The particle breakage of calcareous sands plays a key role in determining the particle shape and shear strength. This process has been analysed in the current study by employing the ring shear tests on calcareous sands sampled from the South China Sea. In this study, a set of parameters, e.g. roundness and aspect ratio, have been employed to quantify the geometrical properties of calcareous sands pre- and after the breakage. These two parameters have been found to correlate well with the vertical loading stress level, shear strain and shear strength. The particle breakage leads to the change of micro-structure during the shear test, as illustrated by the Scanning Electron Microscopy (SEM) images. It is found that during the shearing deformation, particle breakages can effectively reduce, while particle rearrangements can increase the shear strength of the calcareous sands.

Tunnels constructed in gas-bearing strata are affected by the potential leakage of harmful gases, such as methane gas. Based on the basic principles of computational fluid dynamics, a numerical analysis was performed to simulate the ventilation and diffusion of harmful gases in a shield tunnel, and the effect of ventilation airflow speed on the diffusion of harmful gases was evaluated. As the airflow speed increased from 1.8 to 5.4 m/s, the methane emission was diluted, and the methane accumulation was only observed in the area near the methane leakage channels. The influence of increased ventilation airflow velocity was dominant for the ventilation modes with two and four fans. In addition, laboratory tests on methane leakage through segment joints were performed. The results show that the leakage process can be divided into “rapid leakage” and “slight leakage”, depending on the leakage pressure and the state of joint deformation. Based on the numerical and experimental analysis results, a relationship between the safety level and the joint deformation is established, which can be used as guidelines for maintaining utility tunnels.

Hydraulic fracturing is a rock structure transformation method that significantly weakens the mechanical properties of the hard roof strata. Considering the poor hydraulic fracturing effect of special structure such as composite layered rock, this paper carries out hydraulic fracturing numerical simulation experiments and compares the hydraulic fracture morphology and bedding plane interaction mode under different injection rate and injection modes. The experimental results show that the bedding plane can change the trajectory and propagation direction of hydraulic fracture. Under the low injection rate, hydraulic fracturing is conducive to open the bedding plane, but the expansion length of the main hydraulic fracture is easy to be limited. Under the high injection rate, the hydraulic fracture can extend for a long distance. But the fracture morphology tends to be slender and single, which is not conducive to the formation of fracture network. Compared with conventional hydraulic fracturing, stepped variable injection rate hydraulic fracturing can activate more bedding planes, so as to improve the effect of rock strata transformation. The experimental results are instructive in achieving effective control of composite layered rock.

Both ground source heat pumps (GSHPs) and energy underground structures are engineered systems that utilize shallow geothermal energy. However, due to the construction complexity and associated costs of energy tunnels, their heat exchange efficiency relative to GSHPs remains a topic worthy of in-depth investigation. In this study, a thermal–hydraulic (TH) coupled finite element model was developed based on a section of the Torino Metro Line in Italy to analyze the differences in and influencing factors of heat transfer performance between energy tunnels and GSHPs. The model was validated by comparing the outlet temperature curves under both winter and summer loading conditions. Based on this validated model, a parametric analysis was conducted to examine the effects of the tunnel air velocity, heat carrier fluid velocity, and fluid type. The results indicate that, under identical environmental conditions, energy tunnels exhibit higher heat exchange efficiency than conventional GSHP systems and are less sensitive to external factors such as fluid velocity. Furthermore, a comparison of different heat carrier fluids, including alcohol-based fluids, refrigerants, and water, revealed that the fluid type significantly affects thermal performance, with the refrigerant R-134a outperforming ethylene glycol and water in both heating and cooling efficiency.