Construction technology for deep tunnels crossing superhigh-temperature fault zones with high water surges

Abstract

The harsh environment in tunnels with high geothermal temperatures and humidity can adversely impact machinery, personnel, and construction. The main causes of specific problems are the unknown mechanisms of local geothermal formation, inappropriate temperature control measures, and insufficient systematic safeguards. In this study, three work sections relating to a high geothermal tunnel are: the tunnel face, middle-of-tunnel section, and outside-of-tunnel section. A cooling strategy is proposed to offer technical support in achieving comprehensive cooling, overall as well as for each of the sections. First, a comprehensive geological survey explores the mechanism and exact location of the heat source. Secondly, grouting and centralized drainage measures are used to control the heat release of hot water. Enhanced ventilation, ice chillers and other applicable measures are used to control the ambient temperature. Finally, a monitoring and early warning system is established to prevent accidents. This cooling strategy has been applied in the field with good results.

Graphical abstract

Keywords

high geothermal tunnels / cooling strategy / comprehensive cooling measures / safety and security

1 Introduction

The geothermal temperature is influenced by geological structures, magmatic activity, stratigraphic lithology, hydrogeological conditions, etc. A high-geothermal-concentration zone is typically formed near fracture structures, where transport channels that allow geothermal water to rise, in some cases producing heat, become the main source of high temperature groundwater in tunnels [1–3]. Excessive rock temperatures can also adversely affect the health of construction personnel and reduce the mechanical efficiency [4,5]. Consequently, construction difficulty increases, potentially leading to significant delays in the construction schedule.

Before the construction of a high geothermal tunnel, identifying the heat source is necessary. Techniques such as thermal infrared remote sensing can be used to study surface temperature change patterns [6,7]. Overhead detection techniques can be employed to determine the temperature of the surrounding rock and water prior to tunnel workings [8,9]. Following the identification of the heat source, various measures can be implemented to reduce the ambient temperature during construction. To effectively mitigate the problems related to high-temperature environments, Su et al. [10] established a cooling plan, spray, ice, based on ventilation, and they provided support in Qiqihar diversion tunnel, where rock temperature was 110 °C and water temperature was 62 °C. Wen et al. [11] proposed a theoretical method for predicting the tunnel rock temperature by analyzing the internal variations in the surrounding rock of the Nige tunnel, where rock temperature and water temperature were 88.8 and 63.5 °C, respectively. In Anfang tunnel, a grouting method was used to block high temperature groundwater. The water gushing rate was 5–10 L/min per meter after grouting, which significantly reduced the heating of the tunnel by water. In addition, aided by a ventilation volume of 3000 m³/min, the air temperature was reduced from 72 to 30 °C [12]. Various researchers have proposed methods for calculating heat release in high geothermal tunnels. Refs. [13–15] characterized the heat transfer mechanism of a high geothermal tunnel using a two-dimensional finite difference method. Jain et al. [16] obtained a theoretical solution for the transient temperature field in a tunnel, using the separation-of-variables method. Krarti and kreider [17] employed the energy conservation law and proposed an analytical solution for the ambient temperature in high geothermal tunnels.

An automatic monitoring and early warning system can be established in a high geothermal tunnel to comprehensively monitor the temperature, humidity, and environment for ensuring the psychological and physiologic wellbeing of workers [18,19]. Most existing studies have only focused on partial measures for reducing the rock or groundwater temperature in tunnels. However, comprehensive cooling measures and ideal systematic security mechanisms covering the range from the tunnel face to the section outside the tunnel have not been established [20,21].

Through the research, it is found that only simple measures are taken to reduce the ambient temperature in high geothermal tunnels, and there are no complete rules and regulations to cope with all the problems. Therefore, the complete countermeasures are proposed to grasp the mechanism of heat sources and control the heat release in tunnels in a targeted way. Installation of environmental monitoring systems and establishment of a complete system to protect against accidents. This policy can solve all the problems that may occur in high geothermal tunnels.

2 Method for calculating heat release

2.1 Background

A 30 km long geothermal tunnel in South-western China has a horseshoe-shaped cross-section, with an area of 53.5 m². As the tunnel passes through a fault zone, the surrounding rock holds high temperature water at approximately 60 °C. The maximum hot water temperature reached 93 °C [20].

