High Ground Temperature Research Articles

Theoretical analysis of grouting reinforcement for surrounding rock of deep shaft based on stability and water inflow control

To address the challenge of designing grouting reinforcement in a deep shaft to control water, this study established an elastic-plastic analytical formula for the grouted rock surrounding a shaft under the combined action of thermal, hydraulic, and mechanical fields based on the Mohr–Coulomb yield criterion. The various rules and influencing factors of stability and impermeability of the grouted rock surrounding the shaft are calculated and analyzed. The analysis shows that damage to the surrounding rock is aggravated by the action of high ground temperature and high water pressure in deep shafts, and the influence of water pressure is particularly significant. The larger the radius and cohesion of the grouted surrounding rock, the smaller the radius of its plastic zone. With an increase in the elastic modulus ratio between the grouted surrounding rock and the original rock, the radius of its plastic zone increases, so the elastic modulus of the grouted surrounding rock design should consider the original surrounding rock. With a decrease in the permeability coefficient of the grouted surrounding rock, the radial stress decreases, the tangential stress increases, and the radius of the plastic zone increases. The development of plastic zone not only affects the stability but also causes an increase in the permeability coefficient. There is mutual restriction and influence between the shaft water inflow, the permeability coefficient, the radius of the grouted surrounding rock, and the radius of its plastic zone. In the design of grouting reinforcement, the stability of surrounding rock and the control requirements of shaft water inflow should be comprehensively considered and the optimal parameters should be selected based on theoretical calculation. This study provides a theoretical basis for the optimization of grouting reinforcement parameters in a deep shaft.

Study on Mechanical Properties of Ring Sandstone Specimen under Temperature and Water Coupling Dynamic Compression

The excavation of hard rock roadways in coal mines is often in the environment of underground water and high ground temperature, and it is easy to be affected by the dynamic load, which leads to roadway destruction and increases the difficulty of roadway support. The ring sandstone specimens with different inner diameters (0~25 mm) were treated with temperature and water coupled, and the dynamic compression test was produced by the Hopkinson pressure rod device (SHPB). The experimental results indicate that the coupling effect of temperature and water reduces the dynamic performance of sandstone specimens. XRD test results showed that the composition of sandstone specimens did not change before and after warm water coupling, and no new substances were found. Dynamic properties of ring sandstone specimens with different inner diameters weaken with the increase in inner diameters. With the increasing inner diameter of ring sandstone specimens, the energy dissipation per unit volume increases the dynamic compressive strength decreases, and the degree of breakage increases. Fracture morphology, average strain rate, and dynamic peak strain of ring sandstone specimens increase with inner diameter.

Mechanism Analysis of Floor Water Inrush Based on Criteria Importance though Intercrieria Correlation

Floor water inrush has always been an important problem affecting coal mine safety. The water inrush from the deep coal seam floor is the final manifestation of the water inrush channel formed by the expansion and penetration of rock mass cracks under the combined action of a high ground temperature, high ground stress, high karst water pressure and strong mining disturbance. The fault is a structurally weak plane developed in the complete rock layer, which can be regarded as a permeable channel with good permeability. In order to explore the location of confined water from the fault to rock stratum caused by the tensile fracture of a fault during floor water inrush, floor water inrush is regarded as the result of the joint action of water pressure and mining stress. The COMSOL Multiphysics numerical simulation software was used to simulate the advancing process of the working face of the coal seam with a fault structure. Different fault dip models were established for comparative analysis. The objective weight CRITIC based on data volatility was used to analyze the water pressure and principal stress near the fault. The results show that the uplift strength of highly confined water increases with the increase in the fault dip angle, and the weight of water pressure near the fault has a certain relationship with the location of the water body entering the rock stratum from the fault.

