Cement is always used in underground construction to reinforce and improve soft clay, resulting in the formation of a cemented soil–concrete interface. It is of great importance to study interface shear strength and failure mechanisms. So, in order to figure out the failure mechanism and characteristics of a cemented soil–concrete interface, a series of large-scale shear tests of a cemented soil–concrete interface, and corresponding unconfined compressive tests and direct shear tests of cemented soil, were carried out specifically under different impact factors. A kind of bounding strength was observed during large-scale interface shearing. Resultantly, three stages of the shear failure process of the cemented soil–concrete interface are proposed, and bonding strength, peak (shear) strength and residual strength are pointed out, respectively, in interface shear stress–strain development. Based on the analysis results of the impact factors, the shear strength of the cemented soil–concrete interface increases with age, the cement mixing ratio and normal stress, and decreases with the water–cement ratio. Additionally, the interface shear strength grows much more rapidly after 14 d to 28 d compared to the early stage (1~7 d). Additionally, the shear strength of the cemented soil–concrete interface is positively related to unconfined compressive strength and shear strength. However, the trends of the bonding strength and unconfined compressive strength or shear strength are much closer than those of the peak and residual strength. This is considered to be related to the cementation of cement hydration products and probably the particle arrangement of the interface. Particularly, the cemented soil–concrete interface shear strength is always smaller than the cemented soil’s own shear strength at any age.

Artificial ground freezing (AGF) is a widely used method in coastal tunnel construction and reinforcement. With more and more underground construction in coastal areas, clay–sand combined formation, which is common in coastal areas, brings more challenges to AGF. In this paper, the frost–thaw characteristics of soft clay during AFG under the construction of combined formation seepage were studied by model test. It was found that the shape of the freezing curtain changed under the condition of seepage, and the water content of the upper soft soil layer decreased markedly after settlement. Subsequently the micro characteristics of melted soil by CT were also conducted for further mechanism analysis, and it was indicated that the distribution of CT number had obvious segmentation characteristics along the height. Finally, the 3D structure of melted clay was reconstructed, and a method was proposed to calculate freeze–thaw settlement through CT numbers.

The complex condition of an adjacent tunnel in urban city includes high water content, limited construction space, and the presence of an adjacent tunnel. To address these challenges, the artificial ground freezing method is employed to ensure construction safety and stability. Considering the complex problem of temperature field interaction in the freezing construction process of adjacent tunnels, for the first time, this paper proposes a generalized analytical solution for two-dimensional steady-state temperature fields suitable for the annular double-loop freezing system of adjacent tunnels. Based on the polar coordinate heat conduction control equation and the conformal transformation method, the complex geometric arrangement is mapped into a linear system that can be solved, and the analytical solution expression is constructed by combining the heat source superposition principle. In this paper, a numerical model of the adjacent tunnel annular double-loop freezing pipe is established through COMSOL Multiphysics 6.2 software. At the same time, the formula of the analytical method is programmed and solved using Python 3.12, and finally the temperature fields obtained by the two methods are compared. The results show that the analytical solution has good consistency in isotherm distribution, temperature field trend and characterization of frozen core area, which verifies the theoretical rationality and practicability of the constructed model.

In the development and construction of the South China Sea (SCS), coral sand is a kind of common natural construction material. Sanya submarine tunnel is the first application of artificial ground freezing (AGF) in the SCS. Since the tunnel is located at an estuary, high-velocity seepage will have a significant influence on the particle characteristics of coral sand under freezing conditions. Therefore, taking coral sand from the SCS as the research object, the one-dimensional soil column unidirectional freezing test, particle sieving test, and scanning electron microscope (SEM, Hitachi High-Tech Corporation, Tokyo, Japan) test were carried out to investigate the particle breakage and temperature variation characteristics of coral sand under the coupling effect of freezing and seepage. The results show that under the coupling effect of freezing and seepage, coral sand particle breakage was significant. Under none-seepage and 0.5 m/d seepage velocity, the proportion of particles in the 0.5–2 mm size range in the frozen and phase transition zones decreased, while the proportion in the 0.125–0.5 mm size range increased. Through SEM analysis, the coupling effect of freezing and seepage caused serious damage to coral sand particles. Intense freezing could cause coral sand particles to break, while strong seepage could increase the roundness of particles. Seepage would affect the freezing rate and the final stabilization of the freezing temperature; when the seepage velocity was small (0–1.2 m/d), the impact of seepage was not obvious, and when seepage rate was larger (3 m/d), the impact throughout the entire freezing process both reduced the freezing rate and increased the final stabilization of the temperature. This study can provide a reference basis for the research on particle characteristics of coral sand under the coupling effect of freezing and seepage and for engineering and construction in the SCS.