2.2 Calculation of heat release rate from surrounding rock and hot water

The heat sources in tunnels mainly include hot rock, high temperature groundwater, which are mainly related to geological conditions, and construction activities.

(1) Rate of heat release from rock

Heat is transferred from the surrounding rock to the lining by thermal conduction. There is a temperature difference between the outer edge of the lining and the air, and the two transfer heat by thermal convection. This leads to an increase in the ambient temperature of the tunnel and a decrease in the surrounding rock temperature closer to the lining, which ultimately leads to a thermal equilibrium. The heat transfer in the high geothermal tunnel is shown schematically in Fig.1.

The one-dimensional unsteady-state differential equation heat transfer in the rock can be expressed as follows [13,20]:

(1)\frac{\mathrm{\partial }t}{\mathrm{\partial }\tau }=\frac{\lambda }{c\rho }\left(\frac{1}{r}\frac{\mathrm{\partial }t}{\mathrm{\partial }r}+\frac{{\mathrm{\partial }}^{2}t}{\mathrm{\partial }{r}^2}\right),

where λ is the thermal conductivity of the surrounding rock (kW/(m·°C)), c is the specific heat capacity of the surrounding rock (kJ/(kg·°C)), ρ is the density of the surrounding rock (kg/m³), r is the radius of the tunnel (m), t is the temperature (°C), and τ is the time (s).

Based on the Eq. (1), the following discrete equation, applying at any point within the rock, i can be established:

(2)t_n^{i+1}=\frac{F_{\mathrm{o}}}{2}\left[\left(2+\frac{\mathrm{\Delta }r}{r_n}\right)t_{n+1}^i+\left(2-\frac{\mathrm{\Delta }r}{r_n}\right)t_{n-1}^i+\left(\frac{2}{F_{\mathrm{o}}}-4\right)t_n^i\right],

where i is the node number, \mathrm{\Delta }r is the radial distance step of the surrounding rock (m), \mathrm{\Delta }\tau is the calculated time step (s), n is the total number of radial nodes, F_o is the Fourier number, F_{\mathrm{o}}={\displaystyle \frac{a\mathrm{\Delta }\tau }{{\left(\mathrm{\Delta }r\right)}^2}}, a is the thermal diffusivity, and τ is the time.

The heat transfer equation for thermal convection between the edge of the lining and the air can be expressed as follows [22]:

(3)t_1^{i+1}=2F_{\mathrm{o}}\left[\left(1+\frac{\mathrm{\Delta }r}{2r_0}\right)t_2^i+B_i{t}_{\mathrm{f}}+\left(\frac{1}{2F_{\mathrm{o}}}-1-B_i-\frac{\mathrm{\Delta }r}{2r_0}\right)t_1^i\right],

where t_1^i is the temperature of the boundary node at moment i (°C), B_i is the Biot coefficient of the grid, with B_i=2h\cdot \tau /(\rho c\cdot r), t_f is the air temperature in the tunnel (°C), r₀ is the excavation radius (m), and h is the convective heat transfer coefficient.

The complex interface node can be expressed as follows [22]:

(4)\begin{array}{rl}& t_{n+1}^i=\\ & \frac{\mathrm{\Delta }\tau \left[{\lambda }_{n-1}\left(r_n-{\displaystyle \frac{\mathrm{\Delta }r}{2}}\right){\displaystyle \frac{t_{n-1}^i-t_n^i}{\mathrm{\Delta }r}}+{\lambda }_{n+1}\left(r_n+{\displaystyle \frac{\mathrm{\Delta }r}{2}}\right){\displaystyle \frac{t_{n+1}^i-t_n^i}{\mathrm{\Delta }r}}\right]}{({\rho }_{\mathrm{c}}{)}_n\cdot \mathrm{\Delta }r\cdot r_n}+t_n^i,\end{array}

where ρ_c is the density of concrete (kg/m³), and c is the specific heat capacity of concrete (kJ/(kg·°C)).

