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.

Current methods for evaluating the ground settlement due to tunneling in loess do not fully consider the special engineering properties of loess. This paper first studied the ground settlement due to tunnel construction in loess by conducting a uniquely designed centrifuge model test. Then the ground settlements were calculated using a currently widely used method and compared with the measured values from the centrifuge model test. The results indicated a substantial difference between the calculated and measured settlement profiles. For the stratum at shallow depth, the calculated settlement profiles were flatter with smaller maximum settlement S_max at the tunnel centerline and larger settlement far from the centerline than those from the centrifuge model test; while for the stratum near the tunnel crown, the opposite was obtained. The main reason for the difference is because of the large void structure and high compressibility of loess which causes additional settlement at shallow depth. Based on the centrifuge model test results, a new method which properly considers the special engineering characteristics of loess was proposed to evaluate the settlement due to tunnel construction in loess. The calculated settlement profiles using the new method were in good agreement with those from the centrifuge model test. Finally, the new method was used to evaluate the ground settlement caused by the construction of a Metro tunnel in loess and satisfactory results were obtained.

In the New Qidaoliang Tunnel (China), a rear-end collision of two tanker trunks caused a fire. To understand the damage characteristics of the tunnel lining structure, in situ investigation was performed. The results show that the fire in the tunnel induced spallation of tunnel lining concrete covering 856 m³; the length of road surface damage reached 650 m; the sectional area had a maximum 4% increase, and the mechanical and electrical facilities were severely damaged. The maximum area loss happened at the fire spot with maximum observed concrete spallation up to a thickness of 35.4 cm. The strength of vault and side wall concrete near the fire source was significantly reduced. The loss of concrete strength of the side wall near the inner surface of tunnel was larger than that near the surrounding rock. In order to perform back analysis of the effect of thermal load on lining structure, simplified numerical simulation using computational fluid dynamics (CFD) was also performed, repeating the fire scenario. The simulated results showed that from the fire breaking out to the point of becoming steady, the tunnel experienced processes of small-scale warming, swirl around fire, backflow, and longitudinal turbulent flow. The influence range of the tunnel internal temperature on the longitudinal downstream was far greater than on the upstream, while the high temperature upstream and downstream of the transverse fire source mainly centered on the vault or the higher vault waist. The temperature of each part of the tunnel near the fire source had no obvious stratification phenomenon. The temperature of the vault lining upstream and downstream near the fire source was the highest. The numerical simulation is found to be in good agreement with the field observations.