and the soil, and the uplift pile shows a minor influence on the deformation of the existing tunnels.