The average value of the temperature variation in the ith annular region can be expressed as follows [22]:

(5)\mathrm{\Delta }{t}_i=0.5\times (t_i+t_{i+1})-0.5\times (t_i^{\prime }+t_{i+1}^{\prime }),

where t_i is the temperature before ventilation at point i, t_i^{\prime } is the temperature after ventilation at point i.

Heat released by the rock can be expressed as follows [23–24]:

(6)Q_{\text{SR}}=ULh\left(t_1^i-28\right),

where Q_SR is the heat release of the surrounding rock (kW), h is the convective heat transfer coefficient (kW/(m²·°C)), L is the calculated length along the longitudinal direction of the tunnel (m), U is the circumference of the tunnel (m), and the value of 28 °C is the ambient temperature control value in guidelines [23–24].

The calculated parameters for the high geothermal tunnel are shown in Tab.1.

The results of rate of heat release calculation for different surrounding rock temperatures are shown in Fig.2. With increasing surrounding rock temperature, rate of heat release increases approximately linearly. When the surrounding rock temperature increases by 10 °C, the Q_SR increases by 140.8 kW. At a surrounding rock temperature of 60 °C, the Q_SR reaches 329.44 kW.

(2) Rate of heat release of high-temperature groundwater

When high-temperature groundwater gushes out of the tunnel face, the air is heated and humidified, which can lead to tunnel environment deterioration and affect the operations performed by construction personnel.

The rate of heat release by high-temperature groundwater Q_w can be expressed as [23]:

(7)Q_{\mathrm{w}}=h_{\mathrm{w}}A\left(T_{\mathrm{w}}-T_{\mathrm{f}}\right)+M_{\mathrm{A}}\gamma l,

where T_w is the groundwater temperature (°C), T_f is the airflow temperature (°C), h_w is the convective heat transfer coefficient of the exposed area groundwater surface (kW/(m²·°C)), A is the heat release area of the groundwater (m²), γ is the latent heat of vaporization of groundwater at T_w (2.5 kJ/kg), M_A is the evaporation per meter (kg/(m·s)), and l is the calculated length (m).

The convective heat transfer coefficient between hot water and air is calculated using the following Eq [23]:

(8)h_{\mathrm{w}}=Nu\frac{{\lambda }_{\mathrm{w}}}{l},

where h_w is the hot water temperature (°C), Nu is the Nusselt number. λ_w is the thermal conductivity of hot water (kW/(m·°C)).

Nu in Eq. (8) is calculated according to Eq. (9).

(9)Nu=0.664R_{\mathrm{e}\mathrm{x}}^{1/2}{P}_{\mathrm{r}}^{1/3},

where R_ex is the Reynolds number, P_r is the Prandtl criterion.

R_ex in Eq. (9) is calculated according to Eq. (10), and key parameters of Eqs. (8)–(10) are given in Tab.2.

(10)R_{\mathrm{e}\mathrm{x}}=\frac{ul}{v},

where u is the speed of fluid movement (m/s), v is the coefficient of dynamic viscosity (m²/s).

The rate of heat release by high temperature groundwater is directly proportional to the water temperature and quantity, as shown in Fig.3. The average increase in rate of heat release is 10.5% for every 10 m³/h increase in the water volume. The average increase in rate of heat release is 53.2% for every 10 °C increase in the water temperature.

(3) Rate of heat release due to tunnel construction activities

Heat is also generated during tunnel construction, such as by personnel activity, hydration, machine operation and blasting, which can be calculated as follows. The increase in rate of heat release during tunnel construction is the sum of the above four heat sources.

(11)P_{\text{CHR}}=P_{\mathrm{p}}+P_{\mathrm{b}}+P_{\mathrm{m}}+P_{\mathrm{c}},

(12)P_{\mathrm{p}}=\frac{q_{\mathrm{p}}{n}_{\mathrm{p}}{t}_{\mathrm{p}}}{T_{\mathrm{D}}},

(13)P_{\mathrm{b}}=\frac{q_{\mathrm{b}}{m}_{\mathrm{b}}}{T_{\mathrm{D}}},

(14)P_{\mathrm{m}}=\frac{1}{T_{\mathrm{D}}}{\sum }_{i=1}^{n_{\mathrm{m}}}0.55\times 0.8\times 860\times t_{ai}{N}_{ci},

