Postconstruction settlement that occurs before a tunnel is in operation may significantly influence the tunnel’s long-term stability. The current study investigates such a tunnel, a three-lane superlarge section tunnel in Gansu Province, China, to assess the long-term settlement performance of a loess tunnel using reinforcement from vertical jet grouting piles. A three-dimensional finite-element model, validated through field observations, is employed to simulate soil consolidation behavior. Results indicate that the long-term settlement, as determined by the finite-element method (FEM), corresponds with field investigation results. Specifically, most of the Fujiayao tunnel’s long-term settlement (nearly 90%) occurred within the first 60 days after tunneling. Settlement occurred at a relatively rapid consolidation rate and then gradually stabilized within 120 days with a maximum consolidation settlement magnitude of 14.99 mm according to FEM versus 12.89 mm from field observations. Compared to a case without reinforcement, consolidation settlement in the reinforced case was found to decrease significantly over a shorter consolidation period. Furthermore, the relatively large consolidation settlement surrounding the tunnel, as well as consolidation settlement overall, gradually and uniformly declined in an outward direction from the tunnel. The vertical jet grouting technique exhibited a strong reinforcement effect on the loess tunnel’s foundation and can be applied to similar soft foundation tunnel reinforcement projects to greatly improve the stability and safety of tunnels in operation.

Compared with electrical strain gauges, fiber Bragg grating (FBG) sensing technology is a relatively novel method for tunnel structural health monitoring, which has a number of advantages including high accuracy, multiplexing, electromagnetic interference resistance, and good repeatability. In order to study the internal force of the tunnel liner and detect the potential safety hazards, series of strain monitoring tests of a loess tunnel, taking into account the complex stress and strain variation of the loess during tunnelling, were performed by employing the tandem linear FBG sensor arrays controlled by the wavelength division multiplexing (WDM) technology. The concrete strain has obvious linear characteristics over time in the early stage and then gradually tends to a stable value. Moreover, after the necessary temperature compensation, loess tunnel structure safety was assessed through the analysis of real-time strain and internal force of the liner concrete, and the FBG monitoring data and safety assessment results indicate that the safety factors of various liner sections all meet the code requirements, which verify the safety and stability of the tunnel liner structure. The FBG sensors-based in situ monitoring technology can be well applied in the loess tunnel structure safety assessment.

It is well known that the tunnel structure will lose its function under the long-term repeated function of the vibration effect. A prime example is the Xi’an cross tunnel structure (CTS) of Metro Line 2 and the Yongningmen tunnel, where the vibration response of the tunnel vehicle load and metro train load to the structure of shield tunnel was analyzed by applying the three-dimensional (3D) dynamic finite element model. The effect of the train running was simulated by applying the time-history curves of vibration force of the track induced by wheel axles, using the fitted formulas for vehicle and train vibration load. The characteristics and the spreading rules of vibration response of metro tunnel structure were researched from the perspectives of acceleration, velocity, displacement, and stress. It was found that vehicle load only affects the metro tunnel within 14 m from the centre, and the influence decreases gradually from vault to spandrel, haunch, and springing. The high-speed driving effect of the train can be divided into the close period, the rising period, the stable period, the declining period, and the leaving period. The stress at haunch should be carefully considered. The research results presented for this case study provide theoretical support for the safety of vibration response of Metro Line 2 structure.