Geothermal characteristics of the Xianshuihe fault zone and their engineering influence on tunnel construction

The Zheduo Mountain tunnel crosses the Xianshuihe fault zone in Kangding city. The Xianshuihe fault zone is composed of many large-scale faults and features a complex geological structure and strong geothermal activity. Based on geothermal drilling data, hydrogeological tests and the geothermal geological back-ground of the study area obtained via hydrogeological surveys and hydrogeo-chemical methods in the process of tunnel surveying, this paper explores the exposure characteristics and distribution of geothermal water in the study area, analyses its hydrogeochemical and isotopic characteristics, and thoroughly studies the hydrogeological structure of the geothermal water to explore the genetic mechanism responsible for the formation of the geothermal water. The influence of the geothermal water and high ground temperatures on tunnel construction is further predicted, and practical and effective engineering treatment suggestions are proposed.

Comparative study of temperature effects on white sandstone and rockburst-like materials.

Rockburst physical model test, as one of the important means to study deep tunnel engineering, reflects the main influencing factors of rockburst into the model test through similar theory, so as to reveal the formation mechanism, influencing factors and evolution law of different types of rockburst in deep tunnels. In order to study the mechanical properties of white sandstone in deeply buried tunnels at high ground temperatures, materials suitable for conducting rockburst physical and mechanical tests were developed on the basis of the Daqian Shi Ling tunnel project, and similar material ratios were preferentially selected on the basis of white sandstone. Judged by the rock burst propensity, similar materials with low strength and high brittle characteristics, can better simulate the characteristics of white sandstone, and all show a strong propensity to rock burst, is the ideal rock burst similar materials. Uniaxial compressive tests were conducted on similar materials and the original rock at different temperatures, and comparative analysis was performed. Through the study of stress, displacement and modulus of elasticity, it was concluded that the compressive strength of similar materials gradually increased with temperature in the range of 20-100°C, and the vertical displacement at peak strength decreased with increasing temperature. The damage forms of white sandstone and similar materials at different temperatures were comparatively analyzed, and it was obtained that the damage forms of white sandstone and similar materials were basically the same, with a few specimens showing tensile and shear damage, and most specimens showing the form of combined tensile and shear damage. The study of rock burst similar materials and the development of the failure characteristics of rock burst under the action of thermal coupling are of great significance to the mechanism of rock burst generation and prediction.

Analysis of Malignant Expansion of Plastic Zone of Surrounding Rocks in High Ground Temperature Hydraulic Tunnels and the Mechanical Characteristics of the Support Structure

To explore the development process of the plastic zone of hydraulic tunnels and the mechanical characteristics of the support structure in a high ground temperature environment during construction period, methods, including theoretical analysis, numerical calculation, field monitoring test, etc., are used to analyze the generation and formation process of the plastic zone of surrounding rocks in the hydraulic tunnel under high ground temperature environment. The evolution law of mechanical characteristics of the support structure in surrounding rocks is studied. The development process and shape of the plastic zone in the tunnel under various high ground temperatures are simulated. The results show that after the excavation of the high ground temperature hydraulic tunnel, the plastic zone first appears at the arch waist in a crescent pattern, then extends to both sides and the spandrel, and connects with the plastic zone in the arch crown, forming a pattern of circle or butterfly. A higher initial temperature of the surrounding rock indicates a larger range and plastic strain of the plastic zone in the surrounding rock. At the same time, the plastic zone is easier to appear at the spandrel and arch foot and develop to the depth of the surrounding rock. The higher is the initial temperature of the surrounding rock, the greater will be the axial force and bending moment of supported shotcrete, the more obviously the bending moment of arch abutment and arch foot will grow, the greater will be the axial force value of the bolt and the farther the neutral point will be from the tunnel wall. While strengthening the arch waist, the support of the arch shoulder and arch foot should also be strengthened at a high temperature.

