Abstract: There are strict requirements on surface displacement caused by frost heaving and thawing settlement for the shallow tunnels. To improve the frozen wall design of the contact channel in Zhuhai Urban-Airport Interity Railway transit project, the finite difference numerical calculation method is used to simulate the whole process of the artificial ground freezing method based on the thermal-mechanical coupling theory. The optimal design of the thickness of the frozen wall is achieved by comparing the surface displacement of frost heaving, thaw settlement and the deformation of the tunnel segment in models with different frozen wall thicknesses. Results show that: 1) the finite difference numerical model can effectively predict the development of the surface displacement caused by frost heaving and thaw settlement, and an actual deformation prediction value of high accuracy can be obtained with the known error. 2) The frost heaving and thaw settlement of different models almost have the same feature, but the deformation amount and the influence range decrease with the decrease of the thickness of the frozen wall. When the thickness of the frozen wall is less than 2.5 m, the deformation meets the requirements. 3) The frost heaving and thaw settlement of the soil are not simple reciprocal processes. The thaw settlement is usually larger than the heaving deformation, and the averaged excess amount is around 40%. 4) The greater the thickness of the frozen wall is, the greater the frost heave force is. By optimizing the thickness of the frozen wall, the additional stress and deformation of the tunnel segment can be effectively controlled to protect the structure of the existing tunnel. 5) The thickness of 2.5 m is chosen for the improved frozen wall. The research results are directly applied to the construction of the No. 4 communication channel using the freezing method. Combined with the on-site monitoring test, the deformation of each characteristic point is within a reasonable range, indicating that the optimization scheme is practical and feasible, and can be successfully applied in the design of frozen wall thickness in similar projects.

Key words: shallow tunnel, frost heave and thaw settlement, thermal-mechanical coupling theory, frozen wall, thickness optimization