Investigation On Evolution Characteristics Of Temperature Field And Anti-freezing Measures Of High-speed Railway Tunnel In Cold Regions

The freezing damage frequently appears in cold regions and threatens the safe operation of the train and reduces the service life of tunnels,which has become one of the main problems for railway networks in following maintenance period.Meanwhile,the demand for preventing the freezing damage is more urgent with the improvement of the engineering construction grade of high-speed railway.Compared with the common speed railway tunnels,the heat exchange process between air flow inside and outside the tunnel is more intense and complex when the train runs at a high speed into the tunnel.The tunnel structural layer is facing the threat from periodic freezing damage in long terms.As an important part of high-speed railway project,it is important to identify the evolution characteristics of temperature field inside the tunnel under complex factors,such as natural wind,piston wind and hydrothermal coupling,which is of great significance to take some reasonable and effective measures to prevent freezing damage and ensure the safe operation of high-speed railway.To solve the problems above mentioned,the Gaotai tunnel that locates along the JilinTumen-Hunchun passenger dedicated line is taken as the studied background.The freezing damage prevention of the high-speed railway tunnel in cold regions is taken as theme in this paper.On the basis of the in-situ monitoring,a series of the laboratory experiments,numerical analysis and performance evaluation of insulation material are conducted.Then the heat exchange between inside and outside the tunnel is deeply analyzed,and the distribution characteristics of temperature field inside the tunnel under the action of natural wind and piston wind are systematically studied.Furthermore,the performance degradation of insulation materials under repeated freeze-thaw cycles is discussed.Subsequently,based on the coupled hydro-thermal theory model,the key application points of insulation method for lining structures in the prevention of freezing damage of high-speed railway tunnel in cold regions are proposed that considering the performance degradation of insulation material.The main conclusions are as follows:(1)The temperature distribution of tunnel structural layer and air inside the tunnel are related to the air temperature outside the tunnel,wind speed,wind direction,tunnel direction and local topography at the tunne portal.In the cold season,the longitudinal distribution of temperature inside the tunnel is characterized by lower temperature near tunnel portal and higher temperature in the middle.The maximum frozen depth gradually decreased in the monitoring period,the frozen depth of every section inside the tunnel significantly increased and the maximum frozen depth can reach 1.33 m in the winter,which suggests the freezing damage of tunnel will be more serious in the future.(2)The temperature of structural layer and surrounding rock are apparently affected by wind temperature and wind velocity under the action of natural ventilation,especially at tunnel portal.The frozen depth above the vault and below the track increases with the increasing of wind speed when the wind speed is less than 4 m/s.If the wind speed exceeds to 4 m/s,the effect of wind speed on the frozen depth is not remarkable.However,the frozen depth always increases exponentially with the decrease of wind temperature.(3)After the train passing through the tunnel,the air temperature inside the tunnel is redistributed significantly.In the same section,the air temperature inside the tunnel conforms to be parabolic distribution.And the closer to the secondary lining surface,the higher the air temperature is.The temperature inside the secondary lining is affected by the air temperature outside the tunnel and train velocity.And also,the influence of piston action on the temperature near tunnel entrance is greater than that near tunnel exit.At the same speed,the change of the temperature inside the secondary lining under different air temperatures outside the tunnel is related to its initial temperature.Whereas,under the same air temperature outside the tunnel,the faster the train runs,the more obvious of the temperature inside the secondary lining decreases.When the air temperature outside the tunnel is-27.25 ° C(minimum air temperature outside the tunnel on the coldest day),the temperature inside the secondary lining decreased by0.87 ° C after the train passing through the tunnel at the design speed(250 km/h).(4)After the train passing through the tunnel repeatedly,the change process of the temperature inside the secondary lining within 100 m from the entrance and 500 m from the exit of the tunnel are the same as that air temperature outside the tunnel,but the fluctuation range of the temperature is relatively small.Additionally,the repeated movement of the train promotes the development of the negative temperature zone along the longitudinal direction of tunnel,which alters the temperature inside the secondary lining from a positive temperature to the negative temperature near the tunnel entrance.Therefore,it is suggested that the antifreezing measure must be carried out for Gaotai tunnel from tunnel entrance to exit.(5)Repeated freeze-thaw cycles can degrade the performance of XPS board.The maximum reduction rate of compressive strength of XPS board is up to 33% after 175 times of freeze-thaw cycles.The volumetric water absorption of XPS board under freeze-thaw cycles is much greater than that under natural state.The increase of water absorption further increases the thermal conductivity,but the thermal conductivity of XPS still meets the standard requirements after 175 times of freeze-thaw cycles.Therefore,it is feasible to adopt the XPS board to prevent and control the frost damage of high-speed railway tunnel in cold regions.(6)Without considering the performance degradation of XPS board,the XPS board with a thickness of 5 cm laid on tunnel structural layer with the interlayer method can ensure that the surrounding rock does not freeze.When considering the performance degradation of XPS board,the same effect can be achieved by laying the XPS board with the thickness of 6.5 cm.Additionally,in terms of the temperature of surrounding rock in cold season,the insulation effect by laying XPS board on the surface of secondary lining is better than that of interlayer method.For Gaotai tunnel,the thickness of XPS board should be greater than 8 cm when it is laid on the secondary lining surface,which can ensure the secondary lining will not suffer from frost damage.