Macro Freezing Damage Mechanism And Prevention Of High-speed Railway Tunnel In Cold Region Based On Short Period Temperature Fluctuation

Macro water freezing phenomenon seriously threatens the safety of high-speed railway tunnel.With the expansion of high-speed railway tunnel scale in severe cold regions of China,it is increasingly urgent to reveal the mechanism of macro-freezing damage and develop prevention and control technology.Relying on the National Natural Science Foundation of China(51978668,51478473)and the major scientific research project of China Railway Group(2018K019),the formation and evolution mechanism of macro water freezing damage,including hanging ice on lining surface,drainage ditch freezing and frost heave in voids,were revealed on the basis of testing and analyzing the characteristics of temperature field of high-speed railway tunnel.Meanwhile,freezing damage prevention and pre-warning technology were developed.The main contents and achievements are as follows.(1)The continuous automatic measurement of lining surface temperature and numerical simulation analysis were carried out to reveal the temporal and spatial variation of temperature field in high-speed railway tunnel in cold region.The instantaneous and cumulative effects of train wind on temperature field in tunnel were analyzed,and a simplified algorithm of equivalent train wind speed for heat transfer effect was proposed.The temperature of the lining surface of the high-speed railway tunnel was asymmetrically distributed along the longitudinal direction,which was high in the middle and low at both portals.The temperature in the tunnel also showed a trumpet envelope characteristic based on the temperature daily fluctuation outside the tunnel,and its spatial-temporal variation range was controlled by the wind speed inside the tunnel.The range of negative temperature in high-speed railway tunnels was much larger than that of ordinary railway tunnel and highway tunnel,the train wind was the fundamental reason for this difference.The train wind caused the short-term reduction of lining surface temperature,and its cumulative effect could be calculated by the equivalent wind speed.(2)The simulation experiment of ice hanging were carried out,the calculation model of water seepage temperature was established,and the evolution law of seepage water temperature and the evolution mechanism of ice hanging were revealed.The process of formation,development and melting of ice hanging on the lining surface were divided into four states.They were accelerated growth state,delayed growth state,suspended sate and melting-falling state.The state of ice hanging was directly related to the lining surface temperature,water seepage rate and temperature.Under the condition of temperature fluctuation,the growth state of ice hanging transited in the four states.The amplitude and mean value of temperature fluctuation jointly determined the state and duration of ice hanging growth,and then affected the development degree of ice hanging.(3)The calculation model of water temperature in the tunnel ditch was established base on the heat exchange characteristics,and the cooling and freezing mechanism was revealed.The temperature of the water in the tunnel ditch was mainly controlled by the heat exchange between the water body and the ditch wall.The flow distance of the water in the ditch before freezing was controlled by the initial water temperature,flow velocity,wet perimeter and ditch wall temperature.Under the conditions of higher water temperature,higher flow rate,smaller wet circumference and higher wall temperature,the flow distance of water before freezing was larger.For a particular tunnel,whether the ditch was frozen mainly depends on the temperature inside and outside the tunnel and the water flow in the ditch.(4)According to the excretion conditions of water in voids,the classification of voids and the simulation experiment of frost heave were carried out.The evolution process of frost heave in voids was studied by numerical simulation method.The calculation method of frost heave force in voids was proposed by introducing the excretion coefficient,and the influencing factors and rules of frost heave force were revealed.Under the condition of temperature fluctuation of lining surface,the freezing and thawing of water in voids began from the thinnest part,and then developed to the thicker part.The process of freeing-thawing and supply of water in the void leaded to increasing of frost heaving force.The excretion coefficient of the two types of voids was different.The frost heaving force of the void was controlled by the void height and the position of the discharge channel.When the height of the void was larger and the discharge channel was closer to the bottom surface of the void,the frost heaving force in the void is larger.(5)Based on the formation mechanism of macro freezing damage,the strategies and measures to prevent freezing damage were put forward.For the existing high-speed railway in cold regions,a macro freezing injury prediction and pre-warning model based on real-time temperature monitoring was developed.For new tunnels,air curtain technology should be adopted to reduce the impact of negative temperature outside the tunnel on the inside of the tunnel,the scope of freezing damage and the probability of freezing.For the existing tunnel,a temperature monitoring system based on Internet of Things technology was developed,and a macro frost damage pre-warning model was proposed,which was verified in the supporting project.