Review of Research Progresses and Application of Geothermal Disaster Prevention on Large-Buried Tunnels

Abstract

Geothermal disaster caused by high geotemperature is a commonly encountered geological problem in tunnel engineering, especially in large-buried tunnels, which is directly related to the safety, technology, and economy of tunnel construction. It seriously affects the personnel security and the performances of construction equipment and building materials, greatly increasing the construction difficulty, and extending the total construction period, which has become a major issue to be urgently solved in the tunnel construction. This paper first briefly introduces the formation mechanism of the high-geotemperature environment of a large-buried tunnel and analyzes the significant influences of high-temperature on personnel, equipment, and materials in the construction process of tunnel engineering. Then, the worldwide research progress of rock mechanics in high-temperature large-buried tunnels is systematically described, including the thermo-mechanical properties of rock mass, the thermo-mechanical properties of shotcrete, and the rheological mechanism and control technology of surrounding rock. Subsequently, the previous geothermal disaster classification of large-buried tunnels is summarized and evaluated. Finally, the research findings of the key technologies of geothermal disaster prevention and control are presented in detail from three aspects of temperature reduction, thermal insulation, and personal protection, which are of great theoretical and practical significance for ensuring the safety design and construction of tunnels in similar geological environment.

1. Introduction

Generally, the area with dry bulb temperature higher than 28 °C is defined as high-temperature area. High-temperature tunnels usually occur in the regions of geotemperature gradient or abnormal surrounding rock temperature, which are often accompanied by geothermal disaster problems, such as high-temperature surrounding rock and hot water inrush during tunnel construction. The high temperature in tunnels leads to the deterioration of construction environment, and the instantaneous emission of geothermal water may even cause great loss of life and property [1,2,3,4].

The high-geothermal problems in tunnels first appeared in the second half of the 19th century, and seriously hindered the construction of tunnels. For example, the temperatures of surrounding rock of the New Gotthard Tunnel in Switzerland reaches up to 45 °C [5]. Even more, the Apennine Railway tunnel, the Amphang Tunnel, the Mont Blanc road tunnel, the Lyon-Turin Tunnel, the Nueva Lechbourg Tunnel, the Tecolote Road Tunnel, and the Simplon Tunnel are all affected by geothermal disaster during the construction and operation [6]. Meanwhile, along with the western development and the in-depth promotion of the “Belt and Road” strategy in China, the infrastructure interconnection project has become the focus of engineering construction in western region. Among these engineering constructions, there exist numerous tunnels that are long, deep-buried, or cross the active geological structure areas, which means various high-geothermal problems [7,8,9]. It is reported that the temperature in Qinling Line I tunnel is 31~42.1 °C at the depth of 1000~1600 m [10], and the highest temperature in the Sun Village Tunnel in Yunnan Province is up to 43 °C [11]. The area along Sichuan-Tibet Railway belongs to the Mediterranean-Himalayan tropical zone, among which there are about 15 tunnels evaluated to suffer geothermal disaster [12]. Some other typical high-geotemperature tunnels in China are shown in

Table 1. The other tunnels with high geotemperature in China.

Tunnels	Depth/m	Length/km	Temperature/°C
Wuzhishan Tunnel	-	4.45	37
Daiyunshan Tunnel	400	1.562	38.8
Qinling Tunnel of Xikang Railway	1645	18.448	40
Fudang Tunnel	90	7.517	44
Luogu hydropower Station diversion tunnel	-	1.779	47
SaGa tunnel	-	4.941	51.6
Old Village Tunnel of Yunmeng Railway	150	4.46	52
Baishui hydropower station diversion tunnel	-	2.314	62
Dagar Mountain Tunnel	700	7.21	64
Givosiga Tunnel	104	3.974	65.2
Luquan lead factory diversion tunnel	380	7.215	76
Padang Hill Tunnel	33	2.865	76.4
Niangyong hydropower station diversion tunnel	640	0.295	78
Bulunkou-gonger hydropower Station diversion tunnel	300	17.8	105
Qir Khattar hydropower station diversion tunnel	1720	15.64	110

[6].

The geothermal disaster not only directly reduces the feasibility of the project scheme, but also decides the construction safety, efficiency, schedule, and investment of the tunnel project, which brings great challenges to the investigation, design, and construction of tunnels [13]. According to the references and the tunnel construction accidents publicly reported by the State Administration of Safety Supervision of China, there are a total of 176 tunnel construction accidents from 2005 to 2020, wherein the accidents caused by high geotemperature during tunnel construction accounted for 24 [14,15]. This shows that geothermal disaster caused by a high-geotemperature environment has gradually become a key issue in the construction process of deep-buried underground engineering.

In this paper, the formation mechanism of high geothermal environment in large-buried tunnel was primarily explored, and the main hazards on worker health, construction equipment, and building materials were generally described. Secondly, the research progress on geothermal disaster of large-buried tunnels was comprehensively and systematically reviewed, which can be divided into rock thermo-mechanical properties, shotcrete thermo-mechanical properties and rheological mechanism of surrounding rock. Then, the current research of tunnel geothermal disaster classification methods and standards were evaluated. Finally, the key technologies of geothermal disaster prevention were presented in detail in terms of temperature reduction, thermal insulation, and personal protection, so as to provide a certain reference for similar tunnel engineering construction and geothermal disaster prevention.

2. Formation and Influence of Tunnel Geotemperature

2.1. Development Characteristics of High Geotemperature in Tunnels

The formation of high geotemperature in tunnels is mainly determined by heat source and heat conduction channel. Primarily, heat sources can be divided into four categories [16,17,18]: (1) Hot spots are formed in the upper dome of mantle asthenosphere, i.e., deep crustal thermal anomaly; (2) Heat dissipation during cooling of underground magma chambers or high-temperature intrusions in the crust or surface ejecta, i.e., waste heat of magmatic rock; (3) Frictional heat from fracture activity in the Earth crust; (4) Rock radioactive elements decay to generate heat.

At present, the heat source of high geotemperature in-tunnel engineering mainly comes from the thermal anomalies in the deep crust and the decay of radioactive elements in the rock. And the main heat conduction channels of the deep circulation of the deep crustal thermal anomaly include faults and fractures. According to the length, scale, and influence scope of the channel, it can be divided into a main channel and a secondary channel. Wherein, the main channels are generally deep and large faults, while the secondary channels are contact zone, secondary fault, structural fracture, dissolution fracture, karst pipeline, etc.

