Geothermal Tunnel Research Articles

Creep mechanical behavior and damage characteristics of laminated slate under thermal-mechanical coupling

To study the long-term stability of geothermal tunnels within a laminated rock mass, creep tests were conducted on slate with different bedding angles under different temperatures. In this study, we investigated the creep rate and macroscopic failure characteristics of slate with various bedding angles (0–90°). The coupled damage was considered, and combining with the unsteady fractional order theory, a constitutive model that can describe nonlinear behavior of laminated slate under thermal loading was established. The results show that gradual increases in temperature from 20 to 150 °C and loading from 10 to 115 MPa lead to increased rock damage, which leads to accelerated creep and eventual failure of the rock. The long-term strengths of slate with a bedding angle of 0 °C at temperatures of 20, 50, 80, 110, and 150 °C were found to be 126.21, 118.25, 106.18, 94.58, and 85.14 MPa, respectively, and its mechanical properties exhibited anisotropy. The proposed creep model effectively captures the three-stage creep characteristics of high-temperature layered rocks. The results of this study provide theoretical guidance for the long-term stability assessment of geothermal drilling in surrounding rock.

Coupling effect of high temperature and fatigue loading on ballastless track concrete in geothermal tunnel

To elucidate the fatigue evolution mechanisms of ballastless track concrete under high geothermal conditions, the interaction between high temperature and fatigue loading, as well as the mechanisms of concrete performance under the coupling efect of these two types of loading, have been studied. The results showed that static mechanical and fatigue performance of concrete started to decline when temperatures surpassed 60 ℃. When the temperature raised to 80 ℃, the static mechanical performance of the ballastless track concrete decreases by 6.6% to 10.9%, and the fatigue life decreases by 32.1%. On the other hand, the fatigue loading also leads to an increase in temperature due to the displacement and friction of the microstructure within the concrete, and a significant thermal effect is observed at the initial stage of fatigue loading. The fatigue load causes the temperature of the concrete to increase by 4 ℃ to 7 ℃. The principal reason for the accelerated degradation of concrete fatigue performance is the damage caused by high temperature. When the temperature is over 60 ℃, the porosity and pore diameter of concrete increased, the internal pore structure began to deteriorate, and the ettringite within cement hydration products decomposes. Due to the coupled effect of high geothermal condition and fatigue loading, the energy absorption and plastic deformation of concrete decrease, which diminishes the fatigue performance ballastless track concrete in tunnel.

Durability of a Metakaolin-Incorporated Cement-Based Grouting Material in High Geothermal Tunnels.

To improve the durability of cement-based grout in high geothermal tunnel grouting projects, metakaolin was incorporated to replace 8% of the cement. The permeability, chloride penetration, and sulfate and carbonate erosion tests were carried out on the grout cured at varying temperatures (20, 30, 40, 50, and 60 °C). It was shown that with the increase in curing temperature, the impermeability and resistance to chloride penetration initially increased and then decreased, reaching the highest values at 40 °C. Furthermore, the incorporation of metakaolin into grout enhanced the impermeability by 11.11-38.91% and resistance to chloride penetration by 12.23-12.77%. As attacked by sulfate, the surface spalling of the specimen was more obvious with the increase in the curing temperature and immersion time. Conversely, the appearance remained almost unchanged under carbonate erosion. As immersion time increased, both the antisulfate and anticarbonate erosion coefficients declined. The effect of curing temperature on erosion resistance was assessed via the relative antierosion coefficient. When immersed in sulfate, the coefficient achieved the highest value of 1.08 as cured at 30 °C, and in carbonate, the coefficient reached a maximum of 0.99 as cured at 40 °C. Through scanning electron microscopy and binarized images, it was verified that gypsum and ettringite were formed during sulfate attack, leading to volume expansion and cracking, consequently reducing the strength. Meanwhile, in the case of carbonate erosion, calcium hydroxide (CH) and calcium silicate hydrate gel (C-S-H) were dissolved and degraded, resulting in a large number and size of pores in the grout. The research results have certain theoretical significances and reference values for the application of cement-based grouting materials in high geothermal environments.