(15)P_{\mathrm{c}}=\frac{{\alpha }_{\mathrm{c}}{w}_{\mathrm{c}}{q}_{\mathrm{c}}}{{\mathrm{T}}_{\mathrm{D}}}

where P_CHR is the daily total rate of heat release due to construction (kW), P_p is the rate of heat released by a single person (kW), q_p is the heat release of a single person (kW), n_p is the number of workers, t_p is the daily construction time (s), P_b is the amount of rate of heat release by blasting (kW), q_b denotes the heat released during the explosion of each kilogram of explosives (kJ/kg), m_b denotes the daily consumption of explosives (kg), n_m is the number of working machines, t_ai is the actual operating time, in s, of the aith machine per d, N_ci is the rated power of the cith machine (kW), w_c is the daily concrete dosage (m³/s), α_c is the cement content in concrete (kg/m³), q_c is the heat of hydration of cement at 28 d of age, and T_D is the time, 86400s.

The calculated parameters of Eqs. (12)–(15) are shown in Tab.3.

The heat release of the construction in the tunnel is obtained according to Eqs. (11)−(15), and the results of the calculations are shown in Tab.4. The rate of heat release by the tunnel construction amounted to 118.44 kW. At a rock temperature of 60 °C, the heat release rate is 329.44 kW, which is approximately equal to the heat release rate from groundwater at a temperature of 50 °C and a volumetric flow rate of 43.06 m³/s. With increasing water temperature and water volume, heat release of high-temperature groundwater dominates. Therefore, the primary approach is to control the heat released by high temperature groundwater.

3 Comprehensive treatment and cooling strategy for high geothermal tunnels

In high geothermal tunnels, high-temperature rock coexists with high temperature groundwater. This results in the ambient temperature in tunnels exceeding the value of 28 °C specified in guidelines, which affects the health of personnel, mechanical efficiency, and construction process [24]. Therefore, comprehensive approaches have been adopted to improve the environment of high geothermal tunnels.

In the tunnel studied in this work, the presence of high temperature rock and groundwater during tunnel construction led to a maximum surrounding rock temperature of 65 °C. The humidity of the tunnel face approached 99.9%, and the water temperature was 93 °C. The maximum water inflow was 232 m³/h. The monitoring data are shown in Fig.4.

Two 2 × 160 kW fans are used to ventilate and cool the tunnel, while high temperature water near the tunnel working surface is drained to reduce the heat release rate.

The test results are shown in Fig.5. Environmental analysis of the high geothermal tunnel revealed the following: (1) the hottest surrounding rock and water are located in the tunnel face; (2) the ambient temperature in the tunnels gradually stabilizes at 125 m from the tunnel face. Therefore, the ambient temperature varies along tunnel axial direction, suggesting that different measures must be implemented to reduce the ambient temperature at different locations.

In this study, the high geothermal tunnel is divided into three sections: the working section, middle-of-tunnel section, and outside-of-tunnel section. Different measures adopted to reduce the ambient temperature in the tunnel are summarized below. The schematic is shown in Fig.6.

(1) Working section

Advanced geological prediction techniques were applied to capture the temperature field and determine the position of high temperature water within a specific range ahead of the tunnel face. Regarding high temperature water gushing issues, measures such as grouting or thermal insulation drainage were adopted to reduce the heat release rate from hot water. In addition, ventilation enhancement, spray cooling, and ice cooling were adopted to reduce rate of heat release from the surrounding rocks.

(2) Middle-of-tunnel section

A thermal insulation drainage ditch was employed to reduce the rate of heat released by high temperature groundwater to the tunnel environment. Insulation was applied to reduce the thermal conductivity of the air ducts and inhibit the increase in the airflow temperature in the ducts. An air duct with an air leakage rate lower than 1% was used to ensure the air supply to the tunnel face.

(3) Outside-of-tunnel section

Tunnel face cooling can be achieved by increasing the number and power of fans.

