This computational study focuses on the thermo-hydro-mechanical simulations of the behaviors of freezing soils used for artificial ground freezing (AGF) in a metro project. Leveraging the experimental and field data available in the literature, we simulate the sequential freezing and excavation of a twin tunneling that occurred in months during the actual construction of the tunnel. A thermo-hydro-mechanical model is developed to capture the multi-physical rate-dependent behaviors triggered by phase transitions, as well as the creeping and secondary consolidation of the soil skeleton and the ice crystals. We then calibrate the material models and establish the THM finite element model coupled with the rate-dependent multi-physical models, which may accurately predict the surface heave induced by ground freezing throughout the project. To showcase the potential of using simulations to guide the AGF, we simulate the scenario where a simultaneous freezing scheme is employed as an alternative to the actual sequential scheme design. We then compared the simulated performance with the recorded results obtained from the sequential scheme. Finally, parametric studies on the effect of ground temperature, the porosity of the frozen soil, and the intrinsic elastic modulus of the solid skeleton are conducted. The maximum surface heave is inferred from finite element simulations to quantify the sensitivity and the impact on the safety of AGF operations.

Artificial ground freezing (AGF) is an effective technique for ground stabilization in projects such as tunneling and shaft mining. This study examines the impacts of freeze-thaw processes, soil type, and compaction levels on the strength characteristics of sandy and clayey soils and evaluates AGF performance through laboratory-scale physical modeling using liquid nitrogen as the cooling agent. Results indicate that freezing significantly enhances soil strength, but thawing leads to notable reductions. Sandy soils compacted to 95% experienced a 50% decrease in unconfined compressive strength (UCS) after brief exposure to thawing, while clayey soils exhibited a smaller reduction of 30%. Compaction emerged as a critical factor in strength retention, with UCS in sandy soils decreasing by 50% when compaction dropped from 95 to 85%, compared to a 25% reduction in clayey soils. The results also demonstrated that sandy soils froze more rapidly and efficiently, achieving a frozen diameter of approximately 25 cm around a single freezing pipe within 4 h, compared to 15 cm in clayey soils over 8 h. Furthermore, sandy soils required less liquid nitrogen to achieve the same frozen column compared to clay soils, owing to their higher thermal conductivity and lower water retention. These findings highlight the superior efficiency of AGF in sandy soils under controlled conditions, particularly when water seepage is absent, and underscore the importance of optimizing compaction levels and freeze-thaw parameters to enhance the cost-effectiveness of soil stabilization. The study provides valuable insights into soil behavior during AGF, particularly the impact of thawing, supporting its broader application in various geotechnical projects.

Artificial ground freezing (AGF), widely employed in subway tunnel construction, significantly alters the microstructure of surrounding soils through freeze-thaw processes. These changes become critical under subway operation, where traffic-induced dynamic loading can lead to progressive soil deformation. Understanding the dynamic behavior of freeze-thaw-affected soils is therefore essential for predicting and mitigating deformation risks. This study investigates the microstructural evolution of soil subjected to a single freeze-thaw cycle-representative of AGF practice-and subsequent dynamic loading. Dynamic triaxial tests were conducted under a fixed dynamic stress amplitude of 10 kPa and loading frequencies of 0.5 Hz, 1.5 Hz, and 2.5 Hz, simulating typical subway traffic conditions. Microstructural analyses were performed using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Results show that the freeze-thaw cycle leads to a denser yet more disordered particle arrangement, with sharper and more angular particles, as reflected by increased probability entropy and reductions in surface porosity, form factor, and uniformity coefficient. Dynamic loading further causes particles to flatten and align in a more directional manner, accompanied by decreased surface porosity and form factor, and an increased uniformity coefficient. Pore structures become more uniform and less complex. Among various microstructural indicators, total intrusion volume from MIP displays a strong correlation with cumulative plastic strain, suggesting its potential as a micro-scale predictor of soil deformation. These findings enhance our understanding of the coupled effects of freeze-thaw and dynamic loading on soil behavior and offer valuable insights for improving the safety and durability of subway tunnel systems constructed using AGF.

As temporary support in geotechnical and tunneling scenarios, frozen soil bodies are often subjected to varying stress states during different construction stages and techniques and, thus, exhibit stepwise loading and unloading, leading to multi-stage creep. However, experimental and numerical investigations on frozen soil creep behavior have focused primarily on monotonic loading, i.e., single-stage creep. This study expands an existing experimental database on stepwise loaded creep and introduces a unique test series focusing on the uniaxial creep behavior of frozen sand under stepwise unloading and load-unload cycles. Here, similar to stepwise loaded creep, the minimum creep rate is found to remain mostly independent of the loading history, while the corresponding frozen soil lifetime depends on the latter. In contrast to equivalent single-stage creep scenarios, the lifetime becomes longer for stepwise loaded creep and shorter for stepwise unloaded creep. To consider multi-stage creep in the geotechnical design of frozen soil bodies, based on our experimental database and literature data, we test the ability of two versions of an advanced constitutive model to capture the frozen soil creep behavior under varying stress states. Comparison of the extended version, called EVPFROZEN, with the original highlights the advantages of EVPFROZEN in consistently capturing the creep rate evolution and the practically important frozen soil lifetime under complex loading histories. Combining the insights from the novel experimental database with testing and validation of the advanced constitutive model EVPFROZEN advances the efficient and sustainable design of frozen soil bodies in geotechnical applications under multi-stage loading conditions.

