Study On Distribution Characteristics Of Temperature Field And Freeze-thaw Damage Of Structural Layer Of Tunnel In High Altitude Cold Area

As the most representative permafrost area in the world,the Qinghai Tibet Plateau accounts for 75% of the total permafrost area in China.It is a very important strategic and resource reserve area in China.Based on the Jiangluling Tunnel in the permafrost region of the Qinghai-Tibet Plateau,this paper uses literature research,theoretical analysis,numerical simulation and laboratory testing to investigate the characteristics of the temperature field distribution and influencing factors of the tunnel as well as the degradation of the tunnel structure layer under freeze-thaw damage.The conclusions are drawn are as follows:(1)Through literature research,the research status of tunnel temperature field in cold region is summarized from three aspects: theoretical analysis,numerical simulation and model test;Secondly,the classical theory,experimental research and the application of ultrasonic nondestructive testing technology of concrete frost resistance durability are summarized;Finally,the common measures to prevent and treat the freezing injury of tunnels in cold regions are summarized.(2)Based on the basic theories of heat transfer and fluid mechanics,taking the Jiangluling Tunnel as the engineering background,combined with the means of numerical simulation and the principle of orthogonal design,a numerical model of tunnel temperature field in cold region is established,and the influence of different atmospheric environmental factors(including atmospheric temperature,atmospheric pressure,wind speed,wind direction,humidity and solar radiation)on tunnel temperature field is analyzed.From the point of view of numerical simulation,three parameters are selected,including wind temperature,wind speed and duration.Taking the tunnel lining structure temperature and tunnel freezing depth as evaluation indexes,the sensitivity analysis of the influence of atmospheric environmental factors on the studied tunnel temperature field is carried out.The results show that: To evaluate the sensitivity of atmospheric environment to tunnel temperature field in cold region,two reference indexes of freezing depth and lining structure temperature can be used for comprehensive judgment;The order of influence degree of lining structure temperature is as follows: duration > wind speed in tunnel > temperature at tunnel entrance;The order of influence degree of tunnel freezing depth is: duration > temperature > wind speed.(3)The distribution characteristics of tunnel temperature field in high altitude cold region show that the lining structure at tunnel entrance has long been subjected to freezing and thawing action of alternating positive and negative temperatures.In order to improve the freeze-thaw resistance of tunnel lining concrete,aiming at the frost resistance durability of concrete in alpine region,the frost resistance of concrete mixed with different amounts of fly ash and air entraining agent was studied:the concrete quality,dynamic elastic modulus loss and acoustic wave velocity variation under freeze-thaw cycles were studied by nondestructive testing of concrete in the process of freeze-thaw cycles.The results show that: the content of fly ash and air entraining agent have great influence on the frost resistance of concrete;the addition of air entraining agent can improve the frost resistance of concrete;under the same amount of air entraining agent,the frost resistance of concrete with 10% fly ash is better than that with 20%;In the process of freeze-thaw cycles,ultrasonic P-wave velocity and S-wave velocity,dynamic elastic modulus and dynamic shear modulus show similar attenuation with the increase of freeze-thaw cycles,and there is a high correlation.(4)In order to better describe the deterioration law of concrete performance under freeze-thaw cycles,based on the damage deterioration model commonly used to describe concrete freeze-thaw cycles,the fitting and life prediction effects of polynomial,exponential,linear function and Weibull distribution models are compared and analyzed.According to the changes of concrete mass and dynamic elastic modulus obtained from experiments under freeze-thaw cycles,combined with the established damage evaluation parameters of mass and dynamic elastic modulus.The results show that: except for linear function,the other three models can describe the trend of accelerated damage of concrete performance under freezethaw action: in polynomial function,the concept of damage acceleration is clear,the calculation is simple and the fitting accuracy is high;with the increase of freeze-thaw cycles,the exponential function accelerates the damage more obviously;although the form of Weibull function is complex,the fitting accuracy is high and easy to converge;when the frost resistance of concrete is good or the number of freeze-thaw cycles is low,the linear function fitting effect is better when the accelerated attenuation trend of concrete performance is not obvious.