Non-monotonic effect of differential stress and temperature on mechanical property and rockburst proneness of granite under high-temperature true triaxial compression

Complicated geological conditions such as high geo-stress and high ground temperatures often have a significant impact on the mechanical behavior and failure potential of hard rocks in deep engineering. This study aims to investigate the mechanical behaviors and rockburst proneness of granite under high differential stress and high temperatures in deep-buried high-temperature tunnels. A comprehensive experimental study was conducted on the coupling behavior of granite under high-temperature and high differential stress by true triaxial compression tests. The strength and failure mechanism of granite as well as their relationship with high temperature and high differential stress were revealed. The rockburst proneness of granite from a deep-buried high geothermal tunnel was assessed based on the rock's ultimate energy storage characteristics. It shows the strength, failure mechanisms, and rockburst proneness of granite, are significantly correlated to the granite high-temperature high-stress conditions. The temperatures extend the strengthening range of the intermediate principal stress on the true triaxial peak strength of the granite. With increasing temperature and differential stress, secondary vertical cracks develop quickly to induce the macroscopic failure of granite. Under high temperatures, the failure of granite changes from compressive-shear failure to tensile-shear failure at low differential stress. High temperature coupled with high differential stress strengthens approximately 1.14 times the rockburst proneness of granite. The results are important for the safe construction, and disaster assessment of deep-buried high geothermal tunnels.

Performance evaluation of a novel adaptive temperature-controlled lining technology based on thermo-hydro-mechanical coupling analysis

To mitigate the hazards of high geotemperatures in hydraulic tunnels, the performance of a novel adaptive temperature-controlled lining was investigated across various working phases of high geothermal tunnels using the thermo-hydro-mechanical coupling analysis with the finite element software COMSOL. The results indicated an error margin of only 2% to 6% compared to the field measurements, proving the feasibility of the method. Compared with the conventional concrete lining, the adaptive temperature-controlled lining reduced the average inner wall temperature by approximately 55% during construction, decreased the maximum thermal deformation by approximately 37%, transferred the localized energy elevation from the inner wall to the outer lining, and lowered the maximum elastic strain energy density by approximately 62%. During the water-filled operational phase, the radius of temperature influence in the low-temperature zone of the adaptive temperature-controlled lining expanded by approximately 20%, whereas the maximum thermal stress decreased by approximately 76%, the thermal deformation by 50%, and the elastic strain energy density by 88%. Throughout both the construction and operational phases, the adaptive temperature-controlled lining significantly enhanced the temperature control, improved the stress distribution, and reduced the risk of concrete cracking. These findings could offer the innovative engineering insights into the design, construction, and operation of high-geothermal tunnels.

Detailed thermal environment classification of high geothermal tunnel based on thermal comfort indices

High temperature and high humidity in geothermal tunnels pose significant risks to the well-being of construction workers. This study aims to establish critical environmental thresholds to protect workers in these challenging conditions. Field tests were conducted in a high geothermal tunnel in southwest China, continuous measuring parameters such as dry bulb temperature, relative humidity, wet bulb globe temperature (WBGT), skin temperature, and heart rate. The collected data were analyzed, leading to the development of a nonlinear programming mathematical model. This model enabled a systematic classification of the tunnel's environmental conditions and the calculation of threshold values for various environmental parameters. Furthermore, critical thresholds for heat stress indicators such as the discomfort index (DI), heat index (HI), and relative strain index (RSI) were determined. The study categorized the tunnel's hot and humid environment into five distinct zones: comfortable zone, general comfortable zone, safety zone, heat tolerance zone, and danger zone. The findings indicate that in this geothermal tunnel, the comfort zone thresholds are 26 °C for dry bulb temperature, 30 % for relative humidity, and 20 °C for the WBGT index. The safety zone thresholds are 37 °C, 64 %, and 32 °C, while the danger zone thresholds are 43 °C, 81 %, and 40 °C, respectively. For heat stress indices, the safety thresholds for DI, HI, and RSI are 25.42 °C, 26.31 °C, and 0.28, while the danger thresholds are 28.59 °C, 35.30 °C, and 0.38, respectively. This research provides essential insights for safeguarding the health of construction workers operating in high geothermal tunnel environments.