Analyzing Energy Transfer Mechanism during Coal and Gas Protrusion in Deep Mines

Coal is the mainstay of China’s energy supply. With the gradual progress in China’s policy of phasing out backward coal production capacity, the intensive and deep mining of coal has gradually become the new norm. The current mining depth is increasing at a rate of 10~15 m/year. The high crust stress, high gas pressure, high ground temperature, and engineering disturbance stress in deep coal mines can lead to the occurrence of coal–rock–gas dynamic disasters that are complex and show the characteristics of compound dynamic disasters. It is important to understand the evolution and mechanism of deep coal and rock dynamic disasters accurately for the safe development of deep resources. To study the mechanism of occurrence and the evolution of impact–protrusion compound dynamic disasters, we herein analyzed the apparent characteristics of coal–rock–gas compound dynamic disasters in deep mines and obtained the mechanical and acoustic emission characteristics of coal–rock composites through indoor experiments. Then, we conducted in-depth analysis on the non-uniform deformation behaviors and non-uniform stress field of the coal–rock composite and clarified the generation mechanism of local tensile cracks at the coal–rock interface. Subsequently, we established the energy transfer model of the rock–rock–gas composite specimen in the process of dynamic destabilization in the engineering scale mining field and revealed the mechanism of nonlinear energy evolution and release of the coal–rock–gas composite, which has been less reported in previous studies. In this paper, we further clarified the occurrence and development mechanism of coal–rock–gas compound dynamic disasters in the engineering scale mining environment to guide the prevention and control of coal–rock–gas dynamic disasters in deep mines.

Performance Test and Thermal Insulation Effect Analysis of Basalt-Fiber Concrete.

This paper examines the feasibility of applying inorganic thermal-insulating concrete in high geothermal roadways in underground coal mines. This innovative material is based on a mixture of ceramsite, glazed hollow beads, cement, and natural sand, enhanced with varying degrees of basalt fibers. Fibers were used as a partial substitute in the mixture, in the following volumes: 0% (reference specimen), 5%, 10%, 15%, and 20%. Their compressive strength, permeability resistance, and thermal conductivity were studied. A high content of fibers tends to entangle into clumps during mixing, resulting in a significant reduction in the mechanical properties of compressive strength. The appropriate amount of fiber content can improve impermeability, and the permeability height of 5% fiber concrete was reduced by 22.5%. Experiments on thermal behavior showed that an increase of basalt fibers leads to a significant reduction in thermal conductivity. For concrete containing 20% fiber, the thermal conductivity for the reference specimen (0%) in the wet state was reduced from 0.385 W/(m∙°C) to 0.098 W/(m∙°C). There was a slight increase in thermal conductivity when the temperature increased from 30 °C to 60 °C. Despite the reduced mechanical strength, the resulting concrete is well-suited for use in the insulation of underground roadways, as numerical simulations showed that insulating concrete with optimal fiber content (15%) can reduce the average temperature of the wind flow in a high ground temperature roadway of 100 m in length in a mine by 0.3 °C. The final cost-benefit analysis showed that insulating concrete has more economic benefits and broad development prospects when applied to high geothermal roadway cooling projects.

Experimental Study on Permeability Evolution of Deep Coal Considering Temperature

With the depletion of shallow mineral resources, the sustainable development and utilization of deep mineral resources will become a normal activity. As a type of clean energy to promote sustainable development, gas in deep coal seams has attracted wide attention. A better understanding of the permeability evolution induced by mining disturbance and the geological environment is of great importance for underground coal exploitation and gas extraction. In order to analyze the evolution of the mechanical properties and permeability of deep coal that are induced by high ground temperature, coal of the Pingdingshan Coal Mine has been investigated, and the seepage tests were carried out by keeping the confining pressure constant and loading and unloading axial stress under different temperature conditions. The effect of temperature on the peak strength and the initial elastic modulus of coal samples is analyzed. The evolution of permeability, which is estimated with the transient pulse method, based on fractional derivative and fracture connectivity, are discussed by establishing the relationship between fracture connectivity and fractional derivative. Meanwhile, the damage variable that is caused by stress and temperature is introduced and the contribution of thermal damage on coal damage accumulation is discussed. A theoretical model is proposed regarding permeability evolution with temperature and stress based on the Cui–Bustin model, which is verified by experimental data. It has been found that the strength and elastic modulus of deep coal decrease nonlinearly with increasing temperature, which demonstrates that temperature has a weakening effect on the mechanical properties of coal. The fracture connectivity and permeability evolution trends with axial strain are consistent under different temperatures, which decrease slowly in the compaction and linear elastic stages, reach the minimum at the volumetric dilation point, gradually increase in the yield stage, then have a sharp increasing trend in the post-peak stage and, finally, become steady in the residual stage. The damage induced by temperature increases with rising temperatures under different external load conditions. When the external load increases gradually, the thermal damage still accumulates, but the thermal damage variable ratio decreases. The proposed permeability model considering temperature and stress can describe the trend of the experimental data. With axial stress increasing, the influence of temperature on permeability decreases, and its leading effect is mainly reflected in the compaction stage and the linear elastic stage of coal.

