Dynamic Stability of Soft Soil Between Closely and Obliquely Overlapped Metro Tunnels Subjected to Moving Train Loads

The problem in dynamic stability of soft soil between closely and obliquely overlapped metro tunnels during operation will aggravate cumulative settlements which affects the safety of metro systems. Based on Shenzhen Metro Line 5 and Line 11 overlapped tunnels, a three-dimensional vehicle-track-tunnel-soil model has been established. Meanwhile, the dynamic shear strain and its transfer characteristics of the soft soil layers between overlapped tunnels under moving train loads has been studied. In longitudinal direction, the most unfavorable position for the soft soil is located in the place where the maximum overlapped degree exists between two tunnels. In transverse direction, the dynamic shear strain in soft soil between tunnels diffuses obliquely at a certain angle from the arch waist of tunnel and gradually decreases with the increase of distance. It also increases nonlinearly with the number of operating lines. Besides, the dynamic stability of soft soil was evaluated by the cyclic threshold shear strain parameters of Vucetic and the value exceeds the linear cyclic threshold shear strain. The plastic deformation of soft soil under moving train loads will accumulate obviously and the dynamic stability problem cannot be ignored. Therefore, high-pressure jet grouting pile is adopted to reinforce the soft soil between two tunnels. And the result shows that the dynamic shear strain of soft soil after reinforcement is much smaller than that before.