Multiscale model for concrete cured under geothermal environment: Theoretical analysis and experimental validation

Concrete poured as linings of geothermal tunnel would deviate normal curing condition. A multiscale model based on continuum micromechanics, accounting for temperature-dependent factors, is established from the cement paste level to the concrete level. This model predicts the physical properties and compressive strength of concrete and is validated against measured strength, which incorporates the impact of high-temperature curing on the hydration degree, density of C-S-H, and chemical shrinkage of cement. Evolutions of the average hydration degree and compressive strength of concrete were experimentally investigated to this end, and then simulated with multiscale approach. The specimens were subjected to curing temperature 20°C, 40°C, 60°C, and 80°C for first 7 days while maintaining constant relative humidity of 100 %, and then water curing at 20°C for the rest days till tests. The high-temperature curing accelerates the hydration process, but the variance in hydration degree between standard temperature (20°C) and high-temperature conditions diminishes with prolonged water curing. An enhancement in compressive strength is also observed in the early stages of high-temperature curing, succeeded by a decline compared with standard curing after the 50th curing day. Quantitative analysis reveals that volume fractions of hydrates and porosity subjected to high-temperature curing show a substantial increase in the first 7 curing days, and the denser layer of hydration products formed on cement slows down the hydration in later curing ages. The complex evolution of strength in concrete undergoing high-temperature curing is influenced by temporal consideration (the hydration degree) and spatial considerations (the density of C-S-H and the chemical shrinkage of cement). The surge in hydration significantly contributes to strength promotion in early stages. However, the pivotal factor leading to strength deterioration is the increase of porosity when the difference in the hydration degree is less than 0.065.

Prediction of the Temperature Field in a Tunnel during Construction Based on Airflow–Surrounding Rock Heat Transfer

It is important to determine the ventilation required in the construction of deep and long tunnels and the variation law of tunnel temperature fields to reduce the numbers of high-temperature disasters and serious accidents. Based on a tunnel project with a high ground temperature, with the help of convection heat transfer theory and the theoretical analysis and calculation method, this paper clarifies the contribution of various heat sources to the air demand during tunnel construction, and reveals the important environmental parameters that determine the ventilation value by changing the construction conditions. The results show that increasing the fresh air temperature greatly increases the required air volume, and the closer the supply air temperature is to 28 °C, the more the air volume needs to be increased. The air temperature away from the palm face is not significantly affected by changes in the supply air temperature. Adjusting the wall temperature greatly accelerates the rate of temperature growth. The supply air temperature rose from 15 to 25 °C, while the tunnel temperature at 800 m only increased by 1.5 °C. Over a 50 m range, the wall temperature rose from 35 to 60 degrees Celsius at a rate of 0.0842 to 0.219 degrees Celsius per meter. The total air volume rises and the surface heat transfer coefficient decreases as the tunnel’s cross-section increases. For every 10 m increase in the tunnel diameter, the temperature at 800 m from the tunnel face drops by about 0.5 °C. Changing the distance between the air duct and the tunnel face has little influence on the temperature distribution law. The general trend is that the farther the air duct outlet is from the tunnel face, the higher the temperature is, and the maximum difference is within the range of 50 m~250 m from the tunnel face. The maximum difference between the air temperatures at 12 m and 27 m is 0.79 °C. The geological structure and geothermal background have the greatest influence on the temperature prediction of high geothermal tunnels. The prediction results are of great significance for guiding tunnel construction, formulating cooling measures, and ensuring construction safety.

