A thermal insulation layer is often deposited on the lining structure of tunnels in cold regions to solve the problem of frost damage. When the air humidity in the tunnel becomes excessively high, the thermal insulation material tends to absorb water, leading to significant changes in thermal conductivity. Moreover, the temperature differences between the day and night cycles have been observed to be significant in portal sections of cold region tunnels, which facilitate the freeze‐thaw cycle and, consequently, deteriorate the performance of the thermal insulation material. Therefore, the purpose of this study is to determine the changes in the water absorption, thermal conductivity, and microstructure of polyurethane and polyphenolic insulation boards under freeze‐thaw conditions. To this end, an indoor water absorption test was conducted for both the insulation boards till they were saturated, which then underwent a freeze‐thaw cycle test. It was determined that the water absorption and thermal conductivities of these boards increased linearly with the number of freeze‐thaw cycles. In order to explore the change of thermal conductivity of thermal insulation materials after moisture absorption, this study provides insights into the relationship between the thermal conductivities and water contents of tunnel insulation materials under normal and freezing temperatures.

This study investigates the distribution of pore water on the basis of the measured mass moisture content after soaking the tunnel insulation material. This study also analyzes the influence of the distribution of pore water on the thermal conductivity of the material on the basis of this mass moisture content. Scanning images of phenolic and polyurethane insulation boards are obtained by computer tomography (CT). The gray volume moisture content (Gv) is deduced based on the CT scanning images, to determine the distribution of pore water (Gv is the ratio of the volume of the water sample (represented by the gray value) to the volume of the saturated water sample (represented by the gray value) which is the gray volume moisture content of the sample). The correlation between gray volume moisture content and mass moisture content is determined by comparing different algorithms of gray volume moisture content and volume moisture content. The relationship between mass moisture content and thermal conductivity can be determined using a self‐made quasi‐steady‐state tester, whereas the relationship between gray volume moisture content and thermal conductivity can be derived indirectly. Related experimental research can predict the thermal conductivity of thermal insulation materials by using a new perspective and shows the influence of pore water distribution on the thermal conductivity of materials.

The deformation control effect of loess tunnel composite foundation plays an important role in optimization design and reinforcement effect evaluation. Systematically evaluate the adaptability of the composite foundation of jet grouting pile in shallow collapsible loess tunnel. Taking the shallow buried section of Fujiyao Tunnel with a buried depth of 20 m as an example, using MIDAS finite element numerical simulation software, the foundation deformation control during construction and settlement control after construction are systematically studied, the differential deformation control is analyzed, and the reinforcement effect of the tunnel bottom is evaluated. The results show that the uplift displacement can be controlled by changing the pile length and increasing the replacement ratio. The combination of long and short piles can significantly reduce the uneven settlement and plastic zone of the foundation. The uneven settlement of 9 mm can be used as the evaluation index of the composite foundation reinforcement effect in a shallow buried section of the loess tunnel.

In geotechnical engineering, traditional anchor bolts often have problems such as an insufficient bearing capacity, their ease of loosening, and an unsatisfactory support effect under complex geological conditions (such as soft soil or broken surrounding rock), resulting in it being difficult to guarantee engineering stability. In order to solve these problems, this paper studies the supporting performance of a hollow grouting anchor with an umbrella-shaped expansion head with an expansion pipe. Through theoretical analysis, mechanical performance analysis, and experimental analysis, the supporting mechanisms and mechanical characteristics of a hollow grouting anchor with an umbrella-shaped expansion head are systematically discussed. The calculation formula for the maximum pull-out force of the umbrella-shaped expansion head is clarified, and the fixed range of the expansion body section in relation to the loose ring is quantified. Based on the analysis results, the structural parameters and material properties of the bolt were optimized, and the optimization effect was verified by numerical simulation. The results show that the optimized bolt has significantly improved the pull-out bearing capacity, shear resistance, and reinforcement effect on the soil. The maximum pull-out force of the umbrella-shaped expansion head can be increased by up to 35%, and the fixed range of the expansion body section can be expanded by 45%. The research provides an efficient and reliable support solution for geotechnical engineering fields, such as roadway engineering and tunnel engineering, which significantly improves the stability and safety of engineering under complex geological conditions. At the same time, it provides an important theoretical basis and practical reference for the design and construction of similar projects.

Phenolic foam is extensively utilized as a thermal insulation material, and its performance is significantly affected by the ingress of external rainwater. To simulate the material’s environment, we conducted indoor tests to assess its water absorption characteristics under varying water pressure heads. Additionally, we utilized CT scanning to analyze the distribution of water within its pores. The findings indicate that the specimens’ water absorption patterns are quite consistent across different water pressures, with a maximum absorption rate reaching approximately 3200% after reaching a basic level of stabilization. For the novel water-absorbing phenolic material, we processed the CT scan images to enhance clarity and established criteria for determining the gray levels of pore water. It was observed that the phenolic material’s water-absorbing infiltration interface was relatively flat, with localized high infiltration heights at points where large pores interconnected, forming water columns. Microstructural analysis was conducted to elucidate the mechanisms behind the formation of these prominent water columns.