Influence of Flooded Loessial Overburden on the Tunnel Lining: Case Study

Cracks observed in the secondary lining of loess highway tunnel may be caused by various sources, among which flooded surrounding loess is a possible one. This paper studies the influence of flooded surrounding loess on tunnel lining by carrying out in situ investigation, in situ permeability test, and numerical simulations. The statistical analysis of tunnel distress reveals that the length of vault cracks accounted for more than 70% of the total length of tunnel cracks, the cracks in two adjacent sidewalls between the uplink and the downlink were much longer than that of the other two, and the vault cracked more seriously than the sidewalls. Tunnel construction caused ground cracks that allow surface water to infiltrate through the cracks, which floods and deteriorates the surrounding loess, leading to cracking of the tunnel lining. An empirical equation for predicting water infiltration through in situ loess is proposed based on the in situ permeability test, which was used to determine the flooded cases in the numerical simulation. Numerical simulations based on finite-element method were performed to investigate the mechanical response of tunnel lining in different flooded cases. The simulation results show that the tensile stress in the vault and the compressive stress in the sidewalls increased significantly after the surrounding loess was flooded; the vault cracks were due to exerted tensile stress exceeding the capacity of tunnel lining, whereas the sidewall cracks were due to the high localized compression; and the vault cracks prior to the sidewall. The simulation results are generally in good agreement with the in situ observation. This case study may have value for optimal design and long-term operation of future tunnels constructed in similar geological environments. (C) 2017 American Society of Civil Engineers.