Cooling effect and parameter analysis of applying heat insulation layer to tunnel regionalization in construction period based on geothermal level

The construction of high geothermal tunnels presents significant difficulties due to the abnormal rock temperature gradients, including designing a reasonable and cost-effective insulation layer as well as effective cooling measures. In this study, the rock temperature distribution, environmental temperature, and lining surface temperature of the tunnel during the construction period were analyzed through field surveys. A numerical simulation model considering ventilation and heat transfer dynamics was established based on the geothermal measurements and construction characteristics of this tunnel. After validating the 3D numerical model, the effects of installing heat insulation layers according to geothermal levels on the tunnel temperature field control were investigated. Additionally, the impacts of different insulation layer thermal conductivities (λi), ventilation air volume (Wa), and duct thermal conductivities (λd) were analyzed. The results indicate: (1) The comparison of temperature fields in tunnels with insulation layers, which is applied in the range of the rock temperature above 50 °C and without insulation layers was made. The temperature near the excavation face (TAm-1) decreases by up to 0.3 % (0.5 °C), tunnel environment temperature with insulation layer applied (TAm-2) decreases by up to 4.2 % (2.1 °C), the secondary lining temperature (TSec-lining) decreases by up to 10.5 % (4.6 °C), and the wind temperature of air duct outlet (TAir-duct) decreases by up to 0.7 % (0.5 °C). (2) The length of the insulation layer increases to the full length of the tunnel, TAm-1 continues to decline by only 0.4 °C, TAm-2 by 0.5 °C, TSec-lining by 0.9 °C, and TAir-duct by 0.1 °C, which shows that it is the most economical and reasonable to apply the insulation layer only in the tunnel area where the rock temperature is higher than 50 °C. (3) As λi increases, both the tunnel environment and secondary lining temperatures exhibit linear increases, while increasing Wa and λi cause quadratic decreases and increases in tunnel temperature field parameters, respectively. (4) Range analysis of orthogonal experiments reveals that for tunnel temperature field impact level. For TAm-1, the significance order is Wa >λd >λi. And for TAm-2 and TSec-lining, the significance order is Wa >λi >λd, which can provide the more economical scheme for the insulation layer design and cooling schemes in the high ground temperature tunnel.

The Influence of Rock Thermal Stress on the Morphology and Expansion Pattern of the Plastic Zone in the Surrounding Rock of a Deep-Buried Tunnel under High Geothermal Temperature Conditions

The extent of the plastic zone is critical in determining the stability and extent of damage to the surrounding rock in tunnels, crucial for designing support structures and thermal insulation layers. This study focuses on understanding how rock thermal stress affects the expansion of the plastic zone in deep-buried tunnels subjected to high geothermal temperatures, based on the derivation of the boundary line formula of the plastic zone in a high geothermal tunnel, and combined with the test results of the hydraulic fracturing method in a high geothermal tunnel of the Bulunkou Hydropower Station in Xinjiang. The findings indicate that thermal stress in the rock mass slows the growth of the plastic zone but significantly increases its extent. However, the influence of thermal stress on the shape and size of the plastic zone is less significant compared to the lateral pressure coefficient. In conditions of high geothermal temperature and geostress, rock mass thermal stress induces substantial changes in the morphology and extent of the plastic zone, which cannot be overlooked and can lead to significant errors if not properly considered. The theoretical formulas derived from engineering analysis, along with observed patterns of plastic zone expansion, demonstrate practical applicability.

A theoretical model to predict the cooling effect of sprayed polymer under hydro-thermal condition

With the implementation of China’s western development strategy, the number of tunnels located in high geothermal regions has significantly increased. Thus, the cooling of high geothermal tunnels has become a key issue. This paper investigates the cooling effect of sprayed polyurethane foam (SPF) with different properties and under environmental conditions. A theoretical model is proposed to predict the cooling effect of SPF. Accurate characterization of the cooling effect along with the polymer properties and temperature based on the relationship between thermal conductivity and temperature can help to establish relationships such as the thermal insulation-temperature relationship. The results reveal that the experimental and theoretical estimation of the cooling effect of SPF exhibits an error of less than 10 %. Therefore, a theoretical model is developed on the thermal-insulation performance of SPF. The temperature inside the thermal insulation layer (TIL) decreases with increasing SPF thickness and convective heat transfer coefficient (CHTC). There is an optimum foam thickness range (5 ∼ 10 cm) for the most stable cooling effect. In addition, the long-term stable thermal insulation performance proves that the polymer can be considered as a thermal insulation material, as the temperature inside the TIL remains at 30 °C for 30 days. Furthermore, a TIL thickness calculation formula and a TIL thickness sizing table can be completed to guide on-site construction.