In this paper, a cooling strategy was proposed for the three sections of the high geothermal tunnel. The details are as follows: heat sources ahead of the tunnel face were comprehensively surveyed. The rate of heat release from hot water was controlled by drainage and grouting methods. The ambient temperature in the tunnel was subsequently reduced by implementing a comprehensive range of cooling measures.

Environmental monitoring was performed in the tunnel to monitor the tunnel temperature, wind speed and humidity in real time. Fully integrated safeguards were installed to prevent security incidents.

3.1 Comprehensive geological survey

To investigate the temperature distribution in high geothermal tunnels, the ground surface and tunnel interior should be comprehensively surveyed. Various means are used to test areas of geothermal anomalies at the surface of the high geothermal tunnel. In high geothermal tunnels, various overruns are used to check the temperature field in advance of the tunnel face.

(1) The ground surface of the tunnel

The space-air-ground integrated survey method was adopted. First, based on multisource satellite remote sensing data, the surface geothermal anomaly area was initially delineated. Then, a combination of airborne and geophysical exploration methods was used to further examine the geothermal area. Finally, remote sensing data were integrated with boreholes to map the ground temperature field and locate the refined high temperature regional ranges. The space-air-ground integrated survey method is shown in Fig.7.

(2) The tunnel interior

In the high geothermal tunnel sections, there should be an improved geological prediction system implemented ahead of the tunnel face. According to the geological conditions and characteristics of high geothermal tunnels, various geophysical exploration methods have been used for long, medium, and short-distance detections of the surrounding rock temperature ahead of the tunnel face. Additionally, the distribution of unfavorable geological formations, including the integrity of the surrounding rock, water conditions, and high-temperature groundwater ahead of the tunnel face, should be investigated. In this paper, the transient electromagnetic method (TEM) and tunnel seismic prediction (TSP) method were used to characterize the geology ahead of the tunnel face. The prediction results are shown in Fig.8.

The TEM is based on the principle of electromagnetic wave reflection and can be employed to detect the location and amount of groundwater ahead of the tunnel face. As can be seen in Fig.8, the red areas represent large inversion resistivity and the absence of groundwater. The green and blue colors represent smaller inversion resistivity and the presence of groundwater. Notably, the inversion resistivity was low from 14–25 m ahead of the tunnel face. Therefore, it was predicted that bedrock fissure water and a layer of hot water occurred in this section.

The principle of the TSP method is the generation of elastic waves toward the tunnel working face and the identification of faults (damage zones), concentrated joint bands, and damage zones based on elastic wave reflection. TSP-based detection was performed at the tunnel face for advanced geological prediction purposes. The pressure and stress wave velocities of the surrounding rock decreased in a section ahead of the tunnel face, where the Poisson’s ratio changed only slightly and the dynamic Young’s modulus decreased. There were also multiple reflection interfaces in the particular area. Hence, it could be inferred that the surrounding rock strength in this section was low, and joint fissures were well developed. The rock mass of the entire tunnel face was relatively broken there [25,26].

3.2 Control of heat release

(1) Geothermal formation mechanism

To better understand the geothermal formation mechanism, 105 temperature measurements and 50 groups of water quality analyses were completed in the high geothermal tunnel area. The ground and airborne geophysical exploration distances were 55.98 and 820 km, respectively, and the thermal infrared remote sensing interpretation area was 950 km². Based on measurements, the primary heat source extends widely and encompasses localized melting within the deep-seated shell of the suturing belt and cenozoic igneous rocks in the high geothermal tunnel region. When rainwater and melted snow infiltrate into the subsurface through deep faults and crush belts, hot water is generated in the northern plateau region, at an altitude of 3500–4300 m, owing to the abundant heat in the suturing belt. When hot water is conveyed to a fracture intersection or an area with intensive fractures and decompression, it ascends to the surface, giving rise to springs near river gullies. The geothermal formation mechanism is shown in Fig.9.

A large amount of high temperature groundwater is distributed along the suture zone. Geothermal anomalies can be revealed by drilling. The maximum temperature is 93.51 °C, and the geothermal gradient generally ranges from 10 °C to 35 °C per 100 m, with a maximum local value of 42.2 °C per 100 m.