2.2. 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].

3. Research Progress of High-Temperature Rock Mechanics in Large-Buried Tunnels

High geotemperature not only affects the physical and mechanical parameters of tunnel surrounding rock, shotcrete, and supporting material structure [36], but also produces temperature stress in the lining structure due to the uneven distribution of temperature. These reduce the bearing capacity and durability of the tunnel lining structure system [37]. Therefore, it is of great significance to study the mechanical characteristics of tunnel surrounding rock, shotcrete, and support structure under the condition of high geotemperature and large buried depth to ensure the stability of the tunnel.

3.1. Thermo-Mechanical Properties of Rock Mass

Du et al. [38] conducted an experimental study on the mechanical properties of granite at different temperatures, and it is found that the mechanical properties of granite was not obviously influenced within 400 °C. Beyond that, the peak stress, Poisson ratio, and elastic modulus sharply decreased. Zhu et al. [39] compared the mechanical property differences of granite, tuff, and breccia after different high-temperature treatments, and analyzed the changes of peak stress, peak strain, and elastic modulus with temperature. Xu et al. [40,41] systematically studied the mechanical properties, structural effects, and strength characteristics of high-temperature granite. Zhu et al. [42] presented a generalized rule describing the effects of high temperature on marble by an extensive review of Chinese publications. Based on the experimental research on the thermophysical characteristics of borehole surrounding rock in granite under high temperature and pressure, the equations of elastic modulus E and Poisson ratio μ of borehole surrounding rock in the range of normal temperature ~600 °C were fitted, which are shown in Equations (1) and (2) [43].

$$E=78.366\exp \left(-0.0015T\right)$$

(1)

$$\mu =0.0699\ln T+0.0028$$

(2)

Zuo et al. [44,45] and Chen et al. [46] studied strength, fracture characteristics, permeability, and thermal damage of Beishan granite after heat treatment, and pointed out that the relationship between crack number and temperature was accorded with the Gauss curve relationship, i.e.,

$$n=n_0+\frac{A}{w\sqrt{\pi /2}}\exp \left[-2{\left(\frac{T-T_0}{w}\right)}^2\right]$$

(3)

where n is the number of cracks in hot cracking; T is the temperature; n₀, A, w and T₀ are material constants associated with granite. Wherein, T₀ = 260.91011 °C.

In addition, Zhao et al. [47,48,49] and Wan et al. [50] studied the mechanical properties, permeability and thermal fracture of “Luhui” granite under high temperature and pressure, as well as the compressive strength, tensile strength, and permeability of granite after rapid cooling. Kumari et al. [51,52,53] explored microfracture, mechanical property deterioration, and the effects of mineral particle size on the strength of Strathbogie granite under different cooling modes. Kant et al. [54] analyzed the thermophysical properties of Central Aare granite and the irreversibility of thermal fracture. Chaki et al. [55] explained the propagation, evolution, and failure characteristics of thermal cracks in the process of rock damage, established a damage model, and revealed the failure mechanism of thermal damage of rock. Liu and Xu [56,57] derived the thermal damage evolution equation and the one-dimensional thermodynamic coupled elastic-brittle damage constitutive equation (Equation (4)) based on the thermal damage of granite.

$$\sigma =E_0\epsilon \left[1-D\left(T\right)\right]\left[1-D\left(\epsilon \right)\right]$$

(4)

where E₀ is the elastic modulus at 20 °C, and D is the damage variable.

Zhang et al. [58] introduced the concept of thermo-mechanical coupling factor and proposed a one-dimensional nonlinear thermo-mechanical coupling constitutive model to reflect the stress-strain relationship of rocks at different temperatures, which is shown in Equation (5).

$$\sigma =\mu E_0\epsilon \left(1-D_T\right)\left(1-D_F\right)$$

(5)

where μ is the function of strain ε and temperature T, which characterizes the coupling effect of temperature and force, and is called thermo-mechanical coupling factor; D_T is the thermal damage variable; D_F is the damage variable caused by force.

Hu et al. [59] adopted the change of elastic modulus of rock to express the macroscopic damage caused by high temperature, and deduced the total damage expression of rock under the thermo-mechanical coupling effect, which can be expressed by Equation (6).

$$D=1-\frac{E_t}{E_0}\exp \left[-\frac{1}{m}{\left(\frac{\epsilon }{{\epsilon }_f}\right)}^m\right]$$

(6)

where D is the damage variable; E₀ and E_t are the elastic modulus of rock after treatment at room temperature and high temperature, respectively; m is the parameter characterizing the evolution of rock damage under load; ε is strain; and ε_f is the peak strain of rock.

Similarly, Xu et al. [60,61,62] combined Drucker-Prager and Hoek-Brown criteria with Weibull distribution function respectively to deduce the thermal-mechanical coupled damage model, as shown in Equations (7) and (8).

For Drucker-Prager criterion,

$${\sigma }_1=E{\epsilon }_1\left\{1-\delta +\delta \exp \left[-{\left(\frac{{\alpha }_0{I}_1+\sqrt{J_2}}{F}\right)}^m\right]\right\}+2\mu {\sigma }_3$$

(7)

where ${\sigma }_1$ is axial stress; ε₁ is axial strain; E is the elastic modulus; δ is the damage variable correction factor; m and F are Weibull parameters; α₀ is the material constants; I₁ is the first invariant of stress tensor, J₂ is the second invariant of stress deviator tensor; $\mu$ is Poisson’s ratio; ${\sigma }_3$ is confining pressure.

For Hoek-Brown criterion,

$${\sigma }_1=E{\epsilon }_1\exp \left\{-{\left[\frac{m{\sigma }_c{\sigma }_{1}E{\epsilon }_1}{F_0\left({\sigma }_1-2\mu {\sigma }_3\right)}+\frac{{\left({\sigma }_1-{\sigma }_3\right)}^2{E}^2{\epsilon }_1^2}{F_0{\left({\sigma }_1-2\mu {\sigma }_3\right)}^2}\right]}^n\right\}+2\mu {\sigma }_3$$

(8)

where ${\sigma }_1$ is axial stress; ε₁ is axial strain; E is the elastic modulus; n and F₀ are Weibull parameters; m is material constants; ${\sigma }_c$ is uniaxial compressive strength; $\mu$ is Poisson’s ratio; ${\sigma }_3$ is confining pressure.