Strength characteristics of cement slurry in high geothermal tunnel environment

The early-age strength of grouting material is crucial for tunnel support under high geothermal temperatures. Grouting materials (such as the most commonly used cement slurry) hydrate and solidify under the condition of variable-temperature curing after entering the real stratum. However, the development process of the early strength of cement paste under the condition of variable-temperature curing is still unclear. This aim of this study was to elucidate the environmental effects of high-ground-temperature tunnels on the early strength development of cement stone. Cement slurry was subjected to variable-temperature curing under three different temperatures (T = 40°C, 60°C and 80°C) and two different relative humidities (H = 5% and 95%) through design experiments. Compressive strength tests were conducted, along with microscopic analysis using X-ray diffraction, scanning electron microscopy and nuclear magnetic resonance. The results indicate that the strength of cement stone under variable-temperature curing was lower than that under constant-temperature curing at the same temperature. The influence of variable-temperature curing conditions on the strength variation of samples with a high water/cement ratio at later age was greater for curing at 80°C. The research results are helpful to understand the influence of environmental effects of high-ground-temperature tunnels on the properties of grouting materials.

A self-satisfying cooling system based on cold energy storage for the locomotive in a high geothermal tunnel

Effective thermal management of locomotive systems is crucial for ensuring the safe operation of trains through high geothermal tunnels. By taking advantage of the frequent alternation of high-temperature tunnels and cold climate environment, a self-satisfying cooling system based on cold energy storage is proposed, in which a phase change heat exchanger between air and phase change material (succinic acid with 20 wt% expanded graphite) is developed to store cold energy outside tunnels and improve air-cooling efficiency inside tunnels. Numerical models are established to examine the thermal environment in a typical tunnel and the effectiveness of the proposed cooling system. The results indicate that because of the heat exhausted from the train and the thermal dissipation from the surrounding hot rock, the air temperature in the tunnel can rise above 50 °C. The proposed cooling system can release the cold energy from phase change material to keep the air outlet temperature below 40 °C, increasing the energy efficiency and reliability of locomotive operation. By examining the effects of locomotive operating conditions and orthogonal analysis of multi-factors, the heat exchanger structure and other operating conditions are recommended for the self-satisfying cooling system. Finally, the cooling system’s effectiveness is confirmed for the full-line operation of the locomotive along the Ya’an-Nyingchi section of the Sichuan-Tibet Railway in both summer and winter climatic conditions. This study is significant because it provides a novel design scheme for locomotive’s thermal management in high geothermal tunnels and other similar engineering conditions with natural cold sources.

Research on ventilation cooling design driven by human thermal response in high geothermal temperature tunnel construction

Constant high temperature conditions in tunnel not only significantly reduced the labor efficiency of workers but also affected their physical and mental health. However, the traditional design of ventilation and cooling for high geothermal tunnel construction only considers the control of environment temperature and ignores the thermal comfort state of the workers, which will increase the potential risk of tunnel safety accidents. Therefore, this paper adopted the methods of scene testing, theoretical analysis and numerical simulation to establish a modified evaluation model applicable to the thermal comfort of workers in high geothermal tunnels. By analyzing the temperature distribution pattern of heat-sensitive parts of workers engaged in different labor intensity under different air supply parameters, the design strategy for ventilation and cooling of high-temperature tunnels was refined on the basis of ensuring that the environment temperature and thermal comfort meet the requirements. The results show that ventilation design parameters that satisfy the environmental temperature control criteria are equally adequate for worker thermal comfort at light-intensity work. However, when the ambient temperature is higher than 40 °C, the thermal comfort of workers engaged in medium-intensity work still does not meet the requirements, even if the ambient temperature already meets the criteria. It is recommended that the wind speed be increased by at least 8.3 %, 15.6%and 28.6 % for ambient temperatures of 40 °C, 45 °C and 50 °C, respectively. More effective cooling techniques should be tried to further enhance thermal comfort for workers engaged in heavy workloads especially in hot environments exceeding 40 °C.

