Fractures in marine sediments are critical zones for hydrate formation. The kinetics and morphological characteristics of hydrates within sandstone fractures are comprehensively investigated in this study by employing a high-pressure visualization reaction vessel to examine their formation, dissociation, and reformation processes. The results are presented below: (1) In 3 mm Type I fractures, the induction time is longer than that observed in the other two fracture widths. Hydrates predominantly form on the fracture walls and gradually expand toward both sides of the fracture. (2) Gas enters the fracture from multiple directions, causing the hydrate in Type X fractures to expand toward the center from all sides, which shortens the induction time and increases the quantity of hydrate formation. (3) An increase in fracture roughness promotes nucleation of the hydrate at surface protrusions but inhibits the total quantity of hydrate formation. (4) Hydrate dissociation typically propagates from the fracture wall into the interior, exhibiting a wavy surface morphology. Gas production is influenced by the fracture width, with the highest gas production observed in a 3 mm fracture. (5) Due to the memory effect, the hydrate induction time for reformation is significantly shorter, though the quantity of hydrate formed is lower than that of the first formation. This study aims to provide micro-level insights into the distribution of hydrates in sandstone fractures, thereby facilitating more efficient and safe extraction of hydrates from fractures.

The technology for freezing shaft sinking is widely used for shafts to pass through deep, unstable alluvia with the continuous exploitation of mineral resources. Due to the technique using the sectional excavation and shaft lining construction adopted in deep alluvia, the radial stress at the inner edge of a frozen wall is incompletely unloaded. In this paper, a mechanical model was established for a frozen wall with its inner edge radially incompletely unloaded. A parameter, α, expressing the degree of being unloaded was introduced, and then a new method of designing and calculating the thickness of the frozen wall was proposed. The range of parameter α was estimated based on the frozen wall–shaft lining interaction forces from field data from a given project. The results indicate that the range of α can be chosen to be from 0.05 to 0.15 in deep alluvia. The design thickness of the frozen wall can be reduced by at least 5% for the frozen wall with the inner edge radially incompletely unloaded. The design thickness is significantly influenced by the strength and elastic modulus of the frozen soil and the elastic modulus of the surrounding unfrozen alluvium. The design and calculation method of frozen wall thickness can provide new ideas for guiding the design of frozen walls in deep alluvia.

As coal resource development progresses deeper underground, the increasing depth of mine shafts poses significant challenges to the safety and stability of traditional shaft construction methods, further compounding operational difficulties. In this context, cast-in-situ concrete shaft walls in a slurry environment have emerged as an effective solution. The strength of these shaft walls is a crucial parameter for assessing their safety. To explore this, experiments were conducted on slurry preparation and mortar casting (used here as a substitute for concrete) under three different conditions: slurry environment, pure water environment, and dry environment. The cast specimens underwent compressive, tensile, shear, and microscopic observation tests to analyze the strength development patterns of the mortar specimens in these varied casting environments. The study yielded several key findings: As the casting environment becomes more complex, the strength of the mortar specimens gradually decreases. Specifically, specimens cast in a slurry environment exhibit strengths approximately 15% to 20% lower than those cast in a dry environment, although both environments show similar trends in strength development over time. Across all casting environments, the initial strength loss of the specimens is significant, while the rate of strength loss decreases in the later stages; the strength loss is minimal in specimens cast in a pure water environment and reaches its maximum in those cast in a slurry environment. Additionally, in specimens cast in a slurry environment, air void diameter tends to polarize, and the distribution of air void is denser compared to the other two environments. In conclusion, cast-in-situ mortar in a slurry environment exhibits the lowest strength and the greatest strength loss compared to specimens cast in dry and pure water environments. Nonetheless, the strength development trends over time remain similar across all conditions, providing theoretical and technical support for the construction of shaft walls in slurry environments.

Artificial ground freezing (AGF) method is a ground reinforcement and waterproofing technique employed in underground engineering to address complex geological conditions. Such strata typically encompass fractured rock mass with intricate fracture networks, where groundwater permeates fractures to form high‐velocity dominant flow. Variations in fracture conditions significantly impact both the development of frozen walls and subsequent construction safety; however, current AGF‐based numerical simulations rarely consider the influence of fracture seepage on the freezing process. To address this gap, a thermal‐hydraulic coupled model for freezing pipes acting on fractured rock mass incorporating fracture phase transition is developed in this study. The effects of fracture aperture, flow velocity, and inclination on the thermal‐hydraulic field and frozen wall evolution are simulated, and the results are validated against existing physical model test data. The duration for the two scenarios ranged from 7.0 to 55.9 days and 5.1 to 55.9 days, respectively. Fracture inclination modifies the affected zone around freezing pipes; thus, the frozen wall development rate of a 40° inclined fracture decreases by 92.5% compared to a horizontal fracture. However, the phase transition extent is a dominant factor for heat transfer in fractured rock mass compared to the aforementioned fracture conditions. Temperature distributions within fractures from numerical simulation closely match physical model test results, while the discrepancy in frozen wall development rate diminishes as fracture flow velocity increases.

The mechanical properties of high-pressure frozen ice are critical design parameters for deep artificial ground freezing and ice sheet drilling operations, making their investigation fundamentally significant. In this study, ice specimens were prepared at −10 °C under freezing pressures of 10, 20, 30, 40, and 50 MPa. In situ axial loading simulation experiments were conducted to investigate their mechanical behavior and macroscopic deformation characteristics during failure. The experimental results indicate that the deviatoric stress–axial strain curves of the ice specimens exhibited a rapid yet smooth transition before and after reaching the peak deviatoric stress, with all samples exhibiting ductile failure. The peak deviatoric stress initially increased and then decreased with increasing freezing pressure, reaching a maximum value of 8.61 MPa at a critical transition pressure of 20 MPa, eventually declining to a minimum of 1.66 MPa at 50 MPa. The residual deviatoric stress decreased significantly with increasing freezing pressure, declining from approximately 3.5 MPa at 10 MPa to 0.85 MPa at 50 MPa. The peak tangent modulus demonstrated a fluctuating trend with increasing freezing pressure, ranging from 1.76 to 2.37 GPa. As the freezing pressure increased, the failed ice specimens transitioned from a densely cross-cracked state to a highly transparent phase, and finally to a sparsely cross-cracked morphology.

The vertical additional force induced by the drainage consolidation settlement of the topsoil is one of the main factors for shaft wall fracture. To date, the number of aquifers of topsoil rises with the depth increasing of shaft, which would lead to a more serious additional force effect. Thus, studying the vertical additional force law of the shaft when drainage settlement occurs in its surrounding topsoil containing multiaquifers is of great significance to predicting the shaft stress and guarantying shaft safety. In this study, mechanical analysis of the topsoil with the shaft crossing multiaquifer was carried out, and the settlement of each aquifer and aquiclude was calculated by separating the single-slope drainage consolidation and the double-slope drainage consolidation. Then, the calculation model of vertical additional force was established due to the settlement caused by the reaction of the additional force on the topsoil containing multiaquifers, and the calculation model of shaft wall stress was also developed. Verification of this model was conducted by comparing the filed measurement data of the shaft wall strain and the theoretical data calculated by the stress model. Finally, the effect laws of drainage velocity, central aquifer thickness, location, and number on the additional force were obtained and analysed. This paper is expected to provide theoretical support for predicting the additional force and shaft wall stress during its service time.