The above research results indicate that heat conduction in rock mass will not only produce large numbers of cracks from the micro level, but also degrade the macroscopic mechanical properties of rock mass.

3.2. Thermo-Mechanical Properties of Shotcrete

In 1971, Lankard tested the compressive and tensile strength of concrete after curing in a high-temperature environment, and obtained that concrete with different water content had different strength [63]. Erguna et al. [64] conducted mechanical tests of high temperature concrete with different cement content, and discovered that there were fewer effects on cement content. Fares et al. [65] used the microscopic detection technology to vibrate concrete in different ways, and obtained that the strength of concrete would degrade with time by observing the change of internal microstructure along with high-temperature curing time. Kim et al. [66] studied the mechanical law of steel fiber reinforced concrete at high temperature by changing the size and the type of concrete, and established the corresponding relationship as presented in Equation (9), which can be used to roughly predict the strength of steel fiber reinforced concrete.

$$f_{DPT,T}=\left\{\left[\left(0.0008\frac{l}{d}-0.03\right)F-0.113\right]T+100\right\}{f}_{DPT,{15}^{\circ }C}$$

(9)

where f_DPT,T is the DPT tensile strength (MPa) of the heated SFRC at temperature T, F is the fiber volume fraction (%), and T is the maximum temperature (C). f_{DPT, 15 °C} is the DPT tensile strength (MPa) of SFRC at 15 °C. The aspect ratio of fiber is denoted by l/d.

Cui [67] simulated the 7d and 28d bond strengths of shotcrete and rock in the environment of high-temperature tunnel. It was concluded that the hydration speed was accelerated in the high-temperature environment through the three-dimensional video microscope. Li et al. [68] pointed out that the rock samples cured in high temperature environments can improve the early strength of concrete and promote the hydration reaction.

Later, it is found that adding admixtures to concrete is helpful to improve the mechanical properties of concrete. Tan and Liu [69] conducted compressive tests on concrete by changing the values of temperature, water binder ratio and admixture. Through the analysis of the results and monitoring data, it was shown that the high temperature was helpful to improve the early strength of concrete, while reducing the water binder ratio and adding admixtures can improve the late strength of concrete, and the effect of silica fume, fly ash, and slag in the admixtures was reduced successively. Mu [36] studied the influence of different admixtures on the mechanical properties and durability of concrete, which showed that proper addition of fly ash, slag powder, and water-reducing agent can effectively compensate the late compressive strength of concrete and improve its durability, but reduce the late tensile strength, elastic modulus, and carbonation resistance of concrete. Ji [70] studied the influence of different temperatures, humidity, admixtures, and curing agents on the mechanical properties of concrete through experiments, further analyzed the action mechanism of admixtures on the mechanical properties of concrete in combination with microscopic analysis methods, and proposed the design method and maintenance scheme of shotcrete. Aiming at the performances of concrete support structure in deep-buried long tunnel under high temperature, Ma et al. [71] studied the influence of temperature model and fly ash model on concrete strength by using regression analysis method, and summarized that fly ash was the main factor for the formation of concrete strength. Additionally, the high-temperature curing range of 46~95 °C can promote the growth of concrete strength.

With the development of mathematical statistics technology, scholars generally believe that the study of mechanical properties of concrete should not consider the influencing factors of temperature only, but also take other relevant factors into account for diversified analysis. Culfik [72] carried out relevant tests using the two-factor influence method and found that appropriate reduction of the water-binder ratio of concrete and the use of appropriate amount of silica fume could improve the internal bonding of concrete in high-temperature environments from the micro-level observation. On this basis, Mousa [73] conducted further research on silica fume and high-temperature concrete, and concluded that replacing 10~20% cement with silica fume and 3~6% aggregate with rubber could improve the high-temperature performance of cement concrete. Yoon et al. [74] observed the changes of aggregate volume in concrete under high temperature through the principle of thermal phase formation, and found that the different expansion coefficients of cement and coarse aggregate resulted in great expansion stress, causing the reduction of concrete strength under high temperature.

Kizilkanat et al. [75] studied the influence of thermal conductivity and moisture resistance factors of different types of concrete in high temperature, and deduced the second-order simplified equation with high correlation (Equation (10)).

$$Y=a\times T^2+b\times T+c$$

(10)

where Y indicates the thermal conductivity or moisture resistance factor; T indicates the exposed temperature; a, b, and c indicate constants.

3.3. Rheological Mechanism and Control Technology of Surrounding Rocks

Considering thermal behavior, Rankoth et al. [76] proposed a detailed scheme for tunnel by simulation method, which is a powerful tool to execute relevant research in the field of geotechnical engineering [77,78,79]. Figure 1 is the completed model with the heat transfer boundary conditions of a half tunnel, and a detailed cross-section is presented in Figure 2. Specifically, the length of the modeled second lining block is 12.2 m, and the thickness is 0.3 m. The thicknesses of the invert, rock layer, and shotcrete are 0.45 m, 2 m, and 0.15 m, respectively. The length of the previously concreted lining is 6.1 m.

Kumar and Singh [80] used elastic theory to analyze the thermal stress at the openings of lined and unlined cylindrical and spherical underground caverns, and pointed out that the support pressure and lining stress only depend on the temperature distribution inside the lining. Zhao [81] obtained a rheological model reflecting the rheological properties of gneiss by laboratory triaxial compression rheological tests at different temperatures, and analyzed the relationship between the parameters of the rheological model and temperature. Through the triaxial tests of granite under the conditions of high temperature and high pressure, Xi et al. [82] and Zhang et al. [83] found that there were obvious critical values of stress and temperature in the rheological process of granite, which was about 300~400 °C. If the threshold is exceeded, the rock shows obvious accelerated creep characteristics.

