Study On The Spatial And Temporal Variation Pattern Of Tunnel Melting Circle In Permafrost Region Based On Coupled Hydro-Thermal

In China’s mammoth economic development,the eastern infrastructure construction is gradually saturated,due to the implementation of the "Sichuan-Tibet Railway" and "the Belt and Road" and other century projects,the focus of infrastructure construction gradually shifted to the west,the scope of transportation infrastructure construction further expanded.In order to ensure the smooth traffic in the western region,the construction of tunnels in high altitude and cold regions is inevitably increasing,but because of the unique climate in cold regions,the construction and operation of tunnels in this region are very susceptible to frost damage,such as hanging ice,lining cracking,etc.,which has a great impact on the safety of vehicles in driving,so it is significant to carry out research on the spatial and temporal changes in the melting circle of tunnels in cold regions during construction and operation.In this paper,based on Jiang Luling tunnel,the study on the evolution of melting circle of surrounding rocks in cold area tunnels based on hydrothermal coupling during construction stage and various temperature environments was carried out through data collection,field monitoring data analysis,theoretical modeling and numerical analysis,and the sensitivity of melting circle influence in cold area tunnels was analyzed.The main results and conclusions are as follows:(1)Applying the concept of shape factor of heat transfer,ignoring the high-pressure influence of gas generated by blasting,equivalently considering the influence of heat generated by blasting on the surrounding rock melting circle,a temperature field control equation considering heat sources(hydration heat,blasting heat)is established.(2)Based on Jiang Luling tunnel,the study of the temperature field evolution law of the surrounding rock during the construction stage was carried out,and the results showed that: shotcrete as a permanent support structure can also effectively avoid direct contact with air in the surrounding rock,which inhibits the development of the melting circle depth,while the melting circle depth increases by 10 cm for every 5 cm increase in shotcrete thickness,and the melting circle depth changes in the same pattern when applied at the same time interval.(3)primary and secondary mold construction stage,the timing of application has the greatest impact on the depth of the surrounding rock melting circle,too early application of primary and secondary mold construction,shotcrete hydration reaction has not yet completed,will accelerate the development rate of melting circle,increase the melting depth,too late application will lead to long exposure to the air,and then the formation of too large melting circle;primary mold construction of different entry temperature melting circle changes are similar.The insulation layer can effectively reduce the depth of the tunnel melting circle and avoid the repeated freezing and thawing around the tunnel caused by the alternation of seasons during operation,which is important for the construction and operation of tunnels in cold areas.(4)Research on the change of tunnel thaw circle depth under different temperature environment,the results show that: the construction season has the greatest influence on the thaw circle;the higher the initial ground temperature,the greater the change of thaw circle depth,when the initial ground temperature is-1℃,-2℃,-3℃,the thaw circle depth is 4.76 m,2.63 m,1.84 m respectively,while the initial ground temperature is too low,the thaw circle from the inner surrounding rock back freezing rate accelerates,forming Considering the influence of temperature change inside the cave due to the heat generated by construction machinery and personnel during excavation,it was found that reducing construction heat could effectively reduce the melting circle and speed up the process of refreezing.(5)To study the influence of different factors on the melting circle of surrounding rock,the melting depth of surrounding rock under each stage increases with the increase of thermal conductivity,initial ground temperature and cave temperature,and decreases with the increase of specific heat capacity;in the excavation stage,according to the sensitivity calculation,it can be seen that the sensitivity of melting depth to each influencing factor is from large to small: specific heat capacity,cave temperature,thermal conductivity,initial temperature;in the shotcrete stage,for 0.25 m,0.30 m thick shotcrete,the melting depth of surrounding rock increases with the increase of thermal conductivity and initial temperature,For 0.25 m and 0.30 m thick shotcrete,the sensitivity of melting depth to each factor is as follows: initial ground temperature,specific heat capacity,thermal conductivity,and cave air temperature;for primary molded concrete at 5℃ and10℃,the sensitivity of melting depth to each factor is as follows: application timing,specific heat capacity,thermal conductivity,and cave air temperature;for secondary molded concrete at 0.03 m and 0.05 m,the sensitivity of each factor is as follows The sensitivity of each factor under the thickness of insulation layer is the same as that of the primary moulding stage,and the sensitivity of temperature inside the cavern is extremely small with insulation layer,which shows the importance of insulation layer to the surrounding rock from the side.