Research on a Spontaneous Combustion Prevention System in Deep Mine: A Case Study of Dongtan Coal Mine

ABSTRACT Due to the influence of high ground stress, high ground temperature, high water inflow and other problems in deep mines, coal spontaneous combustion (CSC) is seriously harmful. This paper studied the variation law of three kinds of indicator gases in the process of coal heating in deep mines, and established a multi-parameter gas indicator prediction system for CSC. Secondly, in view of the shortcomings of the negative pressure bundle tube. A positive pressure beam tube monitoring system for early warning of CSC was developed, the accuracy of sampling data was verified on site, and the high temperature area in the well was further identified by isotope radon measurement. Combined with the positive pressure beam tube monitoring system, to realize the joint detection of uphole and downhole. Finally, according to the detection results of the fire area, the fire prevention methods for small space fire area, multi-coal seam and multi-source air leakage complex fire area and large-area concealed fire area are determined. The above-mentioned CSC early warning and prevention system was applied in Dongtan (DT) Coal Mine, and a good fire prevention effect was achieved. In this paper, the CSC prevention and extinguishing system established has important significance in the prevention and control of mine fires.

Study of the microstructure and oxidation characteristics of residual coal in deep mines

Residual coal is prone to spontaneous combustion under the influence of high ground temperature, and the risk increases with mining depth. This results in gradual increases in smoke and dust emissions, which seriously affect the ecological and geological environment of the mining area. To avoid spontaneous combustion of the residual coal, protect the resources and ecology of mining areas, and investigate the influence of the deep thermal environment on the microstructure and oxidation properties of coal, the pore evolution characteristics and functional group migration of coal in a thermal environment were studied by N2 adsorption and infrared spectroscopy. A temperature-programmed test system was used to simulate the low-temperature oxidation process occurring in deep coal seams, and the heat release characteristics and oxidation kinetic parameters of the index gas were calculated. The experimental results revealed that the pore oxygen storage capacities and coal oxygen contact areas of residual coal in deep mines are larger than those of coal in shallow seams. As the preheating temperature is increased, the rates of CO and CO2 production and the amount of oxygen consumed in oxidation of heat-treated coal increase. The crossing point temperature (CPT) of heat-treated coal decreases. The deep thermal environment accelerates the rate of condensation decomposition of the aliphatic hydrocarbon groups in coal and fracture of side chain hydrogen bonds connecting strong electron-absorbing groups, which increases the exothermic intensity of the oxidation process. The reaction trigger temperatures for aliphatic hydrocarbon groups, peroxyl radicals, associative hydrogen bonds and other active groups in coal are different. With increasing ambient temperature, the types of active groups in coal increase, and the contents accumulate. With an increase in the deep geothermal temperature, the oxidation activity of coal is enhanced, and the coal is more prone to spontaneous combustion. These research results provide a theoretical reference for deep coal clean mining and prevention of spontaneous combustion disasters and have a potential guiding role in environmental protection and ecological improvement of deep mining areas.

