## 2 Formation and Influence of Tunnel Geotemperature
### Influences of High Geotemperature on Tunnel Construction  
High geotemperature occurs in tunnel engineering, forming a high-temperature and humid environment, which have serious negative impacts on the construction personnel safety, the construction materials, the machinery and equipment, and the operation of the tunnel after completion [19; 20; 21]. Specifically,  
1. Hazards of high geotemperature on the personnel safety  
The deterioration of tunnel working environment caused by high geotemperature will not only increase the difficulty of construction, but also drag down the progress of the project, and seriously threaten the health and safety of the construction personnel [22; 23]. Compared with the outside environment, both the temperature and the concentration of carbon dioxide inside the tunnel is higher, and the construction personnel is prone to heat stroke, which includes heat spasm (muscle spasms and pain during and after work), heat collapse (decreased blood pressure, rapid pulse, weakness, confusion or coma, pale skin, and mild rise in body temperature), and heat apoplexy (headache, dizziness, disorientation, elevated body temperature and dry skin). According to the field investigation, about 80% of the operators will develop some of the above symptoms within 2 h after the work in a high-geotemperature tunnel.
2. Damage of high geotemperature on machinery and equipment  
The heat dissipation effect of mechanical equipment in the high-temperature environment is not ideal, and the failure rate increases significantly [24]. If the limit temperature of the machine is exceeded, the machine will be damaged. For example, in the case that rock drill, excavator, loader, ballast truck, and other fuel equipment are used on the high temperature environment [25], the engine \"boiling\" phenomenon can easily occur within 1 h, the power of automobile engine decreases, the braking performance decreases, and the tire blowout increases. Therefore, the working time of the equipment is lowered, both the service performance and life are reduced, and the construction efficiency is seriously slowed down. In addition, the performance of detonating cords, detonators, and other blasting equipment will be affected by high temperature, which indicates higher possibility of blind guns and squibs [26].
3. Deterioration of building materials by high geotemperature  
Take concrete as an example: under the condition of high temperature, the early hydration speed of concrete will be accelerated, the internal hydration reaction is aggravated, both the initial and the final setting time are greatly shortened, but it also leads to incompact internal structure with large numbers of defects [27; 28; 29]. During the long-term usage, internal defects will spread from the inside to the surface, causing the surface to peel [30].  
The large temperature difference in the contact surface of shotcrete and rock layer hinders the bond effect [31; 32; 33]. During secondary lining, there also exists a temperature difference between the new shotcrete and the original shotcrete, which reduces the cohesiability between cement particles, and is easy to cause the formation of internal honeycomb structure failure. Besides, in the presence of high temperature water vapor, the fluidity of concrete will rise, resulting in water damage [34].  
In tunnel engineering, geothermal problems caused by heat radiation or rock temperature conduction will exist for a long-time during operation, requiring regular protective maintenance and repairment. While in the case of high temperature, the service life of the materials used in the tunnel will be greatly reduced, which will cause adverse effects on the safety of passengers, and a significant increase in maintenance costs [35].