Monitoring the behavior of subway tunnels during early operation in saturated soft strata

Field monitoring was performed to investigate the evolution of vibration transmission, pore pressure dissipation, and ground settlement due to metro running at the early operation stage in saturated soft strata. The vibration-induced acceleration attenuates substantially from the rail to the track bed and segmental lining. Then, it gradually declines further to the saturated soft strata. The metro-induced acceleration amplitudes on workdays are 25% higher than those on weekends. The vertical vibration induced by the metro is magnified when close to the ground surface. Once the metro vibration arrives, the excess pore water pressure will increase sharply in a very short time, i.e., less than 10 s. Utilizing real-time monitoring piezometers, the affected domain of the pore pressure where most of the change in pore pressure takes place is restricted to the annulus that extends to a distance of 1 dia from the tunnel boundary, beyond which the change in pore pressure is negligible. Furthermore, the ground continues to undergo settlement at the early operation stage for a duration of 6 months, after which the deformation will become stable. When the curvature of the tunnel axis is sufficiently large, the centripetal force produces a transverse horizontal acceleration component that is more than the vertical acceleration component, which is completely different from that with a straight tunnel axis. The analyzed monitoring results provide an in-depth understanding of the physical issue pertaining to the behavior of subway tunnels at the early operation stage in saturated soft strata.