Shaking table tests on ultra-shallowly embedded shield tunnels with ground-penetrating technology

The dynamic response of the ultra-shallowly embedded tunnels, such as ground penetrating shield tunnels (GPSTs), is markedly different from that of the conventional one. Since GPST is partially exposed above the ground surface and partially submerged, it is crucial to investigate its seismic response. The preliminary results of a series of large-scale 1g shaking table tests, conducted at Tongji University, are discussed. A large-scale tunnel-soil model is developed to accurately reproduce the tunnel joints, the cross-sectional and longitudinal stiffness of the tunnel, and the key soil parameters of the prototype problem. The model tunnel has a total length of 7.7 m, with the embedded depth ranging from -0.5D to 0.5D, where D is the tunnel diameter, while the negative values indicate that the tunnel crown is above the ground surface. The Shanghai artificial synthetic wave is applied as seismic excitation in the transverse direction. Two types of tunnels, standard lining and open-crown lining, are tested. The study focuses on analyzing the acceleration response and the ovaling deformation of the tunnel, to learn the dynamic characteristics of GPST. The results reveal a strong effect of embedment depth on tunnel seismic response. The decrease of embedded depth leads to a significant increase of accelerations. The tunnel exhibits a "whiplash effect", leading to strong shaking of the above-ground lining. The ovaling deformation increases with the embedded burial depth. Overall, the response of the underground lining (D ≥ 0) is governed by soil-structure interaction (SSI), whereas the above-ground structure (D < 0) exhibits a pronounced whiplash effect due to the absence of soil confinement. Such effects need to be properly considered in the seismic design of GPST.