The surrounding rock often exhibits creep characteristics [84,85], it has influence on supporting structure [86], scholars often use theoretical and numerical simulation methods to carry out research [87,88]. Zhang [89] used numerical simulation method to study the high-geotemperature problems existing in the Lari railway tunnel. The size and displacement of the plastic zone, as well as the stress of support structure, around the tunnel at different stages of construction were analyzed, and the critical temperature at which the initial support fails, the development law of the secondary lining and the critical temperature of failure were determined. Tang [90] obtained the calculation formula of mechanical properties of concrete materials for primary support and secondary lining under variable temperature curing conditions through laboratory tests. Besides, the stress characteristics of support structure under different initial temperature of surrounding rock were obtained through thermal-mechanical coupling calculation, and the reasonable support structure for high-temperature tunnel was proposed. Against the background of the tunnel in the high geothermal and deep buried environment, Wang et al. [91] discussed the influence of the thermal insulation layer on the tunnel support structure by combining indoor tests with numerical analysis, and found that the thermal insulation layer has a significant role in optimizing the stress of the secondary lining. Sun et al. [92] studied the mechanical characteristics of reinforced concrete secondary lining of Grade IV and Grade V surrounding rock under different ground temperatures by means of numerical simulation with finite element software. The results indicated that with the increase of geotemperature, the safety factor of the secondary lining shows a downward trend, and it decreases significantly when the local temperature rises from 30 to 50 °C.

4. Research of Thermal Hazard Classification for Large-Buried Tunnels

The correct and reasonable thermal hazard classification is an important criterion to predict the harm caused by high temperature environment to the physical health of personnel on the basis of fully mastering the tunnel heat source environment, and it is the premise for stable construction and strong guarantee of construction quality under the high temperature environment.

Unfortunately, the current research on the thermal environment tunnel is still in the preliminary stage. For the classification of thermal hazards, different indexes can be used to evaluate the meteorological conditions of tunnel operation environment by referring to the mine regulations under the same conditions. The international thermal environment assessment indicators mainly include the following:

(1)

Wet Bulb Globe Temperature (WBGT). WBGT was proposed by Yaglon and Mainard in 1957. It is an empirical index indicating the thermal intensity of human exposure to the production environment. WBGT adopts three parameters: natural wet bulb temperature (t_nw), 150 mm diameter black bulb temperature (t_s reflects radiant heat) and dry bulb temperature (t_a), which can be calculated by the following:

For indoor operation: WBGT = 0.7 t_nw + 0.3 t_s

For outdoor operation: WBGT = 0.7 t_nw + 0.2 t_s + 0.1 t_a

(2)

Required Sweat Rate index. It evaluates the thermal environment by calculating and analyzing the necessary sweating rate of the human body.

(3)

Physiological Measurements. It is a method to evaluate the impact of core temperature, skin temperature, heart rate, mass loss, and other physiological indicators on the human body.

(4)

Effective Temperature (ET index). It was put forward by Yaglou and Houghten in 1923, and is the thermal sensation index generated by human body under the combined effect of air temperature, relative humidity, and wind speed [93]. In 1946, Bedford [94] proposed to take radiation into consideration to revise ET index, i.e., CET index.

(5)

Predicted Four-hour Sweat Rate (P₄SR index). It is a method based on the experiment of sweating volume changes caused by young men wearing single clothes under different meteorological conditions and labor intensity, which can be evaluated by predicting the 4 h sweating rate.

(6)

Heat Stress Index (HSI Index). It takes air temperature, humidity, radiant heat, wind speed and labor intensity into account, and regards the physiological reaction generated by human functions as the basis for calculation. On this basis, a series of new indicators were proposed, among which a relatively perfect index is the Effective Heat Strain Index (EHSI). The calculation is mainly based on the heat exchange equation of radiation, convection, and evaporation, combined with the physiological reactions, such as metabolism and sweating of human functions.

Different evaluation indicators have their own characteristics and limitations. Although the intensity of thermal environment and its impact on human physiological response can be accurately obtained from these indicators, it still needs to be supplemented. For example, WBGT needs to revise the value of human body in different clothing or labor protection clothing when working in the thermal environment, and Physiological Measurements index needs more accurate results of different physiological reactions of human body. Therefore, combined application of different indicators can make more accurate evaluation of thermal environment, especially extreme thermal environment, and effectively prevent possible thermal hazards [95].

With regard to rules and regulations, TB 10204-2002 Code for Construction of Railway Tunnels, JTGF 60-2009 Technical Code for Construction of Highway Tunnels, and SL 303-2004 Code for Design of Construction Organization of Water Resources and Hydropower Projects stipulate that the temperature in the tunnel shall not exceed 28 °C. According to Several Provisions on Geotemperature Measurement for Geological Exploration of Coal Resources in China [96], the rock temperature range from 31 °C to 37 °C is the first-grade thermal hazard, and the second level thermal hazard area is the rock temperature above 37 °C. As required, cooling measures must be taken in the secondary heat hazard area.

The WBGT index in ISO7243 is adopted in the newly revised GB/T 4200-2008 Classification of High Temperature Operation in China [95] as the management standard of labor safety and health classification to evaluate and classify the thermal intensity and grade of high temperature operation environment, which is shown in

Table 2. Classification of high-temperature operation.

Contact Time/min	WBGT Index/°C
	25~26	27~28	29~30	31~32	33~34	35~36	37~38	39~40	41~42	≥43
≤120	Ⅰ	Ⅰ	Ⅰ	Ⅰ	ⅠⅠ	ⅠⅠ	ⅠⅠ	ⅠⅠⅠ	ⅠⅠⅠ	ⅠⅠⅠ
≥121	Ⅰ	Ⅰ	ⅠⅠ	ⅠⅠ	ⅠⅠⅠ	ⅠⅠⅠ	ⅠV	ⅠV	-	-
≥241	ⅠⅠ	ⅠⅠ	ⅠⅠⅠ	ⅠⅠⅠ	ⅠV	ⅠV	-	-	-	-
≥361	ⅠⅠⅠ	ⅠⅠⅠ	ⅠV	ⅠV	-	-	-	-	-	-

.

Allowable duration of high-temperature operation is shown in

Table 3. Allowable duration threshold of high-temperature operation.

Working Temperature/°C	Light Labor/min	Moderate Labor/min	Heavy Labor/min
30~32	80	70	60
>32	70	60	50
>34	60	50	40
>36	50	40	30
>38	40	30	20
>40	30	20	15
>42~44	20	10	10

.

Thermal Environment-Evaluation of Thermal Load of Operators Based on WBGT Index (GB/T 1998) [97] divides the thermal environment into four levels: good, medium, poor, and very poor according to the change of WBGT index, which is shown in

Table 4. WBGT Index evaluation standard.

