| Freezing damage occurs frequently in tunnels in cold regions,which brings many challenges to traffic safety and tunnel maintenance.However,the formation,mechanism and influencing factors of freezing damage are extremely complex,which brings many difficulties to the study of tunnel freezing damage in cold regions.In recent years,with the construction of a number of major transportation facilities in cold regions in our country,many scholars have conducted a lot of research on the coupled hydro-thermo-mechanical of frozen soil in cold regions,but there are still few studies on the longitudinal frost heave force of tunnel entrances.This paper adopts a combination of theoretical research,model test and numerical simulation to conduct an in-depth study on the longitudinal frost heave force of the tunnel portal section in cold regions.The main contents and results are as follows:(1)The fluid dynamics-solid mechanics hydro-thermal-mechanical coupling model is selected to explore the distribution characteristics of the temperature field,longitudinal frost heave force and moisture field of the tunnel entrance section in the cold region.The model uses soil water potential as the driving force of water migration in frozen soil,takes into account the heat carried by water migration,and uses soil volume changes to calculate the longitudinal frost heave force.(2)According to the similar theory,the freezing and thawing indoor model test of the tunnel entrance section was carried out,and the axial strain of the tunnel lining and the temperature of the model soil were collected and analyzed.The test results show that during the freezing and thawing process,the model soil temperature shows a slow V-shaped change with time,which can be roughly divided into five stages: unfrozen stage,phase change stage,frozen stage,phase change stage,thawing stage;During the freezing and thawing process,the axial strain of the tunnel lining changes significantly,indicating that the lining has an axial frost heave force.The lower the temperature,the greater the axial frost heave force,which first increases and then decreases.In addition,the longitudinal frost heave force of the lining at the entrance of the tunnel is significantly greater than that of the inner part,and the longitudinal stress of the inner part of the lining is negative and in a compressed state.(3)Adopting COMSOL Multiphysics finite element analysis software,a three dimensional simulation model based on fluid dynamics-hydrothermal coupling model and solid mechanics sequence coupling is established,which realizes the three-field coupling solution of hydro-thermal-mechanic and analyzes the distribution characteristics of the three-field hydro-thermal-mechanic.The simulation results show that: The mathematical model can well reflect the change characteristics of the model temperature field,which is consistent with the experimental results;The longitudinal frost heave force at the entrance of the tunnel has a similar variation with the test results,but there is a big difference in the value of some stresses,which is mainly caused by the uneven frost heave locally.At the same time,near the portal section of the tunnel,the longitudinal frost heave force of the tunnel lining increases obviously,which si similar to the tangential frost heave force distribution of the pile foundation.In addition,more obvious stress concentration occurred at the entrance of the tunnel.The unfrozen water content will decrease sharply after the phase change,the ice content will increase rapidly,and the speed of both will show a vertical linear change.As the temperature further decreases or increases,the rate of change of unfrozen water content and ice content gradually slows down.(4)According to the research of different working conditions,it is found that the working condition with large initial water content can delay the heat transfer.When the ambient temperature is the lowest,the maximum temperature of the part far away from the tunnel portal in the model is significantly higher than that under the working condition with small initial water content.In addition,under the action of the temperature gradient,water continuously migrates to the freezing front,causing the ice content to increase continuously,and the soil volume expansion becomes more and more intense,which directly leads to the greatest frost heave force near the tunnel portal.Therefore,it is necessary to strengthen the strength and protection of the tunnel portal.This paper uses a combination of model test and numerical simulation to compare and analyze the temperature field,longitudinal frost heave force and moisture field results of the tunnel entrance section in the cold zone,and verify the reliability of the model test,which can provide theoretical basis for research on tunnels in the cold regions. |
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