To isolate the heat source and prevent geothermal heat release, the femoral hot water collection, insulation heat pipe drainage, and grouting method was adopted. Control of the heat source is the primary measure for preventing environmental deterioration.

(2) Hot water collection

Through advanced drilled holes in the tunnel face, hot water was directly drained into the outside-of-tunnel section using a thermal insulation pipe. Additionally, drainage could reduce heat release in the high geothermal tunnel. A back-pressure valve was adopted in the tunnel face, and an antishock device was installed in the orifice pipe.

(3) Thermal insulation pipe drainage

Thermal insulation pipes (diameter 200 mm) were used for hot water discharge in the tunnel, and the pipe diameter was selected according to the water discharge. Pipes are wrapped with insulation materials, which can effectively reduce the rate of heat release of the pipes to the air.

(4) Drill grouting

When a large amount of femoral hot water is exposed through tunnel face drilling, the water outlet must be quickly sealed to reduce the impact of the heat source in the tunnel. Orifice pipes and back-pressure valves were installed at the water outlet and sealed with grouting. Photos of the on-site heat source control are shown in Fig.10.

3.3 Combination of drainage and grouting

To reduce the rate of heat release in high geothermal tunnels, the first measure entails high temperature groundwater control. To this end, the actual high temperature groundwater conditions of the tunnel face must be considered. Based on the development degree of high temperature groundwater, ground temperature, environmental temperature, engineering geology, etc., a combination of drainage and grouting methods was conducted under different conditions and at different levels. The application standards of the joint method were determined as follows.

(1) The total rate of heat released by the surrounding rock and hot water should be controlled within 1300 kW. At a surrounding rock temperature of 65 °C and a water temperature of 50 °C, the water surge should be controlled within 60 m³/h. At a water temperature of 70 °C, the water gushing volume should be controlled within 30 m³/h. Curtain grouting should be employed when the abovementioned limits are exceeded. Where the water gushing volume is less than the limit, the tunnel can be constructed normally.

(2) In the event of localized femoral water occurrence in the frontal region, directional grouting should be implemented with intermittent grouting techniques.

(3) In scenarios where a substantial water volume is present and the water area remains uncertain, the adoption of sequential dynamic intermittent grouting is recommended.

(4) In the presence of faults or concentrated joint bands ahead of the tunnel face, comprehensive curtain grouting should be implemented in situations where high temperature groundwater is encountered [27–29]. High temperature hot water control measures are shown in Appendices A–C in Supplementary materials.

3.4 Comprehensive cooling

The rate of heat released in the tunnel with high-temperature rock and groundwater exceeded 1000 kW. Increasing the number of fans and ducts will increase the average air velocity in the tunnel. Therefore, ventilation is the most effective measure for reducing the temperature in high geothermal tunnels [30,31]. The comprehensive temperature reduction measures are shown in Fig.11.

(1) Ventilation cooling

In ventilation cooling, air is blown into the tunnel, absorbing heat to physically transfer energy out of the tunnel. When the temperature outside the tunnel is high and the ventilation distance is large, the air temperature at the outlet of the air duct near the tunnel face can approach 28 °C, and cooling is difficult to achieve. Ice, water sprinkling, refrigerators, and other applicable measures can be adopted to reduce the air temperature.

(2) Ice cooling

The effectiveness of ice cooling measures mainly depends on the melting of ice to absorb heat. Through calculation, melting of 1 m³ of ice can remove 3.58 × 10⁵ kJ of heat. Field measurements showed an air temperature reduction of 3–5 °C, within 3 m, around the ice. However, in general only local temperature reduction can be achieved by this method.

(3) Cold water fusion

Cold water could be transferred from outside the tunnel and directly combined with high temperature groundwater in the tunnel face. The high temperature groundwater releases energy to the cold water, producing an intermediate temperature of the mixture. In the high temperature sections of the tunnel, comprehensive cooling technology that employs cold water could be further supplemented by ventilation. Notably, natural water with a low temperature (15 °C) is sufficient all year round in the upstream valley.

