Field tests on longitudinal stress relaxation along shield tunnel in soft ground

In order to study the longitudinal stress relaxation properties along shield tunnels in soft ground, two years of field tests in Qianjiang Tunnel are carried out using monitoring cells to obtain the relaxation characteristics of tunnel segments and the contact surface between adjacent lining rings, and to obtain the time-dependent response of longitudinal bolts connected adjacent rings. The results show that the longitudinal stress evolution process inside the segments in soft ground experiences four stages: drastic fluctuation, dynamic stabilization, gradual attenuation and final stabilization. The longitudinal stress relaxation inside the segments lasts for at least 1 year, and causes stress reduction from 3 to 1.2 MPa gradually. Similarly, the development of longitudinal stress between adjacent lining rings experiences four periods as well. After 18 months of longitudinal stress relaxation, the stress value on the interface between adjacent rings decreases from 2 to 1 MPa. The axial forces of bolts decrease first, then temporarily stabilize, and finally increase, and the axial forces of longitudinal bolts are still not stable after two years of dynamic adjustment. Therefore, the origination of longitudinal stress relaxation along the shield tunnel in soft ground is the time-dependent response of contact adjusting between adjacent rings, interaction of lining and soils, and axial forces relaxation of bolts, and coupled-interaction with each other. The stress relaxation along shield tunnel will easily lead to stiffness degradation, shear resistance reduction, and leakage water, etc. The above characteristics should be considered carefully in the future.