Following recent rapid developments in tunnel engineering in China,the heavy structural maintenance work of the future is likely to pose a great challenge.Newly developed vibration-based health assessment and monitoring methods offer good prospects for large-scale structural monitoring,hidden surface detection and disease pre-judgment.However,because the dynamic properties of tunnels are sensitive to the coupling and damping effects of the surrounding soil,there is little relevant research on tunnel structures.Using the PiP(pipe in pipe)model,the intrinsic tunnel modes and their response characteristics are investigated in this paper,and the degree to which the identification of these characteristics is influenced by mode superposition and the soil coupling effect are also considered.The response features of these flexible wave modes are found to be barely recognizable in a tunnel-soil coupled system,while the phase velocity of the torsional wave can be determined by combining phase spectrum analysis and the HHT(Hilbert-Huang transformation)method.A new structural health assessment method based on the torsional wave speed is therefore proposed.In this method,the torsional wave speed is used to determine the tunnel structure’s global stiffness based on a newly developed dispersion algorithm.The calculated stiffness is then used to evaluate the tunnel’s structural service status.A field test was also carried out at a newly built tunnel to validate the proposed method;the tunnel structure’s Young’s modulus was obtained and was very close to the designed value.This indicates that this method is an effective way to assess tunnel service conditions,and also provides a theoretical basis for future applications to health assessment of shield tunnels. Following recent rapid developments in tunnel engineering in China, the heavy structural maintenance work of the future is likely to pose a great challenge. Newly developed vibration-based health assessment and monitoring methods offer good prospects for large-scale structural monitoring, hidden surface detection and Because the dynamic properties of tunnels are sensitive to the coupling and damping effects of the surrounding soil, there is little relevant research on tunnel structures. Using the PiP (pipe in pipe) model, the intrinsic tunnel modes and their response characteristics are investigated in this paper, and the degree to which the identification of these characteristics is influenced by mode superposition and the soil coupling effect are also considered. The response features of these flexible wave modes are found to be barely recognizable in a tunnel -soil coupled system, while the phase velocity of the torsional wave can be determined by combining phase sp ectrum analysis and the HHT (Hilbert-Huang transformation) method. A new structural health assessment method based on the torsional wave speed is assumed. In this method, the torsional wave speed is used to determine the tunnel structure’s global stiffness based on newly developed dispersion algorithm. The calculated stiffness is then used to evaluate the tunnel’s structural service status. A field test was also carried out at a newly built tunnel to validate the proposed method; the tunnel structure’s Young’s modulus was obtained and was very close to the designed value. This indicates that this method is an effective way to assess tunnel service conditions, and also provides a theoretical basis for future applications to health assessment of shield tunnels.