(4) Mechanical refrigeration cooling

When using mechanical cooling, heat is released into the outside air through the cooling system (cooling tower). The implementation method is as follows: a natural cooling source (river water) can be used to replenish the water in the cooling water tower, which is sent to the tunnel through a pump group and pipeline as the heat sink, or energy absorber, of the refrigeration unit and then returned to the external cooling water tower. After refrigeration of the unit and the air is cooled down in the refrigeration machine, the cold air is blown to the tunnel face to achieve the purpose of improving the ambient temperature.

A refrigerator was placed in the tunnel approximately 200 m away from the tunnel face. The air outside the tunnel was blown toward the tunnel face in the external circulation mode of the air-receiving belt. The cooling capacity can be determined according to the heat inside the tunnel. The refrigerator provides a refrigeration and dehumidification mode and exhibits a small width. The transport length of cold air exceeds 200 m, and the refrigerator can be operated at 70 °C in a high-humidity, dusty, and vibratory environment. The working principle of the refrigerator is shown in Fig.12.

Considering the abovementioned cooling measures, one can conclude that a single cooling measure has limitations. Therefore, two or more cooling measures should be adopted to achieve comprehensive cooling in a high geothermal tunnel. At different rock and groundwater temperatures, corresponding cooling measures are proposed for the different sections of the high geothermal tunnel to control the heat source. Cooling measures for the different sections are summarized in Tab.5.

3.5 Monitoring and early warning

Many studies have indicated that a high-temperature environment can adversely affect health, safety, and production [32,33]. When the temperature exceeds 30 °C, there is an increased risk of fainting or death due to heat stroke. When high-temperature groundwater is encountered during excavation, high-temperature water vapor always exists. Moreover, harmful gases, such as H₂S, may occur. Therefore, omnidirectional intelligent monitoring of the high geothermal tunnel environment is needed to ensure safe construction of a high geothermal tunnel.

Remote intelligent monitoring technology was used in the high geothermal tunnel to automatically monitor the ambient temperature. The system comprised 8 temperature and humidity sensors, 7 wind speed sensors and 3 collectors. The range provided by the temperature sensors is from −20 to 80 °C, the operating environment temperature ranges from −50 to 150 °C, and the accuracy is ±0.5 °C. The range of the wind speed sensors is 0–30 m/s, the operating environment temperature ranges from −50 to 150 °C, and the accuracy is ±0.1 m/s. The frequency range of the collectors is from 1 s to 1 d, and we set the collection frequency to 10 min/time. The sensors and collectors were connected with an S485 communication cable, and the lora signal was used to transmit data between the collectors. The collector in the tunnel cavern adopted a 4G signal to upload all data to the cloud platform, thus realizing remote intelligent monitoring.

Additionally, wireless transmission was installed along the longitudinal direction of the tunnel to effectively reduce the cost and any interference with construction. Notably, remote intelligent monitoring technology can be used to collect and upload data automatically. When a preset threshold value of the software platform is exceeded, an alarm is triggered. The remote intelligent monitoring system for the high-geothermal tunnel is shown in Fig.13.

Based on the monitoring results for the high geothermal tunnel environment, the temperature near the tunnel face was controlled at approximately 28 °C, and the relative humidity in the hole ranged from 40% to 70%. People in the working section were not affected by the environmental temperature and humidity, which validates the effectiveness of the cooling policy.

Monitoring data for a similar tunnel, the Sang Zhuling Tunnel, located in southwest China, with a total length of over 16000 m and a maximum surrounding rock temperature of 89.9 °C, were collected and analyzed [34–36]. The curve obtained from the literature reveals that the secondary lining temperature changes with ventilation time during construction, as shown in Fig.14. When the ventilation time is 1 d, the lining temperature drops from 60 to 56 °C, and under a ventilation time of 15 d, the lining temperature reaches 42 °C.

Numerical simulations were conducted in ANSYS software to study the trend in the influence of the ventilation time on lining safety during high geothermal tunnel construction. Based on a similar tunnel, the simulation parameters were determined. The numerical simulation parameters are listed in Tab.6.

Finally, the safety of the secondary lining was assessed based on the existing specifications for tunnel structures. Tab.7 shows that the safety factor of the lining increases with increasing ventilation time. The main reason is that with increasing ventilation time, both the lining temperature and the additional temperature load acting on the lining decrease. With more than 16 d of ventilation, the lining temperature was 75% of that with 1 d of ventilation, increasing the factor of safety from 2.53 to 3.8. Therefore, active cooling measures should be implemented to improve the safety factor of the lining [24].