Effects of longitudinal ventilation and GHEs on geothermal energy extraction and HRC in high geothermal tunnels

With the excavation of deep tunnels, the problem of high geothermal heat is becoming more and more prominent. However, high geothermal heat can be utilized as a renewable energy source. In order to effectively reduce thermal damage and utilize geothermal energy, it is necessary to study the effects of longitudinal ventilation and ground heat exchanges (GHEs) on the heat-regulating circle of the surrounding rock. In this study, an experimental platform was designed and constructed in a laboratory to test different geothermal heat exchanger flow rates and ventilation air velocities and to monitor the heat extraction from the geothermal heat exchanger, the ventilation heat extraction, and the temperature field of the surrounding rock. The experimental results show that when the temperature field tends to be stabilized, the flow rate and ventilation velocity of the GHEs are closely related to the temperature distribution of the surrounding rock. The initial heat storage and heat replenishment capacity of the enclosing rock have a great influence on the heat extraction efficiency. Ventilation has a great influence on the heat extraction efficiency of GHEs, and the optimal wind speed range is 1.5∼2 m/s considering the cooling and heat extraction efficiency. When the wind speed is less than 1.5 m/s, the heat extraction efficiency of GHEs can be improved, and the total heat transfer can be increased. When the wind speed is greater than 1.5 m/s, the situation is reversed. The thermal entropy theory for circular tunnels is not fully applicable to arched tunnels containing GHEs.

A study on the sulfate erosion deterioration law and damage model of shotcrete in high geothermal tunnels

AbstractWhen building a tunnel in an environment rich in high‐temperature hot water, it is particularly necessary to pay attention to the influence of sulfate ions in underground hot water on tunnel shotcrete. In order to study the sulfate erosion mechanism and mechanical properties of shotcrete in a real high‐temperature hot water environment, this study was carried out by setting the curing temperature (20, 40, 60, and 80°C), humidity (55% RH, 95% RH), and erosion age (0, 15, 30, 60, and 90 d) as the test influencing factors; a full combination of dry‐wet cycle test was carried out, and the specimens under different conditions were analyzed macroscopically and microscopically. The results show that with the increase of the number of dry‐wet cycles, the quality of shotcrete increases first and then decreases, and the mechanical properties gradually decrease. In the early stage of erosion, the erosion product is mainly ettringite, and the macroscopic damage is aggregate spalling. In the later stage of erosion, the erosion product is mainly gypsum, and the macroscopic damage is expansion damage. Compared with standard curing, a certain degree of high temperature curing has little effect on the sulfate attack resistance of shotcrete, but when the curing temperature exceeds 60°C, the concrete is seriously damaged. Finally, by constructing the damage model of sulfate attack shotcrete, the variation of compressive strength of shotcrete with age after sulfate attack under different curing conditions was successfully predicted.

An innovative PCM-modified lining system for energy tunnel: From concept to numerical investigations

The rich geothermal stored in rock mass would result in high temperature to tunnels. This not only makes condition worse to workers, but also wastes geothermal energy and increases cost to adjust the internal environmental temperature. This motivates the present feasibility study on an innovative PCM-modified lining system of conventional energy tunnel, which generally makes use of ground source heat pump (GSHP). That is, the PCM-modified concrete regulates the thermal properties of tunnel linings installed with pipe loops of GSHP. To verify the concept of the new system, a finite element model (FEM) describing it is established, based on a case of high geothermal tunnel. Simulation of the heat conducting process of tunnel linings starting from hydration of cement during cast-in-place concrete, thermal conductivity from surrounding rock mass, circulating of GSHP, phase change within the PCM-modified linings, and boundary effect since ventilation are considered in the modeling. The results demonstrate that the new system would mitigate the level of thermal discomforts during construction stage, and it can improve the stability and efficiency in energy extraction in operation stage. Parametric study on the GSHP with PCM-modified linings gives that the better heat reduction performance can be obtained with the smaller intermittent ratio (IR) of GSHP, smaller temperature range of phase change, and larger latent heat, and higher exchanger rate can be obtained with the larger IR and larger latent heat.

