Field Measurement Analysis on the Damping Characteristics of Floating Slab Track in a Small-Radius Metro Tunnel Section

Due to the influences of terrain, geology and underground buildings, subway tunnels usually use curved lines. However, there is relatively little research on the vibration attenuation of small-radius curved subway tunnels. The present study takes a small-radius section tunnel of Ningbo Metro Line 5 as an example, the vibration damping characteristics of floating slab track at the curved section are revealed by conducting on-site measurements on the vibration response of subway tunnel with fixed track or floating slab track. The research show that: The floating slab track can reduce the vibration acceleration of subway tunnel walls, but causes an increase in the vibration accelerations of rail and track bed. The floating slab track only has a damping effect on the acceleration level of the subway tunnel wall when the frequency is above 1.414 times the natural vibration frequency of floating slab. Below this limit frequency, the one-third octave acceleration level will increase. The vibration response in the small-radius subway tunnel is overall high-frequency vibration. The frequency range of rail is very wide, the center frequency of the vibration dominant frequency can be up to 4000 Hz. The vibration dominant frequency of the track bed from the floating slab track is larger than that of the fixed track, with a center frequency of around 3000 Hz. When the vibration is transmitted to the tunnel wall, the center frequency drops to below 1200 Hz. Vertical vibration is still the main vibration form of the small-radius subway tunnel, however, the lateral vibration is significant since the peak lateral acceleration can reach half of the peak vertical counterpart, and the metro vibration response on the superelevation side is more significant than that on the underelevation side. The present research can provide a reference for the vibration reduction design of small-radius curved subway tunnels.