Average Energy Metabolic Rate Grade	WBGT Index/min
	Good	Medium	Poor	Very Poor
0	≤33	≤34	≤35	>36
1	≤30	≤31	≤32	>32
2	≤28	≤29	≤30	>30
3	≤26	≤27	≤28	>28
4	≤25	≤26	≤27	>27

. This standard is applicable to the evaluation of the average heat load of 8 h working days, not to the heat load of less than 1 h.

On the other hand, some scholars combined the practical environmental factors of tunnels to classify the thermal hazard from other perspectives; for example, the survey data of Gaoligong Mountain Tunnel, Hou et al. [98] considered the five influencing factors of lithology, fault, ground temperature, main water storage structure and main rivers, and assessed the thermal hazard risk in the tunnel area through GIS spatial analysis, which divided four areas of heat hazard extremely dangerous area, heat hazard relatively dangerous area, heat hazard dangerous area, and heat hazard free area. Chen and Chen [99] comprehensively evaluated the thermal hazard level by taking into account the internal temperature of surrounding rock, operation time and intensity, adaptability of operators, and divided the thermal hazard into four grades. In combination with the special climatic conditions of the Lari high-temperature tunnel, Xie and Yu [100] divided the geothermal disaster according to the construction and design. This classification method can consider factors such as operation intensity and operation time. Ren et al. [94] established an evaluation system for underground high-temperature workplaces by analyzing the human heat exchange process and influencing factors of workers, and conducted qualitative and quantitative screening and analysis of relevant indicators. Cao et al. [101] discussed the evaluation of thermal environment indicators, and suggested to master the natural meteorological conditions in the local high-temperature season and temperature difference to analyze the factors that cause high temperatures, so as to provide targeted guidance for heatstroke prevention and cooling and effectively protect the life, health, and safety of workers.

5. Key Technologies of Geothermal Disaster Prevention and Control in Large-Buried Tunnels

5.1. Temperature Reduction Technologies

The main temperature reduction methods used in high-temperature tunnels are: (1) Ventilation; (2) Watering, cold water, spray, and ice; (3) Refrigeration and air conditioner and mobile refrigeration station. These kinds of measures can basically meet the requirements of general high-temperature tunnels when they are used in combination. This paper mainly introduces the research progress of two cooling methods of ventilation and refrigeration, as well as their applications in high-temperature tunnels.

Zhou et al. [102] stated that the temperature and speed of wind have a significant impact on the tunnel temperature distribution. Liu [103] established a numerical model of 30 m construction tunnel to simulate sidewall forced ventilation, mixed ventilation and vault forced ventilation, respectively. By adjusting parameters such as air supply speeds and duct positions, the influences of ventilation methods and parameters on the cooling effect were understood. Long [104] compared the influences on temperature field and cooling effect under different wind supply temperature, wind supply speed, wind duct diameter, and different roadway parameters. Lv [105] simulated the 1300 m long-high-temperature construction section of Sangzhuling Tunnel to observe the temperature field at different ventilation speeds. When the ventilation speed reaches 35 m/s, the cooling rate can reach 43.3%, and the temperature difference by ventilation can reach 22 °C, which is shown in

Table 5. Temperature comparison before and after ventilation.

Ventilation Speed (m/s)	Temperature (°C)
	Before Ventilation	Ventilation after 300 min
20	51	36.1
25	51	32.5
30	51	30.3
35	51	28.9

.

Yang [106] set different ventilation speeds for simulation to analyze the cooling effect of ventilation speed on tunnel face when the wall temperature is 54 °C. Zhang et al. [107] analyzed the heat exchange law in the tunnel under ventilation, and established a prediction model for the tunnel face wind temperature, which was proved accurate by the comparative analysis between the prediction and the measurement of Qinling Tunnel. Underpinned by the Gaoligong Mountain High Temperature Tunnel, Zeng et al. [108] studied the ventilation by using the tunnel convection heat conduction model, where the ventilation effect of the shaft was considered, and the influence of different ventilation frequency and ventilation speed on the ambient temperature in the tunnel were analyzed, providing a reasonable cooling scheme during the tunnel operation period.

In addition to ventilation, refrigeration can also achieve the purpose of tunnel cooling. As presented in literature [109], the research progress of cooling technology abroad is showing as follows:

The first mine air conditioning system in the world was established in 1920 at the Morau Yolihejin Mine in Brazil, where centralized refrigeration stations were built on the surface. As early as 1923, Pendleton Coal Mine in England used a refrigerator to cool the air temperature of the working face. A refrigerating machine on the surface of the Radlod coal mine was installed in Germany in 1924. The MorroVelno mine in Brazil and the Robinson Well in South Africa were cooled in the 1930s by centrally cooling the air flow into the shaft. Large air flow cooling equipment was settled under the Loeberg mine in 1953. In the 1960s, centrally air-conditioning large mines were employed in South Africa. After a decade, refrigeration cooling is gradually popular in the Soviet Union, Japan, and other countries. South Africa firstly used ice as a carrier cooler in November 1985, and the cooling capacity was up to 628 MW. In 1989, an air conditioning system underpinned by compressed air refrigeration was built at a gold mine in South Africa. In the same year, the vortex tube air refrigeration device was developed in Poland, which was tested in the coal mine heading face and achieved certain air-conditioning effect.

As a large mining country, the development of refrigeration and cooling system in China is relatively late, and the main experience is shown in

Table 6. The development experience of refrigeration and cooling system in China.

Period	Development
1970s	Pressure air ejector and vortex control cold device were developed [109].
1975	Huainan Jiulonggang Mine has designed the first mine local cooling system in China [110].
1987	Shandong Xinfen Mining Bureau designed the first underground centralized cooling system in China.
1991	Suncun Mine has established a centralized refrigeration system on the ground with three refrigeration units, but no pressure heat exchangers. The ground system has not been put into operation [111].
1993	The scientific research institute of Pingdingshan Mining Bureau and the former No. 609 Research Institute of China Aviation Industry Corporation jointly developed KKL101 fluorine free air refrigerator for mining [109].
1995	Shandong Xinfen Mining Bureau has designed the first mine ground centralized cooling system in China, with a design cooling capacity of 7400 kW, which is the largest mine cooling system in Asia.
2002	Xinhan Mining Group inspected the cooling system of mechanical ice making technology in South Africa, and then set the cooling system of cold and low temperature radiation mine for centralized ice making to deliver ice underground.
2005	The excellent performance of mine explosion-proof screw chiller and control equipment series cooling equipment have been successfully developed [112].
2011	A mine heat dissipation recovery cooling system is invented, including closed cooling tower, heat energy recovery device, heat energy recovery device circulating water pump, pressure exchanger, pressure exchanger circulating water pump, refrigeration cooling device, fan, and underground cooling unit circulating water pump.
2012	A large temperature difference ethylene glycol air conditioning device for the mine was invented.