The Damage Deterioration Mechanism of Tunnel Structure under the Effect of Thermo-Mechanical Coupling and Its Maintenance

The effect of thermo-mechanical coupling is a key factor that needs to be fully considered during the construction, operation, and maintenance of tunnels in high ground temperature areas so that measures could be taken to reduce the frequency of damages and accidents. However, research methods and theoretical results for tunnel support structures under high temperature conditions and layered soft rock environment are insufficient, therefore, to fill in this research blank, this paper aims to study the damage deterioration mechanism of tunnel structure under thermo-mechanical coupling effect and its maintenance. In the second chapter, this paper gave a diagram showing the coupling mechanism of several physical fields, including the stress field, the seepage field, and the temperature field, and proposed control equations for the coupling of these fields during the damage deterioration process of the tunnel structure. In the third chapter, this paper applied two yield failure criterions, namely the Mohr-Coulomb criterion and the maximum tensile stress strength criterion to the analysis of the evolution of compression shear failure and tensile failure, and built scientific evolution equations for the permeability coefficient and the thermal conductivity coefficient. At last, experimental results verified the effectiveness of the constructed model and the analysis method.

A Primary Support Design for Deep Shaft Construction Based on the Mechanism of Advanced Sequential Geopressure Release

The construction of 1500 m depth shaft in Xincheng Gold Mine, China, faces complex stress conditions such as high geostress (>50 MPa), high ground temperature (>50 °C), high water-pressure (>9 MPa), and highly corrosive. Traditional deep shafts excavated by the sinking and lining method cannot adapt to high geostress problems, such as rock bursts and large deformations, etc., in the deep shaft construction process. To avoid and adjust the high geostress induced the rockburst and large deformations, the mechanism of the advanced sequential geopressure release (ASGR) has been proposed for the ground control in deep shaft construction. In this paper, the safe distance between the concrete lining and the shaft excavation face is determined based on the ASGR mechanism, which can provide the space for geopressure release, and primary support based on rock mass quality and numerical simulation was employed to control the geopressure and deformation. A new support scheme for the deep shaft is proposed, using long bolts to restrain severe deformations, metal mesh, and a double reinforcement bar to improve the induced stress distribution. According to the results, the construction scheme of deep shaft has been improved, and the safe support distance of the proposed scheme is determined to be 12 m, with an interval of three excavation cycles. Compared to the original scheme of shaft lining after excavation, the proposed scheme based on the ASGR mechanism can effectively improve the geopressure release and benefit from controlling the rockburst and large deformation of deep shaft induced by high geostress conditions. The stress distribution in the lining is more uniform, and safety factor of the lining is increased to 2.0, which is benefit the long-term stability of deep shaft.

Influence of a High-Temperature Environment in Deep Mining on the Characteristics of Coal Spontaneous Combustion

ABSTRACT As coal mining continues to progress to deeper levels, the temperature of the coal and rock continues to rise, causing coal spontaneous combustion disasters to become increasingly prominent. This paper studies the spontaneous combustion characteristics of coal in a high temperature environment before and after coal seam mining. The results were as follows: 1) Before mining, under the action of a constant ground temperature, the deep coal seam continuously and slowly released heat, and the higher the ground temperature was, the more heat was released. 2) During mining, after air entered, the coal samples initially maintained the continuous slow heat release. With the continuous entry of air, the coal samples were in an endothermic state. Higher ground temperature provides favorable conditions for coal sample heat absorption and coal spontaneous combustion. 3) When the local temperature rises, the associated hydroxyl groups in the coal sample are gradually activated and become free hydroxyl groups that are more prone to chemical reactions; the methyl groups are destroyed to generate more methylene groups; the aromatic hydrocarbons change significantly after oxygen supply. The long-term high ground temperature environment could lead to an increase in the chemical activity of coal.

Genetic Analysis of Abnormally High Ground Temperature From the Existence of Magmatic Rock Beneath Pingdingshan Coalfield

