Research on Cooling and Thermal Environment of Super-Long Single-Headed Tunnel Construction with High Ground Temperature

In order to solve the problem of difficulty in cooling the super-long working face of high-temperature tunnels, based on the Dechang Tunnel of Chengdu-Kunming Railway, the main causes of heat damage in high-temperature extra-long tunnels and the on-site cooling effects of ventilation cooling, ice cooling and mechanical cooling were discussed by the oretical analysis and on-site monitoring. The COMSOL Multiphysics software was used to compare the distribution law of the ambient temperature field within 100 m of the face under two methods of ventilation cooling and comprehensive cooling（mechanical ventilation + ice cube + mechanical cooling）. The research results show that:（1）When the ventilation distance of single-head construction is 4.6 km, the air temperature at the outlet of the air duct is about 32 ℃; when only mechanical ventilation is provided, the ambient temperature of the tunnel face is about 38 ℃, supplemented by ice cubes and mechanical refrigeration, which basically meets the construction requirements of the face.（2）The average temperature of the tunnel face decreases with the increase of ventilation time in a negative exponential trend, and it is linearly related to the air temperature at the outlet of the air duct; compared with the comprehensive cooling method, the average temperature of the tunnel face is 3 ℃~4 ℃ higher, and the stabilization time of the face temperature is 30~50 minutes less when the temperature is cooled by pure mechanical ventilation.（3）The ambient temperature of the tunnel above the air duct is generally higher than that below the air duct; from the tunnel face to the tunnel opening, the temperature of the upper part of the tunnel presents an increasing-decreasing-increasing development trend, and a high temperature peak occurs at about 15 m from the tunnel face; in terms of the temperature of the lower part of the tunnel section, there is a big difference between ventilation cooling and comprehensive cooling. That is, the ventilation and cooling show a sudden decrease-slow decrease-slight increase trend, and the comprehensive cooling shows a surge-slow increase trend. The research results can provide guidance for the construction of similar high-temperature tunnels.