Research On Base Deformation And Prevention Technology Of Tunnel Portal In Cold Region Under Freezing And Thawing

As the Chinese economy grows and the scale of infrastructure construction increases,China has built many railway tunnels and highway tunnels in cold regions.Compared with ordinary regions,the construction of tunnel technology in cold regions is more complicated.One of the main problems is that seasonal tunnels are generally affected by seasonal freezing and thawing.This periodic freeze-thaw will inevitably cause damage to the tunnel structure.The lining and surrounding rocks near the tunnel entrance are always affected by changes in the outside atmospheric temperature and the air temperature inside the tunnel.Due to the temperature field,it is more likely to cause structural damage.Compared with the interior of the tunnel,the vicinity of the entrance is more vulnerable to freezing damage.Therefore,the focus of tunnel research in cold areas is the near the tunnel entrance.This paper summarizes the research results of domestic and foreign scholars on the temperature distribution of tunnels in cold regions,numerical simulation data,and frost damage prevention data.Based on the engineering background of a cold region tunnel in Tibet,through finite element software simulation calculations,the temperature field distribution law and deformation of the base of the tunnel entrance in the cold region were studied,and the prevention measures were studied.The main contents and results are as follows:The finite element equation of the freezing and thawing temperature field of the tunnel was obtained through theoretical derivation.According to the basic theoretical knowledge of thermoelasticity,the effect of water-ice phase transition was considered,and the stress-strain relationship of the cold tunnel was obtained,and two-dimensional thermal and mechanical numerical models of tunnels in cold region was established.Under the premise of global warming,the change of tunnel temperature field in the next50 years was obtained through finite element calculation,and the melting depth at the base of the tunnel opening in a representative year was calculated.Moreover,the depth of this melting will become deeper and deeper as the climate warms,and the changes of the surrounding rock temperature and the longitudinal depth of the tunnel are analyzed.Through the coupling calculation of the temperature field and the displacement field,it can be seen that in the first year of loading temperature,the maximum thawing deformation of the surrounding rock at the basement is 50 mm,and the maximum freezing and expansion deformation of the surrounding rock at the basement is 80 mm,under the climatic conditions where the global temperature increases year by year The depth of melting and sinking increases,and the deformation of melting and sinking continues to increase.Introduced common measures for deformation prevention of bases of tunnel entrances in cold areas,laying methods of insulation layers and types of common insulation layers,andcalculated and analyzed the temperature field distribution of tunnel doors in cold areas when laying insulation layers.The thermal insulation effect of thermal insulation layer with different thickness is studied and analyzed,The temperature distribution of surrounding rock in the basement of the tunnel during the past 10 years is calculated.Compared with the case where no thermal insulation layer is laid,the maximum depth of thawing settlement has been significantly reduced.Laying a 5cm thick PU polyurethane insulation board between the initial support and the secondary lining,the surrounding rock will still undergo thaw sinking damage.When laying an 8cm thick insulation board,the permafrost at the bottom of the lining is always at a negative temperature,and the thaw ring is kept within the lining,which achieves the purpose of protecting the permafrost and ensures the stability of the tunnel base.