Studies on the genetic mechanisms of abnormally high ground temperatures in coalfields are of great significance for the efficient mining of coal mines and the development and utilization of geothermal water. Therefore, we measured the temperature of 45 Cambrian limestone groundwater samples, thermal conductivity values of 11 Cambrian limestone samples, and the contents of uranium, thorium, and potassium-40 of 40 rock samples, aiming to ascertain the heat source of the abnormally high ground temperature in the Pingdingshan coalfield. The results show that the average geothermal gradient in the coalfield is about 3.32°C/100 m, and the calculated values of the average ground heat flow are about 104.28 mW/m2, which demonstrate the abnormally high ground temperature in Pingdingshan coalfield. We also concluded that the whole radiogenic heat generation in sedimentary and magmatic rocks is 1.53 mW/m2, whose contribution rate to the local geothermal flow is only 1.47%. Hence, we can infer that magmatic rocks exposed in or near the coalfield area may supply the heat source for the abnormally high ground temperature beneath Pingdingshan coalfield. A comprehensive analysis further shows that the latest intrusion time of the magmatic rock mass in the coalfield was 35 Ma; its lava temperature was 900°C, its cooling rate is 20.0°C/Ma, and the calculated residual temperature is more than 200.0°C. These values support the idea that the magmatic rocks may be the primary cause of the abnormally high ground temperature in the Pingdingshan coalfield.

Investigation of a new type of thermal insulation shotcrete with silicone rubber tubes (SSRT)

In order to solve the high temperature problem faced in underground engineering, a method of laying silicone rubber tubes (SRTs) in the anchor net structure to form a new type of thermal insulation shotcrete with SRTs (SSRT) is proposed in this paper. The mechanical and thermal insulation performances of SSRT are investigated, as well as the correlation between SRTs sizes and compressive strength, splitting strength and thermal conductivity. Results show that SSRT has increased the thermal insulation performance of 44–71% compared with ordinary Portland shotcrete (OPS), and the mechanical performance is affected to varying degrees. Based on the experiments in this paper, the optimal SRTs laying process parameters of SSRT are determined, i.e. SRTs with an outer diameter of 8 mm, an inter diameter of 3 mm and a distance of 8–9 mm between SRTs. This investigation provides a new exploration and theoretical basis for shotcrete support and insulation technology in high ground temperature environments.

Numerical Simulation of the Diffusion of Fissure Slurry at High Temperature

Deep-buried tunnel at high-ground temperature is prone to the sudden surge of water when crossing the water-rich formation, resulting in casualties and economic losses. Slurry injection is a common and effective method of governance for addressing this issue. However, there has been little research into pulping at high-ground temperature, with most of the existing research focusing on room temperature conditions. We studied two kinds of slurry commonly used in grouting, cement slurry and cement-sodium silicate slurry. Indoor testing was used in this paper to clarify the effect of temperature on slurry viscosity, and numerical simulation was used to study slurry diffusion in flat cracks at different temperatures. The results showed that in a hydrostatic environment, the spread rate of slurry was independent of temperature and type of slurry. For the same kind of slurry, the higher the temperature, the greater the maximum slurry pressure. The higher the temperature in a flow environment, the faster the slurry diffusion. As the temperature rised, the grouting pressure required for crack sealing decreased at first then rises. In a flow environment, the sealing effect of cement-sodium silicate slurry was superior to that of cement slurry.

Evidence of Warming From Long-Term Records of Climate and Permafrost in the Hinterland of the Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau (QTP) is characterized by its extreme climate and dominated by periglacial processes. Permafrost conditions vary greatly, and the recent changes on the QTP are not well known in the hinterland. Here, we examine the changes in climate and permafrost temperatures in several different regions. Climate data were obtained from three weather stations from 1957 to 2019. Annual mean air temperature (Ta) has gradually increased at .031°C/yr–.039°C/yr. Climate warming has been more rapid in the past two decades, particularly during the cold season (November to February). Precipitation has also been slowly increasing during the instrumental record. However, there is pronounced heterogeneity in the seasonal distribution of precipitation, with very little falling between October and April. Ground temperatures and active-layer thickness (ALT) have been investigated over ∼20 years at five sites representative of the hinterland of the QTP. These sites are located along the Qinghai–Tibet Highway, which crosses the permafrost zone and traverses the mountainous area and basin areas. Annual mean ground temperatures within the active layer (Tal ∼ 1 m depth) indicate recent ground warming at all sites, at rates near .05°C/yr. The ALT at five sites has been increasing steadily by 2–9 cm/yr, with an average of 4.6 cm/yr. The temperature near the permafrost table (Tps) has been increasing at .01°C/yr and .06°C/yr, with an average of .03°C/yr. Permafrost temperatures at 15 m depth (Tg) have been increasing by about .01°C/yr–.02°C/yr. The southern boundary (AD site) of the permafrost has warmed the least among the five locations. In high mountainous areas where permafrost temperatures are low (e.g., KLS site), the annual mean Tg has increased by nearly .02°C/yr. The rate of permafrost warming at a basin site (BLH), with relatively high ground temperatures, was approximately .01°C/yr. The GIPL2.0 model simulation results indicate that the annual mean permafrost temperature at 1 m depth at these sites will increase by .6°C–1.8°C in the next 100 years (to 2100) and that ALT will increase by ∼40–100 cm. We also discuss the impacts of permafrost changes on the environment and infrastructure on the QTP. This study provides useful information to understand observed and anticipated permafrost changes in this region, under different shared socioeconomic pathways, which will allow engineers to develop adaptation measures.