.

Beyond that, Zhang et al. [113] proposed to use high pressure air as the refrigerant for cooling by isentropic expansion of turbine expander. Jin [114] put forward the idea of using constant temperature water source for mine cooling, and the feasibility analysis and calculation are carried out theoretically. He and Xu [115] first proposed a new cooling mode for deep wells using mine water inflow as the cold source, and developed a complete set of technologies and equipment for the HEMS cooling system, which is shown in Figure 3. The cooling effect is 4 to 6 °C of temperature reduction. Yang et al. [116] developed a new technology for underground cooling, which combines artificial cooling water method with ice making technology. Wei and Hu [117] used flue gas waste heat after gas power generation as the power for mine cooling and refrigeration, and firstly proposed the model and theory of using thermo-electric-ethylene glycol cryogenic refrigeration to solve the problem of underground high temperature. Dong et al. [118] proposed a mine air conditioner for latent heat transport of ice slurry.

Zhang and Hu [119] divided the air conditioning system technology into three categories according to the location of the refrigeration station, which were then compared, as shown in

Table 7. Comparison between air conditioning system technology.

Location of Refrigeration Station	Advantage	Disadvantage
Outside	Convenient installation, management, and operation; Safe and reliable; Convenient heat removal; Without hole digging in big section in mechanical and electrical copper chamber; Ground natural cold source in winter; Facilitate adjustment of cold energy.	High pressure water processing difficulties; Long cooling pipe and large loss; Large diameter pipe required; Pipeline corrosion by salty water needed for refrigerating medium Complex air conditioning system
Inside	Short cooling pipe and less loss; No high-pressure water system; Convenient heat removal Simple refrigeration system and convenient adjustment of cold energy.	Large section mechanical and electrical chamber needed; Special request for refrigeration equipment; Inconvenient equipment installation, management and operation; Poor safety.
Both outside and inside	Return water temperature rise of primary refrigerant and less loss; Removal of condensation heat by primary refrigerant; Reduction of primary refrigerant circulation.	Complex system; Scattered refrigeration equipment and difficult for manage

.

Yan et al. [120] designed a tunnel local cooling system using air-cooled chillers, which can transfer the heat in the construction area to the non-construction area through cold water pipes. Through comparative calculation, the cooling power of the local cooling scheme is far greater than that of the direct ice delivery scheme, and the cooling efficiency and effects are better.

In order to improve the stability and accuracy of the optimized cooling effect, experts also began to pay attention to the establishment of the mathematical model for the cooling system. Braun [121] optimized the air conditioning system on the basis of single components and the overall system, and proposed a parameter identification method, namely “recursive least squares” method. Olson [122] adopted an integer nonlinear method and SQP (sequential quantitative programming) algorithm to optimize the chiller and cooling tower models. Chan and Yu [123] obtained the distribution of cooling capacity in the whole year by TRNSYS, and reasonably combine the chillers so that the combination of rated cooling capacity was close to the peak distribution of cooling capacity. Crowther and Furlong [124] combined chillers and cooling towers to save more energy, and analyzed the total energy consumption change of chillers and cooling towers under three different modes: set cooling water temperature, fixed approximation, and frequency conversion control. Lee et al. [125] also proposed the energy consumption model of the chiller and the cooling tower. Jin et al. [126] gave a mathematical model of the cooling tower based on Merkel theory and efficiency NTU method. Only three parameters need to be identified through measured data, and parameter identification is relatively simple and does not require iterative operation. This model can predict the operation of the cooling tower at any time, which is very helpful for the study of the energy conservation, but it does not consider the operation of other equipment in the system. Lu et al. [127,128] analyzed each energy consuming component in the cooling water system, and established a load related energy consumption mathematical model for each energy consuming component. The parameters in the model were regressed using the offline equation. Ma and Wang [129] proposed the monitoring and optimal control of the air conditioning system, and established a simplified model to predict the response of the system energy consumption under the change of control settings and operating conditions. The model was optimized using genetic algorithm, and the results show that the optimized system can save 2.55% energy consumption per day. Yu [130] listed the mathematical relationship between the system energy consumption and the factors affecting the energy consumption of air-conditioning system, and calculated the optimal value of each parameter. However, the changes of the system during the operation of chilled water flow and cooling water flow are not considered, and the regression results are not universal. Xu [131] established a mathematical model for the energy consumption of the air conditioning system, and optimized the operating parameters under a certain load, so that the air conditioning system can always operate in the best state. Liu et al. [132] proposed to take the lowest energy consumption of the air conditioning system as the objective function under the condition of meeting the comfort of the room and the safe operation of the equipment. According to the operating characteristics of various equipment of the air conditioning system under variable working conditions, the optimal chilled water flow and chilled water outlet temperatures were obtained.

5.2. Thermal Insulation Technology of Tunnel

The tunnel geothermal disaster not only has a negative impact on the construction, but also will lead to high costs in the operation if no effective measures are taken during the construction process. Therefore, the selection of construction materials and the design of tunnel structure will play a decisive role in the future operation. This is also an important part of the “active cooling” method in the construction of high-temperature tunnels.

5.2.1. Thermal Insulation Structure

Facing the serious threat of tunnel geothermal disaster, the heat exchange capacity of gas and surrounding rock can be reduced by laying thermal insulation structures, so as to reduce the construction environment temperature.