3.6 Comprehensive safeguards

Systematic security mechanisms enhance the wellbeing and facilitate emergency rescue of workers. Specifically, protection measures were adopted for workers and equipment, and air-conditioned rest areas were established in the tunnel to ensure worker safety and uninterrupted construction. The systematic security measures in the high geothermal tunnel are shown in Fig.15.

(1) Enhancing occupational health training of workers

The safety of tunnel construction and health of management personnel should be guaranteed, and only qualified personnel are permitted to enter the tunnel. Special safety education and training should be provided to tunnel construction and management personnel for understanding the dangers of high temperatures to personnel. The emergency response procedures following exposure to high temperatures should be explained to all personnel concerned.

(2) Personnel protection

In a high temperature environment, protective clothing should be worn during construction when centralized cooling is difficult or the local cooling effect is insufficient. Personal cooling suits are provided for the protection of workers in the high-temperature sections. Plastic bottles and bags filled with water can be frozen using ice machines. The workers entering the tunnel can carry ice cubes and ice water.

(3) Operation lounge and infirmary

When exposed to a high-temperature and high-humidity working environment for a long period, workers may suffer depletion of physical strength, and may even suffer shock or death. Therefore, a lounge space should be established at the construction site for workers to cool down and rest. Notably, a lounge should be installed at the tunnel entrance.

Medicine and liquids for heatstroke prevention and emergency response, including ice cubes, gauze, and disinfectant, should be made available in the infirmary. In addition to an emergency kit and lockers, saline and dextrose solutions should be provided to address electrolyte imbalance after heavy sweating.

(4) Emergency rescue plan and exercise

A table-top exercise should be performed to rehearse the emergency plan. The main participants should be the project manager, heads of departments of the construction organization, head of the emergency rescue team, and emergency rescue attendants. A verbal exercise should be conducted to examine the implementation of emergency plan requirements, and to enhance abilities in solving problems and in delivering coordination among duty divisions during an emergency.

4 Conclusions

In this study, a high geothermal tunnel was divided into three sections. A comprehensive cooling strategy was proposed, and the geothermal formation mechanism of the high geothermal tunnel was investigated. Targeted methods were employed to reduce the rate of heat release from heat sources, and multiple measures were adopted to reduce the ambient temperature. The ambient temperature and humidity were monitored in real time by a monitoring and warning system. The establishment of systematic security mechanisms is a prerequisite for safely crossing high-geothermal sections. The study conclusions can be summarized as follows.

(1) In a high geothermal tunnel, advanced geological prediction was adopted to investigate the geothermal formation mechanism, the temperature field ahead of the tunnel face, and the location of high-temperature ground water. This provided a foundation for the formulation of a comprehensive cooling strategy.

(2) Full-face grouting, which can be used to effectively control the water inflow in a tunnel and reduce the heat released by high temperature water, was used in the high geothermal tunnel. In the event of local high-temperature groundwater gushing in the tunnel, insulated drainage pipes could effectively release heat.

(3) Measures for maintaining a comfortable working environment are suggested as follows. First, measures should be implemented to increase the air volume during the construction of a high-geothermal tunnel. Second, ice cooling and local fans should be used. When the temperature exceeds 60 °C, refrigeration should be adopted to reduce the temperature. Moreover, installed duct should be insulated.

(4) In the high geothermal tunnel, a remote intelligent monitoring system was developed to gather tunnel environment data. Upon surpassing predefined thresholds, automatic warnings are issued, enabling designated personnel to take corrective measures to personal protection. Establishing systematic security mechanisms stands as a prerequisite for safely navigating high geothermal sections.

(5) A case study was conducted on a high geothermal tunnel in South-West China, with groundwater at 93 °C and surrounding rock temperatures at 70 °C. Implementing the proposed cooling strategy during construction successfully regulated the temperature near the tunnel face to 28 °C. This controlled environment allowed uninterrupted work activities for construction personnel despite the challenging tunnel temperatures.

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