Analytical method for thawing analysis of surrounding rock in seasonal cold region tunnels

Thaw and frozen heave are both the important factors for the stability of surrounding rock in cold region tunnels. A theoretical model is estublished to study the stress distribution when the surrounding rocks melt in seasonal cold region tunnels. The model is based on the condition that frost heave occurs firstly in the surrounding rocks, and the deflation in volume and the compression progress of thawing surrounding rocks under load when the frozen surrounding rocks melt are considered. When the frozen surrounding rocks melt and the volume reduces, the surrounding rocks outside the thawing range move to the tunnel, the thawing surrounding rocks are compressed and deformation occurs. After the balance is reached, other frozen surrounding rocks also enter the thawing range. They will also melt and the surrounding rocks outside them also move to the tunnel. The melting surrounding rocks are compressed again and the new balance is reached at last. The phenomenon is repeated, until the entire system is stable. In order to simplify the analysis, it is assumed that all progress is completed one time and the deflation in volume when the frozen surrounding rocks melt is approximately equal to the frost heave. The lining and thawing surrounding rocks are regarded as a composite retaining structure. Its deformation is regarded as the compression process of thawing surrounding rocks. The results of the example show that when the frozen surrounding rocks thaw, the maximum principal stress in lining will reduce and the stress in thawing range will also decrease distinctly. The stress in the surrounding rocks without thaw increases due to the change of geometrical sizes. In addition, the influence law of the linear strain of frost heave, geostress and thawing radius is analyzed. The proposed model can well reflect the actual situation in the field. It may provide a certain reference for the thawing study of surrounding rocks in cold region tunnels and well guide the design of cold region tunnels.