Shao et al. [133] obtained the theoretical solution of thermos-elasticity including temperature field, displacement field and stress field by using the method of dimensionless and differential equation series solution, and providing a theoretical basis for tunnel thermal insulation. Shao [134] deeply discussed the thermos-elasticity of two-dimensional steady cylinder. Zhou et al. [135] analyzed the influence of seasonal wind temperature, different temperature, and laying thermal insulation layer on the heat transfer of high temperature tunnels. Li et al. [136] developed a full-scale temperature simulation test instrument, and carried out simulation tests under the conditions of with or without thermal insulation layer. On this basis, the change law of the radial temperature field of the tunnel surrounding rock was analyzed, providing a design basis for the tunnel insulation. Li [137] used ANSYS to compare the temperature field distribution of the tunnel with or without the thermal insulation layer, and found that the thermal insulation layer can effectively inhibit the heat transfer from the surrounding rock to the tunnel. Xia et al. [138] calculated the insulation layer thickness under different working conditions by combining theoretical analysis with numerical simulation of temperature field. Based on the heat conduction theory of multi-layer cylinder wall and combined with the Stephen formula, the implicit expression of the theoretical formula for the thickness of double-layer insulation layer was also derived, which is shown in Equation (11).

$$\begin{array}{l}\frac{1}{\left(r_1+h_1+h_2+h_3\right)\alpha }+\frac{1}{{\lambda }_3}\ln \frac{r_1+h_1+h_2+h_3+d_u}{r_1+h_1+h_2+h_3}=\\ \frac{1}{r_1\alpha }+\frac{1}{{\lambda }_1}\ln \frac{r_1+h_3}{r_1}+\frac{1}{{\lambda }_2}\ln \frac{r_1+h_3+h_2}{r_1+h_3}+\frac{1}{{\lambda }_1}\ln \frac{r_1+h_1+h_2+h_3}{r_1+h_2+h_3}\end{array}$$

(11)

where λ₁ is the thermal conductivity of concrete; λ₂ is the thermal conductivity of the thermal insulation layer; λ₃ is the thermal conductivity of surrounding rock; h₁ is the initial lining thickness; h₂ is the thickness of the thermal insulation layer; h₃ is the thickness of the second lining; α is the convection heat transfer coefficient between the air and the inside of the secondary lining; r₁ is the equivalent radius of the tunnel; d_u is the maximum melting depth of permafrost.

Liu et al. [4] conducted numerical simulation on the Sangzhuling high-temperature tunnel, and pointed out that the optimal insulation layer thickness from the perspective of preventing lining cracking was 0.06 m. Bai et al. [139] calculated the corresponding cold energy supplement amount of styrene and polyurethane insulation materials for high-temperature tunnels under different insulation layer thicknesses, and found that the cooling effect by increasing the thickness of the insulation layer was limited. Wu and Wang [140] compared the advantages and disadvantages of different insulation layer laying methods, as shown in

Table 8. The comparison of different insulation layers.

	Sandwich Type	Attachment Type	Double Type	Separation Type
Definition	Laying thermal insulation layer between primary support and secondary lining	Laying thermal insulation layer on the inner surface of the secondary lining	Laying thermal insulation layers between primary support and secondary lining and on the inner surface of the secondary lining	Setting air layer between the inner surface of the secondary lining and the thermal insulation layer
Advantage	No requirements for flammability of thermal insulation materials; No additional fireproof layer and protective layer requirements; Blocking the heat and protecting the secondary lining.	Easy to repair and replace the layer; Low requirements for the compressive strength of materials.	Better heat insulation effect; More suitable for tunnels with high temperature.	Simple installation process and high construction efficiency; Convenient replacement and maintenance; Relatively low cost; Simple structure.
Disadvantage	Certain requirements for the compressive strength of materials; Difficult to repair and replace the layer; Easier to absorb water and become wet.	High flame retardancy and no toxic gas requirements; Fireproof layer and protective layer requirements; Low safety of secondary lining; Vulnerable to damage; Affecting the health examination of the secondary lining.	Complex construction process; Ineconomic due to high cost; Difficult to maintain and repair; High requirements for compressive resistance and flame retardancy.	Strict installation accuracy; High requirements for surrounding rock grade; Easy to damage the integrity of waterproof layer and thermal insulation layer

, then the fuzzy analysis method was used to select the insulation layer materials. It was recommended that the thermal insulation material should be silicate composite thermal insulation felt or dry process aluminum silicate fiber board, and the insulation layer should be laid in sandwich type.

5.2.2. Thermal Insulation Materials

Chang et al. [141] found that temperature reduction measures, such as ventilation and refrigeration, cost complex processes, consume large power, and are difficult to maintain, while materials with low thermal conductivity can significantly reduce heat conduction and achieve a cooling effect. Xie [142] invented a new type of thermal insulation material-aluminum silicate fiber, which can be used as thermal insulation material for tunnels. As shown in

Table 9. The performance of some thermal insulation materials.

Material	Performance Index
	Density kg/m3	Thermal Conductivity W/(m·K)	Water Absorption %	Temperature Range °C	Compressive Strength MPa	Combustibility
Rigid polyurethane foam	60.4	0.027	3.4	−60~120	≥0.5	B2 flame retardant (GB 8624-1997)
Expanded polystyrene foam	40	0.041	≤4	−80~75	≥0.1	Lower than 75 °C
Extruded polystyrene foam	42	0.030	≤1.5	−85~75	≥0.25	-
Phenolic foam	45	0.038	0.03	−55~90	0.03	B1 Flammability (GB 8624-1997)
Rigid polyvinyl chloride foam	60	0.036	Nonabsor-bent	−30~400	0.15	Incombustible
Dry process aluminum silicate fiber material	188	0.037	Low water absorption	≤1000	Capable of compaction	A incombustible materials (GB8624-1997)
Glass wool series	24~96	0.03~0.04	-	−120~400	-	A incombustibility (GB/T13350-92)

, Liu [143] gave the performance indexes of some thermal insulation materials. It was pointed out that hard polyurethane foam material, dry process aluminum silicate fiber material, and extruded polystyrene foam material were all great heat insulation materials by using the comprehensive evaluation method of analyzing multiple factors in fuzzy mathematics.