Study on the Mechanical Properties and Thermal Conductivity of Cotton Stalk Fiber Heat-Insulating Shotcrete

In this study, heat-insulating shotcrete with excellent performance was developed to effectively reduce the thermal conductivity and meet the strength requirements of shotcrete in high ground temperature roadways. Ceramsite and pottery sand were used to replace 5% and 10% of sand and stone, respectively. The shotcrete was composed of cotton rod fiber treated with polyvinyl alcohol solution (SH adhesive) and other materials in a certain mix ratio. The working, mechanical, and heat insulation properties of the shotcrete were investigated by compressive strength, splitting tensile, thermal conductivity, electron microscope scanning, and X-ray diffraction tests. The results showed that the optimum content of the cotton stalk fiber was 2 kg/m3. The compressive strength and the splitting tensile properties were excellent, and the strength attained the maximum value. The performance of shotcrete increased with an increase in the cotton stalk fiber content. The thermal conductivity curve decreased smoothly when the cotton stalk fiber content exceeded 2 kg/m3. Furthermore, when the cotton stalk fiber content was 0–2 kg/m3, the internal cotton stalk fiber was tightly wrapped by the hydrate, and the mechanical properties were enhanced. Moreover, when the cotton stalk fiber content was 2-3 kg/m3, a clustering phenomenon occurred, and the compressive and tensile strengths were reduced. The hydration reaction of the cement mixed with cotton stalk fiber was normal, and the reinforced reticulated porous structure was formed with ceramsite in the concrete matrix, improving the strength of the concrete and reducing the thermal conductivity.

Influence of mechanical wind heated by ground temperature on ambient temperature in tunnels: Numerical modeling, case study and analysis

A numerical heat transfer model of a tunnel in a cold region is proposed, and the accuracy of the model is verified by a laboratory model tunnel and field test. The effects of natural airflow velocity, natural airflow temperature, mechanical airflow velocity and jet fan operation time on ambient temperature in tunnels with high ground temperature are investigated. The result shows that mechanical ventilation can effectively reduce the range of ambient temperature below 0 °C in tunnels (RATB) with high ground temperature in a cold area. The lower the natural airflow temperature, the better the temperature control effect of mechanical ventilation. The RATB of Zilashan tunnel will exceed 5000 m without considering the influence of mechanical ventilation with natural airflow velocity at 3.0 m/s. The RATB decreases linearly with mechanical ventilation under different natural airflow velocities, natural airflow temperatures and jet fan operation time. Increasing the mechanical airflow velocity cannot effectively reduce the RATB considering the influence of mechanical ventilation with natural airflow velocity higher than 2.0 m/s. When considering mechanical ventilation to prevent freezing injury to the tunnel, the jet fan operation time shall not be less than 3.0 h. The longer the operation time of the jet fan, the faster the attenuation rate of RATB. When the jet fan operation time is 5.0 h, the attenuation rate of RATB is 8.6 times greater than when the jet fan operation time is 1.0 h with natural airflow velocity at 1.50 m/s. The existence of an insulation layer will greatly increase the antifreeze length of rock and antifreeze length can be controlled within 650 m under proper ventilation conditions in the Zilashan tunnel.