Performance Analysis and Optimization of Coupled Cooling System for Auxiliary Ventilation and Partial Thermal Insulation in High Geothermal Tunnels

A high temperature is the key factor limiting the safe development of deep mine tunnels. By confronting the phenomenon of serious heat exchange between airflow and the surrounding rocks in the tunnel excavation area, a conceptual model of coupled cooling of auxiliary ventilation and partial thermal insulation is proposed. The performance of a coupled cooling system was investigated and optimized by using the scale model test with a 1:10 geometric scale and the orthogonal test. The results suggest that the average temperatures of the work zone and its central point decrease by 1.5 °C and 3.3 °C, respectively, while partial insulation layers are used. According to the sensitivity analysis for a single factor, as the ventilation duct outlet (VDO) moves away from the working face (WF), the temperature gradually increases, leading to a local high temperature area. When the ventilation duct height is arranged in the middle of the insulation layer, the cooling effect is optimal and the highest average temperature difference is 4.4 °C. The thermal equilibrium temperature can be further decreased by lengthening and thickening the insulation layer. In addition, the range analysis shows that the ventilation velocity has a greater impact on the thermal environment of the tunnel working area than the ventilation duct location and insulation layer length. The coupled cooling method can save on cooling capacity and effectively alleviate the high-temperature problems of the tunnel excavation area.

Heat Hazards in High-Temperature Tunnels: Influencing Factors, Disaster Forms, the Geogenetic Model and a Case Study of a Tunnel in Southwest China

The construction of extensive tunnels in regions characterized by high geothermal activity presents significant challenges and inherent risks that affect both the safety and operational efficiency of construction personnel. This study investigated the factors influencing geothermal fields in shallow crustal rock formations through a comprehensive examination of existing literature and a detailed analysis of case studies. In addition, this study categorizes the geogenetic models of high-temperature heat hazards into three major classifications. Research findings indicate that several key factors significantly influence the geothermal fields. These factors, which include the deep geothermal background, heat transfer conditions, and localized additional heat sources, are paramount in shaping the geothermal field. Notably, it is observed that among these factors, the presence of additional heat sources, particularly the circulation of underground hot water, poses the most considerable threat to safety and operational efficiency. Moreover, this study utilizes a representative high geothermal tunnel in Southwest China to conduct a field investigation. This investigation assesses the potential for high-temperature thermal hazards within the tunnels, evaluates the geological conditions, verifies the factors governing the geothermal field, and outlines specific measures for the prevention and control of high geothermal tunnels. In conclusion, this study provides a structured analysis of lessons learned from these experiences, along with practical countermeasures for addressing high-temperature thermal hazards during the various stages of tunnel construction. The findings of this research serve as a valuable reference for those investigating the mechanisms behind geothermal disasters in tunnel construction. Furthermore, they offer practical guidance to ensure the secure and efficient excavation and sustainable operation of tunnels in the challenging geological environments characterized by high geothermal temperatures.

Experimental Research on the Performance Characteristics of Grouting Slurry in a High-Ground-Temperature Environment

Grouting materials with good thermal insulation and reinforcement properties are the key factors in solving the temperature control problems of high geothermal tunnels using curtain grouting, as the existing grouting materials are unable to take into account the working performance and thermal insulation properties of high-temperature environments. In view of the above problems, this paper configures a high geothermal tunnel red-mud-based grouting material (RMGS) using red mud, carries out tests on the working performance (viscosity, setting time, and compressive strength) and thermal insulation performance (thermal conductivity and specific heat capacity) of the grouting materials at different temperatures (20, 40, 60, and 80 °C), and analyses the variation rules and micro-mechanisms of the various properties at different temperatures. The results show that the increase in temperature will accelerate the viscosity development and condensation of the grouting material and will also lead to the acceleration of the attenuation of the thermal conductivity of the three types of grouting material and the reduction in specific heat capacity. In addition, the appropriate temperature can improve the compressive strength of the material. The increase in temperature will accelerate the hydration reaction speed of the grouting material and will also lead to the development of the internal pore space of the material, which affects the macroscopic properties of the material and is the reason for the effect of the temperature on the performance of the grouting material. In terms of application, the cement slurry is suitable for grouting in a static water environment, the cement–water glass bi-liquid slurry is suitable for grouting in a dynamic water environment, and the RMGS is suitable for grouting in a high-ground-temperature environment.