Since Artificial ground Freezing (AGF) appeared in the 1880s in the mining sector in Europe, it has been used for various construction applications worldwide. In recent years, it has been increasingly popular in urban projects due to its versatility and applicability to complicated site conditions. So far, it has been used to stabilize substrata to nearly 1,000 m below the ground surface, which is considered not possible for many other ground improvement technologies. Due to the growth in field applications, the practice and theories related to AGF have become more mature in the most recent two decades. The improvement in understanding of this topic is a result of lessons that have been learned through numerous projects, as well as a variety of comprehensive studies that have been completed. This paper reviews the existing practice, the recent development on AGF and the challenges of AGF.

Artificial ground freezing (AGF) has emerged as a prominent treatment method due to its ability to mechanically strengthen the soil while reducing its permeability. However, its implementation has raised concerns about its impact, particularly with respect to frost heave and subsequent thaw-induced displacements. These soil movements can cause subsidence and pose a significant threat to the integrity of surface structures. Overburden pressure plays a crucial role in AGF and determines the amount of heave generated. This paper presents an analysis of the existing literature about soil freezing and thawing. The aim is to offer an understanding of these processes, specifically with regard to their application in AGF. This paper explains the behavior of soil during freezing, with particular emphasis on the influence of overburden pressure. It also investigates frozen soils' thawing and freeze-thaw (FT) cycles' long-term effects on soil properties. AGF offers improved soil strength and reduced water permeability, enhancing construction project stability. However, the interplay between the temperature, soil composition, and initial ground conditions during freezing is complex. This thermo-hydro-chemo-mechanical process strengthens the soil and reduces its permeability, but it can also induce frost heave due to water expansion and ice lens formation. Overburden pressure from the overlying soil limits ice lens growth. FT cycles significantly impact soil properties. In fine-grained soils, FT cycles can lead to over-consolidation, while rapid thawing can generate high pore pressures and compromise stability. Importantly, FT acts as a weathering mechanism, influencing soil properties at both the microscopic and macroscopic scales. These cycles can loosen over-consolidated soil, densify normally consolidated soil, and increase overall hydraulic conductivity due to structural changes. They can also weaken the soil's structure and deteriorate its mechanical performance.

Artificial ground freezing (AGF) is a ground improvement technique enabling the construction of underground structures in challenging geological conditions. After constructing an underground structure within the groundice cofferdam, the soil undergoes a thawing process that can impact the structure stability and waterproofing properties of the lining. Minimizing or preventing potential damage, as well as avoiding delays in construction, can be achieved through a rational design of thawing regimes. In this paper, we present a semi-analytical model for the thermal behavior of ice-wall during its natural or artificial thawing. The process is described by three independent one-dimensional mathematical problems: the thawing of the outer surface of the ice wall, the thawing of its inner surface, and the thawing of soils around the freeze pipes (in the case of artificial thawing). The proposed approach facilitates the calculation of natural and artificial thawing times and the power required for artificial thawing. The efficiency of the model is demonstrated by comparison with numerical simulation results. This makes the approach suitable and desirable for engineering practice. Importantly, the model allows for seamless analysis of several combinations of influencing factors to select thawing parameters aligned with the requirements of different construction projects.

The advancement of massive construction in urban subway projects contributes to the increased use of the artificial ground freezing (AGF) method in the construction of cross passages due to its reliability and environmental friendliness. However, the uplift or subsidence of the ground surface induced by the frost heave and thawing settlement of the soil can be a problem for existing buildings, and the current design method places way too much emphasis on the strength requirement of the freezing wall. In this study, FLAC3D was employed to develop a series of state-of-the-art numerical models of the construction of a typical subway cross passage by the AGF method, utilizing freezing walls with different thicknesses. The results of this study can be used to examine the ground deformation arising from the AGF method and the influence of the thickness of the freezing wall on the AGF method.

The control of freezing temperatures throughout the artificial ground freezing (AGF) process is always difficult. An overly high temperature of the circulating refrigerant may lead to insufficient frozen soil strength, while an overly low temperature may cause unnecessary energy waste, and even excessive pore ice may damage the soil structure and reduce the frozen soil strength. What's more, overly freezing may damage buildings on the surface. Therefore, it is of great significance to study the optimum freezing temperature (OFT), which is very important for better and more energy-efficient employment of the AGF method. In this paper, we use uniaxial compression and direct shear tests to obtain dynamic mechanical parameters in the soil freezing process. After the analysis of varying mechanical parameters by the entropy weight TOPSIS principal component analysis method, the results show that the interval range of OFT for saturated and unsaturated sandy gravel is [- 10 degrees C, - 15 degrees C] and [- 15 degrees C, - 20 degrees C], respectively. The findings indicate that, in the AGF method, a lower temperature is not always preferable. According to the results, constructive measures to optimize the temperature field distribution in the AGF method are proposed. The research results will contribute to the assessment of the safety and efficiency of AGF projects.