Wu [144] found that polyurethane thermal insulation board was the most suitable thermal insulation material, and designed a series of experiments to verify its thermal insulation effect. Yang [145] compared the thermal insulation effects of thermal insulation layers with different thermal conductivity and concluded that the lower the thermal conductivity, the better the thermal insulation. Jiang et al. [146] tested the mechanical properties and thermal conductivity of the vitrified bead thermal insulation mortar, and used ANSYS to simulate the stress of the vitrified bead thermal insulation mortar used as the thermal insulation layer in the tunnel. Zhang and Ou [147] proposed to use high-performance heat insulation lightweight aggregate concrete to replace traditional concrete and heat insulation materials, so that new concrete can be used as both structural layer and heat insulation layer. The use requirements under various working conditions can be adapted by changing the characteristics of gel and aggregate. Wu et al. [148] analyzed the thermal insulation effect of silicate thermal insulation composite felt and rigid polyurethane board, and found that both had the best thermal insulation effect at the inverted arch. Cui et al. [149] studied the thermal insulation effect of the rubber plate damping layer and found that the thermal insulation effect is the best at the right arch shoulder of the tunnel.

5.3. Personal Protection Technology

In order to solve the damage caused by tunnel geothermal disaster to operators, it is economically feasible to study the personal protection technology and equipment.

Ma and Zhang [150] summarized the research status of thermal protection of textiles at home and abroad, mainly introducing the fiber materials, fabric thermal protection technology, and product development that can be used for thermal protection at home and abroad, and the evaluation methods and standards of fabric thermal protection. Taking the construction characteristics, functional requirements and costs of shield tunnels into account, Yuan et al. [151] designed a personal protective equipment integrating cooling, noise prevention, communication, and positioning functions by using semiconductor refrigeration, active noise reduction, wireless positioning communication, and other technologies. Liu [152] took the protective cooling clothing based on semiconductor refrigeration technology as an example to illustrate that personal protection technology and equipment should be studied in terms of the combination of microclimate inside clothing and wet heat exchange, refrigeration devices and clothing, as shown in Figure 4.

Thermal protective clothing can greatly protect workers in high temperature environment, since it is generally composed of shell layer, waterproof layer, and thermal insulation layer [153,154]. In order to design thermal protective clothing with better thermal insulation effect in a shorter period, more and more researchers are studying the heat transfer process of thermal protective clothing. Gibson first proposed a single-layer porous medium model to study the heat dissipation performance of thermal protective clothing [155]. Torvi put forward a heat transfer model of thermal protective clothing considering thermal radiation based on the physical properties of materials, the thickness of gaps between material layers and between material layers and air layers [156]. On this basis, Mell et al. [157] further proposed a heat transfer model for multi-layer textiles, and Chitrphiromsri and Kuznetsov [158] built a coupled model of thermal protective clothing for porous media. Considering the influence of the air layer on the heat resistance, Son et al. [159] established a heat transfer model for multi-layer textiles. Ghazy and Bergstrom [160] established sandwich structure, and carried out more systematic discussion on each link of thermal protective clothing-air layer-skin system. Song et al. [161] introduced the thermal manikin system to establish the heat transfer model of the thermal protective clothing and human skin system in the high-temperature environment, making the research more practical.

Phase change protective clothing is a kind of passive cooling personal protection technology. Generally, phase change materials are placed in the clothing to absorb heat from the human body through the change of material phase state [162]. Because of its advantages, such as low cost, simple structure, recyclable use, convenient wearing and good refrigeration effect, phase change cooling clothing has become a hot research topic at home and abroad. Foreign countries began to develop liquid cooling clothing mainly in the 1950s, and the first liquid cooling clothing was designed and developed by Burton and Collier for the Royal Air Force in 1962 [163]. In the 1970s, the United States used CO₂ as the phase change material to achieve body insulation by gas-solid phase, and then researchers tried to apply CaCl₂·6H₂O, SrCl₂·6H₂O, and polyethylene glycol to clothing, which was demonstrated to have a certain temperature regulating effect [164]. Chang et al. [165] designed a kind of thermal protective clothing in detachable arm guard form by using Na₂SO₄·10H₂O to absorb heat during high temperature. Moreover, there are also other cooling vests with mixed liquid medium or dry ice as phase change materials [166].

During the research of phase change protective clothing, Chou et al. [167] filled the cooling bag with different phase change materials for testing, and the internal and external temperature of the protective clothing, as well as the temperature of the human skin surface, were measured in the high temperature environment. The results show that the cooling effect is good, and wearing thermal protective clothing can significantly reduce the surface temperature of human skin. Bennett et al. [168] studied the influence of the number of cooling bags on the cooling effect and human perception. The results showed that the more the number of cooling bags in the protective clothing, the better the cooling effect, but the quality will also increase, giving a sense of weight, which is not conducive to the normal activities of personnel. To improve the comprehensive performance of protective clothing and balance the relationship between functional requirements and comfort requirements, thermal protective clothing combining low density and low thermal conductivity aeroneneneba gel materials has appeared in recent years [169]. It can effectively resist thermal radiation in a high-temperature environment, such as an explosion or sudden fire, reduce heat damage caused by physical, chemical, and other external factors to human body, lower stored heat of human body, and relieve heat stress symptoms, which has broad prospects for development.

6. Conclusions

In response to a series of problems in tunnel construction and operation caused by high geotemperature, this paper summarizes and displays the existing research results from the aspects of geothermal disaster development, geothermal damage mechanism, and the key prevention technologies, which can provide certain references for subsequent related research. The main research conclusions are as follows:

(1)

From the macroscopical perspective, both the physical and mechanical properties of rock mass show a decreasing trend after treated by high temperature. The anisotropic thermal expansion is the main cause of thermal damage to rock mass. High temperature degrades the bond strength of concrete, which can be improved by adding admixtures. Through the numerical simulation method, the rheological mechanism of surrounding rocks in high-geotemperature tunnels can be well reflected.

(2)

The current thermal environment classification of tunnels mainly refers to mine regulations under the same conditions. Different evaluation indexes have their own characteristics and limitations. Although the intensity of thermal environment and influence on the physiological reaction of human body can be obtained from these indicators, they still need to be improved to effectively prevent the possible thermal disasters.

(3)

Each geothermal disaster prevention and control method in large-buried tunnels has its unique, applicable conditions. In practical application, the practical situation of tunnels must be investigated, the characteristics and favorable conditions should be fully utilized to find out the appropriate and effective method. In the future, the thermal damage prevention technology of large-buried tunnels will be considered from the perspective of system engineering, economy, and new technology to achieve energy saving, electricity saving, and green environmental protection.