# A Study on the Heat Transfer of Surrounding Rock-Supporting Structures in High-Geothermal Tunnels
## 5 Conclusions  
In this paper, a field experiment conducted in a high geothermal tunnel provides several vital conclusions about the distribution and change rules of temperature during the tunnel construction. Based on the finite difference method, the evolution process of a tunnel temperature field was calculated and compared with field test results. The reliability of the finite difference method in analyzing the tunnel heat transfer problem and the tunnel temperature field prediction method was verified. Finally, through the calculation results under different initial surrounding rock temperature conditions, the heat release of rock and actual curing environment of concrete in a high geothermal tunnel are analyzed. The following major conclusions have been obtained:  
1. After the tunnel is excavated and ventilated, the range of low-temperature rock mass extends itself continuously and reaches a new thermal equilibrium state. The closer to the surrounding rock surface, the greater the influence on the rock temperature by ventilating and cooling. Due to the thermal conductivity of rock being bigger than that of the lining structure, the temperature of rock contact with lining rises; farther away from the contact interface, the rock's temperature is approximately constant. According to the distance away from the sidewall, the rock temperature showed three ways of change: undulate type (\(<\) 2 m), decline type (2-5 m) and stable type (\(>\) 5 m).
2. For the rock temperature field before the completion of the secondary lining structure in the high geothermal tunnel, the thickness of the heat-adjusting layer is noted to be second-order polynomial functions of the initial rock temperature. However, the released heat of the surrounding rock increases near linearly.
3. The temperature of the lining structure dropped sharply within seven days, and then slowly decreased until a stable state. The application of the secondary lining blocked the convective heat transfer between the primary lining and the air in the cave, causing a slight rise of the primary lining temperature, and then the temperature of the primary lining was gradually reduced by the continuous ventilation, finally approaching the air temperature inside the cave again.
4. During the high-geothermal tunnel construction, the concrete actually cures under the variable high-temperature environment. The real curing temperature decreases with time and becomes stable seven days later.
5. The calculated data of the in situ investigation of the Sang Zhu Ling tunnel testifies the accuracy of the temperature field calculation method for high geothermal tunnels. The finite difference model proposed in this paper can be used to guide the design of cooling systems and concrete strength for high geothermal railway tunnels.  
Conceptualization, M.W., Y.H., Q.W. and D.L; formal analysis, C.J.; funding acquisition, D.L.; project administration, M.W.; software, Y.W.; writing-original draft, Y.H.; writing-review & editing, Q.W. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Natural Science Foundation of China (grant numbers: 51578458, 51878568 and 51878567).  
**Acknowledgments:** All authors would like to thank the headquarters of Lhasa-Nyingchi railway construction (grant number: 2017G006-B) for providing support for this field experiment.  
**Conflicts of Interest:** The authors declare no conflicts of interest.